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Human Genetics

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Human Genetics Powered By Docstoc
					                               Human Genetics
                                     Booklet No. 75
                                 Biotechnology: BTS - 5
Contents
Preface
I.     Introduction
II.    Human Blood Groups
           A. Identification
           B. Medically disputed parentage
           C. M-N blood groups
           D. Rh blood groups
III.   Darwin's Theory
           A. Change in natural populations
           B. Theory of Natural selection
IV.    Evolution and Population Genetics
           A. Mutation and selection
           B. Differential selection
           C. The Hardy-Weinberg principle
                  1. Determining frequencies of allelic genes
                  2. Genetie constitution and inheritance of ABa blood groups
                  3. Consequences of Hardy- Weinbcrg equilibrium
                             a. Prediction of allele and genotype frequencies.
                             b. Conservation of genetic variability
                             c. Restoration of Hardy-Weinberg equilibrium
                             d. The dynamic Hardy-Weinberg equilibrium
                  4. Uses of the Hardy-Weinberg equilibrium
                             a. Testing the Hardy-Weinberg equilibrium
                             b. Prediction of genotypes for genetic counseling
           D. Genetic drift
V.     Survey of Human Heredity
           A. Pedigree analysis
           B. Kinds of hereditary traits
           C. Examples of human pedigree
           D. Autosomal abnormalities
                  1. Down's syndrome
                  2. Patau's syndrome
                  3. Edward's syndrome
           E. Sex chromosomal abnormality
                  1. Kleinfelter's syndrome
                  2. Turner's syndrome
VI.    Eugenics
               A. Status of natural selection in man
                       1. Development of medical science
                       2. A differential birth rate
                           3. War
               B. Negative eugenics
                       1. Segregation
                       2. Controlling immigrations
                       3. Marriage restrictions
                       4. Sterilization
             C. Negative eugenics (Genetic variables)
             D. Positive eugenics
                   1. Subsidizing the fit & sperm banks
                   2. Promotion of genetic research
                   3. Early marriage of those having desirable traits
                   4. Genetic counseling
                   5. Protection against mutagens
                   6. Improvement of environmental conditions
             E. Euphenics
VII.   Conclusion

Preface

        Social activists have to deal with various types of people in their work situations and
often strains in human relations become a hurdle in their development work. These problems
come also because of the lack of understanding of the human genetic nature and functions.
This booklet explains the basics of human nature and behaviour.

Dr. K. T. Chandy, Agricultural & Environmental Education

1. Introduction

       Human heredity is naturally one of the most interesting phases of the study of genetics.
A great many other human characteristics have been studied genetically, and since some
knowledge of these may be useful as well as interesting, this booklet catalogues some of these
and gives some information about their mode of inheritance.

        At the outset, it must be stressed that much of our information is incomplete regarding
the inheritance of certain traits, and present conclusion are often tentative. This is because
unlike other species of animals and plants, human are not bred experimentally, therefore, we
cannot apply the type of genetic analysis that have been discussed in the case of animals and
plants. There is difficulty in the study of human genetics due to two seasons.

1. Geneticist is not able to arrange the matings of the human individuals he is studying.
2. The long life cycle of men, production, relatively small numbers of off springs, are
unfavourable factors in term of certain standard research techniques. In spite of these
difficulties, the study of genetics is not impossible. However, before we delve into the
complexities of human genetics, it is necessary to have a concept of human blood groups and
Darwin's theory of Natural selection.

II. Human Blood Groups

       The main components of the human blood are (i) Cells, which are of three main types
viz. RBC (red blood corpuscles), WBC (white blood corpuscles) and platelets, and (ii) the
plasma, which is the liquid portion of the blood. Four types of blood groups are discovered and
designated as O, A, B and AB on an International system.

        The types of blood is classified in the presence or absence of antigens or agglutinogens
and antibodies or aggulations. Antigens are proteinaceous substances at least in part. They are
capable of stimulating the production of specific antibodies in an animal body in response to
foreign antigen. Animal body does not produce antibodies against its own antigens. Clumping
takes place if the RBC and the plasma belong to the different blood groups having different
antigens. Antibodies produced are specific for the antigen which stimulate their production. A
reaction between the antigen and its antibody is the basis of the clumping of blood. Antibodies
which are produced in response to an injection of foreign antigens are termed as acquired
antibodies. There production is stimulated only when an antigen makes entry into the body. In
certain cases, antibodies are produced in the blood even in the absence of their antigens.
These are termed as natural antibodies and there presence depends 'upon the type of blood of
an individual.

A. Identification
        It is seen that if the blood from group A is mixed with that of B, the blood is coagulated or
is agglutinated. The agglutination takes place because of antigen and antibody reaction. The
blood cells of a group contain antigen A and its serum antibody B, so the moment the blood
from group B is transfused into a person of group A, violent reaction starts between the antibody
B (present in the serum of A) and antigen B, brought by the blood of Band vice-versa the patient
receiving such transfusion dies of Shock.

        The blood of AB group contain both antigens but none of the antibodies A or B in its
serum. Since the serum of AB group does not contain any antibody in it besides the blood of its
non group, tolerate the transfusion of blood from all other blood groups. For this season, the
individuals belonging to this group, are called as universal recipients. They however, cannot
donate blood to persons other than those of their own group because both antigens A and B are
present in their blood. Antigen A will react with antibody B, and antigen B with antibody A when
blood from AB is transfused into individuals of either A or B group.

         The group O has both antibodies A and B, but no antigen. Because of lack of antigen, it
does not coagulate blood when administered into the blood of other groups (A,B or AB). Since
all individuals can tolerate the transfusion of O blood group, it is called the universal donor. It
can however only accept its own blood since all others with one kind of antigen or the other will
violently react with the antibodies (A,B) present in the serum of O-blood group.

                  Table 1. Blood group, their antigens and antibodies in plasma

              Sl.No   Blood           Antigen    present Antibody      present    in
                      groups          in RBC             plasma
              1       A               A                  Anti-B or ‘b’ or Bita
              2       B               B                  Anti-A or ‘a’ or Alfa
              3       AB              A and B            No
              4       O               No                 Anti-A and anti-B or ‘a’
                                                         and ‘b’ or Alfa and Bita

B. Medically disputed parentage
       Sometimes dispute arises to the parentage of a child. In such cases, the blood group
may sometimes provide an unmistakable clue as to the parentage of given child. For example, if
the husband and wife both have '0' group, then the child cannot have A,B or AB. Likewise if both
the parent have 'A' then the blood group of their children cannot be 'B' and so on. However,
from the marriage between two parents having 'A 'and 'B' Blood group, children with O, A, B and
AB blood groups may be born because such parents may be heterozygous for both A and B.
Therefore, although a very reliable criterion, the blood group data must be used with caution.
C. M-N blood groups
       After the discovery of ABO blood groups in 1927 two additional antigens called M and N
were discovered by scientists Landsteiner and Levine in human blood. Thus human races may
possess M, N or MN blood groups. These blood groups have no relationship with the ABO
group, and M-N blood group do not contain natural antibodies against other M,N group.

D. Rh blood groups
        Wienen, Levine, Race, Taylor and others discovered another series of multiple allele.
Producing antigens and this series is called the rhesus system or Rh system. If blood from a
rhesus monkey is injected into rabbit's blood against the antigen in the monkey's blood, the
monkey blood agglutinates. The same antibodies produce agglutination of blood cells of some
of the human, indicating that these persons also have the same antigen, as that of rhesus
monkey. Such persons are termed Rh+. In others these antibodies do not cause agglutinations
of the blood cells indicating the absence of antigens and these are termed as Rh-. It is seen that
85% of human population are Rh+ (positive).

        The important aspect of Rh blood system is that when Rh+ blood is given to an Rh-
person, immune reaction may result and rhesus antibodies may be formed. Subsequent
transfusion of Rh+ blood to the same receipt will result in the clumping of the donor blood as the
antibodies produce after first transfusion will new react with the Rh+ blood of the donor and the
recipient may die thus, such mis-matched transfusion must be avoided.

        A similar situation may arise when there is a rhesus incompatibility between the mother
and the child. Since Rh+ is dominant over Rh-, the fetus carried by and Rh- mother by her Rh+
husband may be Rh+. At the time of first pregnancy of such mother, the foetal blood cells may
leak through the placenta, mix with the maternal blood at birth and stimulate the production of
antibodies in her blood. These antibodies are less in number during the first pregnancy and do
not harm the foetus and the fIrst child will be safe and normal. The antibodies produced during
subsequent pregnancies (with Rh+ foetus) in mother's blood persist and cross over into the
circulation of any subsequent foetus which if Rh + will undergo haemolysis or destruction of its
cells at birth this reaction takes the from of anaemia, a condition known as crythroblastosis
foetalis. Thus, it is advisable for an Rh- women to marry an Rh- man.

       In case a blood transfusion is needed, no Rh- person should even be given blood of Rh+
person as it will lead to crythroblastosis due to incompatibility of the two blood. Such in
compatibility also occurs in some cases of type '0' woman bearing a type A or B child. She has
antibodies that can react with antigens present in her child. The incompatibility occurs very early
in pregnancy instead of very late as in Rh-system and results in early abortions.

III. Darwin's Theory

        Alteration in gene frequency represent the basin for change in the genetic structure of
natural populations. In essence this is evolution. More is required, however to establish changes
in the gene frequencies in population units over periods of time and thus to account for race and
species formation.

A. Change in natural populations
       Extensive investigation designed to analyse the behaviour of genes in natural
populations are those of Dobzhansky and his associates, involving the various species of
Drosophila, especially, D. pseudo obscura. Several structural variations of the third
chromosome that were associated with different gene arrangements in natural populations of
D.pseudo obscura farmed the basis of study. appropriate comparisons showed chromosome
inversions to be mainly responsible for the different chromosome types. The standard (ST)
chromosome type, for example, differed from a type symbolized CH on the basis of certain
areas of the third chromosome of ST that were inverted with respect to CH. Chromosomes
carrying inversions behaved essentially like other chromosomes in the population. Some
individual flies were found to be homozygous and other heterozygous for a given inversion.

        Natural populations were usually not distinguishable by the universal presence or
absence of particular chromosome type. Quantitative rather than qualitative, differences were
found to be characteristic of differences were found to be characteristic of different geographical
populations of D. pseudo obscura. The relative frequency of different genes or of particular
chromosomes arrangements formed the main criterion for distinguishing one population from
another. Homologous chromosomes, distributed through random mating, tended to establish an
equilibrium in the same way as do the alleles described by the Hardy -Weinberg theorem.

       When certain laboratory condition environments are established to compare different
populations, it became evident that at particular seasons of the year more flies carrying some
chromosome types survived than those of carrying others.

        Flies with certain chromosome types were better fitted to particular temperature and
nutritional conditions than to others and thus were favoured in certain areas and during
particular seasons of the year. The effectiveness of selection was indicated by the relative
frequencies of chromosome types which varied from population to population and from season
to season within a given population.

        In some laboratory experiments, the weaker competitors were not eliminated completely,
but instead an equilibrium was established between the frequencies of the more favourable and
less favourable chromosome types. Dobzhansky explained the failure of the weaker to be
eliminated on the basis of the superiority of the heterozygous combination, i.e. heterosis. The
heterozygous arrangement, which in itself was considered to be favoured, contained both
competing chromosome types and thus perpetuated the weaker competitor which would
otherwise have been eliminated. The term balanced polymorphism was used to describe a
stable mixture of different genotypes in a population.

B. Theory of natural selection
        The concept of natural selection as a directing force in evolution was crystallized by
Charles Darwin (1809 to 1882) and A.R. Wallace in 1858. In his book 'Origin of Species' (1859)
Darwin presented a detailed argument in support of evolution by natural selection. He had been
led to his interpretation largely by his own observations and reflections on the natural
distribution of animals and plants. This theory began to take form in 1838 when he read an
essay on human population, which also applied to animals and plants, written by T.R. Malthus in
1798. Darwin developed a theory based on three facts and three deductions.

1. Facts
        The following were the facts of Darwin's theory: (a). populations increase geometrically
and tend to overproduce, i.e., progeny outnumber parent; (b). in spite of overproduction, the
number in a given population or species remain fairly constant because space and resources
are limited; and (c). Inherent variation exists in all population.

2. Deductions
       The following were the deductions of Darwin's theory.

a. Struggle for existence
         If the all the seeds of any particular plant were to germinate and all seedlings to grow up
into full sized plants, a very wide area would soon be covered by them in course of a few years.
If other plants (also animals) were to increase at this rate, a keen competition for existence,
would be set up at once among them because of supply of food, water and space would fall far
short of the demand. A struggle would soon amuse, resulting in the destruction of huge
numbers of individuals.

b. Variation and inheritance
        It is known that no two individuals, even coming out of the same parent stock are exactly
alike. There are always some variations, however minute they may be, from one individual to
another. Some variations are suited to the conditions of the environment, while others are not
according to Darwin these minute variations are preserved and transmitted to the offspring
although no cause for these variations was assigned by him.

c. Survival of the fittest
        In the struggle for existence the individuals showing variations in the right directions
survive, and these variations are transmitted to the offspring, others with unfavourable variations
perish. This was called 'survival of the fittest'. The survivors gradually and steadily change from
one generation to another, and ultimately give rise to new forms. These new forms are better
adapted to the surrounding conditions.

        Darwin observation on the variation of domestic animals and cultivated plants served
him as a due to the elucidation of his theory of Natural selection. According to it animals and
plants are multiplying at an enormous rate. As no two individuals are exactly alike, the new
forms naturally show certain variations. Some variations are favourable or advantageous so far
as their adaptatible to the condition of the environment is concerned, and other are not so.
Owing to an excessive number crowding together a keen struggle for existence ensues. In this
struggle, those that have favourable variations (therefore better fitted) naturally survive and the
rest perish. Through this survival of the fittest, the species change steadily owing to preservation
and transmission of minute variations, and gradually give rise to new forms. Darwin called this
process natural selection from analogy to artificial to artificial selection. It is the environment that
selects and preserves the better types and destroys the unsuitable forms.

       Darwin's work supported the concept of evolution but it left unclarified many points
concerning the mechanics of the process. The competition of numerous later experiments has
served to modify and define Darwinism, but the basic theory, is firmly established. Investigation
of population genetics have laid the foundation for a explanation of race and species formation.

IV. Evolution and Population Genetics

        The basic mechanics of evolution lies in the field of genetics there can be no evolution
without hereditary variation in the population. T'hrough selection there is a tendency to smooth
out the variations which arise in any population living in stabilized environment. Some
characters will be weeded out, some will become more widespread. In the course of time a
population will tend two become homogeneous throughout, and there would no longer be any
variety to serve as a basis for further evolutionary change were it not for the fact that hereditary
material is capable of change. Such adaptive changes are not frequent, as we have learned, but
they do occur at a steady rate throughout the ages, providing new material which becomes the
basis for continuing evolution.
A. Mutation and selection
        The mutation of the gene lies behind all evolutionary changes. New phenotypic effects
may be produced through changes in the gene position or changes in chromosome number, but
without gene mutation these changes are necessarily limited in scope. Though 99% of the
mutations are regarded as harmful, the one percent which happens to be beneficial, forms the
basis for most evolutionary developments. Through selection there is a differential multiplication
of the few mutation which contribute to the welfare of the race. Should this selection be relaxed,
either through natural or artificial influence, the accumulating harmful mutations which are
ordinarily not perpetuated would soon bring about disorder and degeneration in place of
adaptation. Further, mutation combined with natural selection forms a powerful force for the
strengthening and continued evolution of species. Mutation without natural selection exerts a
deteriorating influence. The combination of mutation and selection is necessary not only for
improvement, but for the continued existence of the race.

B. Differential selection
        Variation in frequency of distribution of specific genes may occur in populations living in
different environments because of differential selection. For example, the sickle-cell trait in man,
seems to more prevalent in certain races than in others. A study of the geographical distribution
of the gene offers a clue to a solution of the problem. Its frequency is greatest in certain areas of
equatorial Africa, in a small region of India and in parts of Greece. These are the areas of the
world where malaria has been very prevalent. Allison and other have found a very high co-
relation between high frequency of the sickle-cell trait and the prevalence of malaria. This
suggests a greater resistance to malaria by these individuals. The advantage possessed by
those with two of these genes, and keeps the gene at a high level in the population. Something
about this haemoglobin is not compatible with the needs for the growth of the malarial parasite
in the red blood cells. This shows how a gene which may be entirely disadvantageous in one
environment can have some advantage in another environment and selection can take place on
a different basis.

C. Hardy. Weinberg principle
        In 1908,two scientists G.H. Hardy and W. Weinberg made a very important contribution
to the study of population genetics. They developed a simple mathematical method of analyzing
the frequencies of alleles in populations. This principle holds that in a population alleles tend to
establish an equilibrium with reference to each other. As an example, if two alleles occur in
equal proportions in a large, isolated population and neither allele has a selective advantage
over the other, they will be expected to remain in equal proportion generation after generation. It
would make no difference if one allele was dominant and the other recessive or if there was an
intermediate expression. Since this is true, it is possible to calculate the approximate
frequencies of the alleles from a sample which is representative of the population as a whole. Of
course, we will never find a large population where there is no selective in advantage of one
allele over another and no variation in rate of mutation, but the principle is of great value since
there are many population which approximate these conditions.

1. Determining frequencies of allelic genes
        By means of the Hardy-Weinberg principle it is possible to determine the frequency of a
particular recessive allele in a population and the number of heterozygotes carriers of the allele,
as well as the number of homozygous dominant individuals.
        To calculate recessive allele take a representative sample of the population and
determine the percentage of the people who show the recessive phenotypes. The square root of
this percentage will give the approximate percentage of the recessive gene in the population
assuming that there are only two variant alleles.

       The frequency of the dominant allele then is easily determined since it represents the
balance of the genes at this locus in the population.

       From the percentages, it is easy to separate the people who show the dominant trait into
heterozygous and homozygous individuals. The square of the percentage of the dominant allele
in the population represents the homozygous person who slow the dominant trait, and the
balance of those showing the dominant trait, are therefore heterozygous. For example, some
persons have the ability to roll the tongue into a distinct V-shape when tongue is extended from
the mouth. A dominant gene seems to be responsible for the tongue -rolling ability, while its
recessive allele brings about an inability to roll the tongue.



2. Genetic constitutions and inheritance of ABO blood groups
        In the ABO group there are 4 subdivisions namely A,B,O and AB and a person may
belong to any of these subdivisions. The view that the inheritance of ABO blood groups is
governed by three multiple alleles situated at the same locus. It was propounded by Bernsterin
who assigned the term A,B,O for these three alleles. It is not known as to which of these four
blood groups represents the normal one. Ordinarily in any species, individuals with normal traits
are the most numerous and on this basis the groups '0' and 'A' which are fairly common, may be
convenient, the class '0' is generally regarded as representing the normal. In that case the
groups 'A'and 'B' might be considered as having arisen from group '0' as a result of two
dominant mutations, one for each group. The mutant genes may be represented by the symbol
A and B respectively, both originating in the same locus from one of the normal genes in class
'0'. The normal gene representing the class '0' is indicated by the symbol II. The classes 'A' and
'B' may be conveniently represented by the symbols IA and lB. Thus the genotypes of the four
blood groups can be represented as:

Class 0        -nor + +
       A       -IA IA or lAi
       B       -IB IB or lBi
       AB      -IAIB

        Since (+) is recessive, group '0' must be homozygous for (+). Since gene A is a
dominant allele group A might be either homozygous (A/A) or heterozygous (+ / A). Similarly
group B might be either B/B or + /B. Group AB is always a hybrid and has the composition of
A/B. If both the parents in a family belong to class O, they must both have the genotype +/+.
Naturally all their progeny also must be of the + + i.e. class O. On the other hand if both the
parents belong to class A and both are hybrids with the constitution A/+, then the chances are
that some of their children might be of class O and some of their own class because a cross A/+
X A/+ will produce off spring in the Mendelian ratio of 1 (A/A) : 2(A/+ ) : 1 ( + I A). Therefore, if
both the parents belong to the group A, the children would to be either of group A or O. (Table
2)

                       Table 2. The relation or blood group inheritance
                 Sl.No Blood groups in Blood           group Blood      group
                        parents             which can occur which    cannot
                                            in children     occur        in
                                                            children
                1       OxO                 O               A, B, AB
                2       OxA                 O, A            B, AB
                3       OxB                 O, B            A, AB
                4       AxA                 O, A            B, AB
                5       BxB                 O, B            A, AB
                6       AxB                 O, A, B, AB     …
                7       O x AB              A, B            O, AB
                8       A x AB              A, B, AB        O
                9       B x AB              A, B, AB        O
                10      AB x AB             A, B, AB        O

         Similarly the Table 3, gives the genotypes of the parents and the possible genotypes of
their offspring showing impossible groups as a result from the mating of parents

           Table 3. Genotypes and phenotypes of parents and offspring or different
                                      blood groups
      Sl.No Parents                   Offspring                       Impossible
                                                                      group
             Group       Genotype     Group          Genotype
      1      OXO         O/O X O/O    O              O/O              A, B, AB
      2      OXA         O/O X A/A    A              A/O              O, B, AB
      3      OXA         O/O X A/O    O, A           O/O, A/O         B, AB
      4      OXB         O/O X B/B    B              B/O              A, AB, O
      5      OXB         O/O X B/B    O,B            O/O, B/O         A, AB
      6      O X AB      O/O X B/O    A,B            A/O, B/O         O, AB
      7      AXA         A/A X A/A    A              A/A              B, AB, O
      8      AXA         A/A X A/O    A,A,O          A/A, A/O, O/O B, AB
      9      AXA         A/O X A/O    A              A/A, A/O         B, AB O
      10     AXB         A/A X B/B    AB             A/B              A,B,O
      11     AXB         A/A X B/O    A, AB          A/O, A/B         B,O
      12     AXB         A/O X B/O    B, AB          B/O, A/B         A,O
      13     AB X AB     A/B X A/B    A,B,AB         A/A,A/B,B/B      O

3. Consequences of the Hardy-Weinberg equilibrium
        A population that reproduce by random mating, in which no forces are disturbing the
allele or genotype frequencies during the life cycle, has a number of properties that are a
consequences of the H. W. equilibrium.
These are :

a. Prediction of allele and genotype frequencies
       They key consequence of the H. W. law is the fixed relationship between allele
frequencies and genotype frequencies for autosomal loci that define the gene pool of a
population in H. W. equilibrium i.e.

                      a2 = X (RR)
                      2ab = Y (Rr)
                      b2 = Z(rr)
        By knowing the allele frequencies we can determine the genotype frequencies in the
population. This fundamental binomial property of the frequencies of alleles and genotypes in
diploid species may also be used to relate the allele frequencies in one generation to the
genotype frequencies in the next generation.

b. Conservation of genetic variability
        The static H.W. equilibrium will continue indefinitely as long as there are no disturbing
influences that alter random mating and as long as the normal Mendelain segregation ratios
occur in the offspring of all mating types. Consequently, any genetic variability that exists in the
population will be maintained as long as the H. W. equilibrium persists.

C. Restoration of H. W. equilibrium
        If the allele frequency is disturbed at any point in the life cycle by the intervention of
mutation, drift, selection or migration, the parental genotype frequencies may not be in H.W.
equilibrium. But the genotype frequencies in the next generation of parents will return to the H.
W. equilibrium values after just one generation of random mating if the disturbing forces have
been removed from inl1uencing the population.

d. The dynamic H. W. equilibrium
        When the allele frequencies are disturbed the population does not return to the former
equilibrium, but seeks a new equilibrium in one generation. But an alternative kind of equilibrium
may be more typical in natural populations, in which the allele frequencies remain stable from
one generation the next even though the genotype frequencies are disturbed during the life
cycle. Most natural populations are likely to experience this kind of dynamic equilibrium because
a balance of the forces of mutation, genetic drift, selection and migration is usually operating in
a real population. An idealized population in which these factors do not play a role in
reproduction rarely, if ever exists in nature.

4. Uses of the H. W. Equilibrium
       Some of the important uses of the H.W. equilibrium are mentioned here.

a. Testing the H.W. equilibrium
        Deviation of the observed genotype frequencies from the expected frequencies
predicted by the H. W. law are a measure of the failure of a population to reproduce without the
influence of mutation, drift, selection or migration. Rejection of the H. W. equilibrium for a
population is the first step in establishing the presence of forces that are altering allele
frequencies from generation to generation.

b. Prediction of genotypes for genetic counseling
        Knowing that the H. W. equilibrium is true allows us to I complete the probability of a
particular genotypes occurring in an unstudied individual. If the individual is known to be a
carrier of a deleterious recessive allele, it is possible to use the H. W law to compute the
probability that such a person will marry another carrier If the frequency of the deleterious allele
is less than 0.33, we know that it is more likely that the known carrier will marry an individual
who is homozygous for the normal allele. The probability that the mate is a carrier (2 ab) will
decrease as the frequency of deleterious allele approaches zero.

D. Genetic drift
       One factor which may be of evolutionary significance is known as genetic drift. This is
the term applied to the fluctuation of gene frequency which is due merely to the chance
assortment of genes in meiosis and fertilization and is unrelated to the benefits or detriments of
the genes involved. The effects of such genetic drift are, of course, much more pronounced in a
small drift are, of coarse, much more pronounced in a small isolated population group than in
large groups where the magnitude of the number would level 9ff random variations in gene
distribution.

         An excellent case of genetic drift in human population is shown in a study made by H.
Bentley glass of a group of people in Franklin County, Pennsylvania known as the dunkers.
These people are descendants of a religious sect known as the Baptist Brethren, who lived in
the Rhineland region of Germany near Krefeld. In 1719, a group of 28 people migrated to
Pennsylvania. Later others of the same sect came from Germany to join them. They have
remained relatively isolated from others in America, with their own customs and manner of
dress distinct from those around them. among a number of characteristics of the dunkers which
were studied the ABO blood groups will serve to illustrate the possible influence of genetic drift
in this rather small isolate. Comparisons were made with the people in the Rhineland of West
Germany and with people in the eastern United States. The proportion of the different blood
types in shown in Table 4 presented here.

           Table 4. Blood types among Dunkers, Rhineland Germans and American

  Sl.No Group people       No. of people     Percentage of ABO blood groups
                                             O           A           B              AB
  1       Dunkers          228               35.5        59.3        3.1            2.2
  2       Rhineland        3,036             40.7        44.6        10.0           4.7
  3       U.S.A            30,000            45.2        39.5        11.2           4.2

        The distinctly higher percentage of those with type A blood among the Dunkers appears
to give good evidence of the influence of genetic drift. Somewhere along the line, probably in
the early generations after migration, the ancestors of today's Dunkers produced children with a
greater abundance of the gene for the a antigen than the exact proportions of probability would
indicate. There is no reason to believe that selection or anything other than pure chance was
involved. This has come down through the generations with the higher proportion of type A
blood which is shown in Table 4.

        Other studies indicate that genetic drift may be a factor in determining gene frequency in
a population, even when selection is also operating. In a study of certain Africa tribes where
selection should be about the same, Foy and his coworker in England found that the sickle-cell
blood trait showed considerable variability among the f = different tribes as well as among
isolates within the same tribe., This study indicates that genetic drift as well as selection may
operate to determine gene frequency in a population.

V. Survey of Human Heredity

        Though the branch of human genetics was established early as 1901, no significant
work has been done in the early years as man was found to be an unfavourable object for
genetic studies because of the following hindrances. (i) The life span of man is so long that it
does not permit the geneticist to study more than 5 or 6 generations in a family. In Drosophila,
several generations can be obtained and studied in a few months and in bacteria, in a week. (ii)
The number of offspring produced by man is far less than that produced by most insects and
plants. In man, thus very few samples of the various genetic possibilities can be obtained for
study. (iii) Man does not live in a uniform environment through his life. this varying natural and
emotional environments greatly influence his inherited characters, making the study of the latter
some what complicated. (iv) Marriages cannot be controlled by geneticists who wishes to study
the offspring of a specific mating. (v) Majority of human beings are genetically heterozygous for
many characters. Therefore, it is difficult to get homozygous or pure strain.

A. Pedigree analysis
        The pedigree analysis is the study of traits as they have appeared in a given family line
for several past generations. These days pedigree records are properly maintained and the
inheritance of many human traits like polydactyly, syndactyly, idiocy, intelligence, skin colours,
haemophilia, colour blindness and so many other diseases have been studied by pedigree
analysis.

       A family pedigree chart conventionally has circles for females individuals and squares for
male individuals and, so on. A marriage is indicated by a horizontal bar connecting a circle and
square, and symbols for offspring are shown suspended from a line drawn perpendicular to the
marriage bar. Individuals on the same line are from the same generation. Heterozygous are
customarily designated by colouring half of the symbol block. Carries of a sex-linked recessive
gene is designated by a black dot in the middle of the symbol.

       Occasionally, an arrow pointing at a particular affected individual indicated that the
disease was brought to the notice of geneticists by the persons indicated by the arrow such a
person is known as proband or propositus (for male) and proposeta if female.

        It is found that many quantitative variation of human characteristics can be explained
only by multiple gene inheritance. Multiple genes also participate in reducing the penetrance or
altering the expressivity of many human variations which depend primarily on a singly gene
difference. Finally, surgery is often employed early in life to remedy inherited defects. This, of
course, does not alter the genes, but it may make it very difficult to determine whether or not a
person possessed a certain characteristic at birth. E.g. club footedness, may be hereditary, but
skilled orthopedic treatment and surgery in early childhood can correct the condition, so that an
adult may not even know that he was born with clubfeet, Hence, the method of inheritance of
human characteristics presented here, must be taken with reservation as to their absolute
accuracy. They may be 0 value, as a preliminary guide in the survey of family histories which
may extend our knowledge of human heredity.

B. Kinds of hereditary traits
       The hereditary traits can be grouped into seven categories on the basis of their visibility
or expression. These are:

1. Dominant traits
        Dominant traits always appear in almost every generation possessing them. Various skin
and exoskeletal traits like piebald (White spotted skin); tylosis (thickened skin); ichthyosis (scaly
skin); epidermolysis (blistered skin); red, beaded hair hypotrichosis (hairlessness) are some of
the dominant traits.

2. Autosomic recessive traits
       These are invisible characters, characterised by the irregularity of their appearance in
pedigree. These recessive characters appear only when they become double (homozygous ).
Otherwise they may skip one or several generations and appear when recessive 'genes
becomes homozygous. If both parents are heterozygous for the recessive gene then upon
mating they will show usual Mendelian ratio of 3(dominant) : 1 (recessive). These traits include
albinism, cretinism, alkaptonuria etc.

3. Sex -linked recessive
       These characters pass from generation to generation only through gametes. The colour
blindness, haemophilia and various disease come under this category.

4. Autosomic dominant traits
        The characteristics of an autosomal dominant trait are: (a) every affected person in a
pedigree must have at least one affected parent (b) each generation in a pedigree should have
individuals who express the trait, (c) since the X -chromosome is not 1 involved, father to son
and mother to daughter transmission should occur as frequently as father -to- daughter and
mother -to-son transmission and (d) approximately equal numbers of males and females in a
pedigree should express the trait. E.g. Huntington chorea.

5. Physiological traits
       This category includes various excretory traits-like alkapotonuria, phenylketonuria, blood
group traits like crythroblaslosis foetalis and other biochemical disorders.

6. Mental traits
       These traits affect the development and work of nervous system. Feeble mindedness,
epilepsy, idiocy etc. belong to this category.

7. Pathological traits
       The pathogical or abnormal traits (concerning itself with the acute diseases) belong to
two categories namely teratological in which defects or deformities are due to developmental
upset and nosological which occurs as diseases of one kind or other.

C. Examples of human pedigree
      Some of the examples of human pedigree related traits are enumerated below.

1. Graying of hair
        There is no complete agreement as to the cause of this phenomenon, for one theory
holds that it results from the production of a substance known as leucokeration, while another
maintains that it is due to the formation of air bubbles in the hair shaft. We do know that the age
at which the hair begins turning grey is influenced by heredity. It is difficult to establish the exact
method of inheritance, however, because of the environmental effects of diet and other factors.
In certain family pedigrecs premature grayness is inherited as and autosomal dominant.

2. Baldness
        Baldness is a characteristic which without doubt can induced by environmental agents
like diseases, but the majority of people who are bald become so because of their genes. The
condition is inherited as a result of sex influenced gene which is dominant in male and recessive
in women. The pattern of baldness which develops and the age at which it begins is also
influenced by r heredity.

3. Vision
        Nearsightedness i.e. myopia, is prevalent hereditary defect of the eyes. It may be
brought about by either one or two independent factors. When the eyeball is too long, a normal
lens will bring distant objects to a focus in front of the retina, thus causing of fuzziness of the
image upon the retina. Such a condition seems to be inherited as an autosomal recessive. An
excessive curvature of the cornea is a less common cause of near sightedness, and seems to
be used by a dominant gene. Similarly, far-sightedness, hypermetropia, results when the
eyeball is short for the curvature of the lens, and this condition seems to be a simple dominant
character. Astigmatism is a defect of vision caused by unequal curvature of the cornea, which
causes objects in one plane to be in sharper focus than objects in another plane. It also seems
to be inherited as dominant.

4. Clubfoot and flatfoot
         Clubfoot results in serious deformity of corrective measures are not taken early in life.
The most common type causes an inward turning of the foot so that the person must walk on
the outer side of t he foot. This due to a shortening of the muscles or the tendons attached to
the muscle on the inner side of the leg. Other types of clubfoot causes the affected person to
walk on the front part of the foot, on the heel, or on the inner edge of the foot. Abnormal
pressure on the embryonic foot with cause clubfooted ness, but some family pedigrees show
that it may result from heredity also. In some families it is inherited as a recessive in others as a
dominant with reduced penetrance.

        Flat foot may result from environmental conditions, but in some families babies are born
with flat feet so that obviously the defect cannot be due to undue pressure during walking or
standing. It is apparently inherited as a recessive trait.

5. Skin abnormalities
        Xeroderma pigmentosum is an inherited condition characterized by an extreme
sensitivity of the eyes and skin to light. A baby that inherits the gene complex for this condition
will be born with normal skin, but exposure even to such light as would normally come into a
room through the windows causes the development of a severe rash on the skin. As the child
grows older certain regions of the rash frequently become malignant, and death nearly always
results before maturity is reached. The gene for this condition is recessive, although freckles
may show in heterozygotes. Similarly, epiloia is a condition inherited as a dominant autosomal
character; Ichthyosis congenita is inherited as an autosomal recessive lethal and ichthyosis
vulgaris (the development of scales on the skin) is inherited as a dominant autosomal trait.'

6. Rickets
        It results in varying degrees of bone deformity because of a deficiency of vitamin D in the
diet. Studies of families living in similar conditions, however indicate that the tendency to
develop rickets is inherited. It appears that susceptibility to rickets is transmitted as a dominant
trait.

7. Arthritis
        This is condition which results in soreness and stiffness of the joints between the bones.
In extreme forms there may be a complete fusion of the bones at the joints. Environmental
factors such as foci of infection, playa part in the development of this common diseases which
cripples so many people, but studies of family histories, show that it is influenced by heredity.
The susceptibility to some types of arthritis seems to be inherited as a dominant

8. Peroneal atrophy
       It is a muscle abnormality and is characterized by a progressive wasting away of the calf
-muscles, usually beginning sometimes between the ages of 10 and 30. In some family
pedigrees the trait is definitely inherited as a sex -linked recessive; in others it is an autosomal
dominant, and in a very few cases it seems to be an autosomal recessive. The recessive
autosomal form of peroneal atrophy is more severe than the dominant form and the recessive
sex-linked from is more severe than either of the other two.

9. Mental disorders
        Of all human factors influenced by heredity one of the most important for the future of
mankind is intelligence, and yet intelligence is so complex and variable, so much a composite of
many talents and aptitudes, that it cannot be defined properly. Nevertheless, there is a variety of
ways in which special aptitudes and general intelligence can be tested. The most useful of these
tests is based on what is called the intelligence quotient (IQ). A person is rated on the IQ scale
under:

               100 -approximately average intelligence
               130 and above -super normal
               70 -50 -feeble minded
               50-20 -imbecile
               below 20 –idiots

        In addition to general intelligence a number of mental disorders and other nervous
defects are in varying degrees hereditary. Many defects of the nervous system may actually
have their variations in other systems, such as endocrine. A few mental disorders can even be
traced to a single gene. Some of the common ones are discussed here.

a. Amaurotic idiocy
        This is a type of mental defect which exists in two forms. In the infantile from, the child is
normal at birth but symptoms of the disease begin to appear within several months there
ensues a gradual decline in mental ability, impairment of vision leading to blindness to
convulsions, progressive muscular weakness and emaciation. Death usually comes before the
second birthday. There is also a juvenile form if the disease which does not begin to have its
effects until a child is around 6 or 7 years of age. At this time there begins a progressive loss of
vision and this is followed : by mental deterioration. Muscular inco-ordination then develops, and
the muscles gradually waste away. Finally there is almost complete mental obliteration, and
death usually occurs before the afflicted individuals reaches 21 years of age. Each of these
forms of amaurotic idiocy seems to result from the action of autosomal recessive genes. there
are two distinct genes, one for each from of the disease, and apparently the two are not alleles.

b. Huntingtons Chorea
       This disease is characterized by an uncontrolled twitching of the voluntary muscles of
the body accompanied by mental deterioration. Unfortunately this condition usually does not
develop until a person is in his thirties and may already have had children. It is inherited as a
simple autosomal dominant, and hence about one -half of the children of an afflicted parent will
develop the disease in the course of time.

c. Schizophrenia
       This is one of the most common mental disease and may to some extent affect as much
as 1% of the population. It manifests itself in varying degrees and ways but it is always
characterized by a tendency to retire from the world of reality, and in some form there is an
almost complete insemibility to surroundings. The onset of this disease is often accompanied by
some mental stress.

       Repeated studies of family pedigrees indicate that there must be an hereditary
predisposition before a person can develop the disease.
       One plausible theory to explain the role of heredity in schizophrenia is concerned with
the blood chemical, serotonin, which is known to be required for normal brain functioning. Those
persons who are homozygous for a certain gene may produce a chemical antagonistic to
serotonin when they are under great emotional stress. The reduced serotonin level of the blood
brings about hallucinations and other symptoms of schizophrenia. A transfusion of blood from a
schizophrenic will bring on temporary schizophrenic symptoms in a normal persons. Persons
heterozygous for the gene may develop a mild form of schizophrenia known as a schizoid
personality, thus indicating that the gene has some degree of intermediate expression.

10. Nervous system disorders
       Epilepsy, spinal ataxia, spastic paraplegia, hypertrophic neuritis, and shakypalsy are
some of the defects involving the nervous system. Only the epilepsy is described here.

        The disease of epilepsy is characterized by sudden seizures known as epileptic fits,
which in the most extreme form run to unconsciousness and muscular spasms. Brain injury is
known to be and environmental agent which can induce the obset of epilepsy, but the majority
of cases arise without such injury and have an hereditary basis. Electroencephalograph which
records the electrical brain waves shows that epileptics show great irregularity in the waves as
against a fairly regular rhythms in a normal person. Sufficient studies have been made to
indicate that the irregular waves are inherited as dominant trait.

11. Blood abnormalities
       Some of the blood abnormalities are different causes and expressions of anaemia,
haemophilia haemorrhuids and high blood pressure.

a. Pernicious anaemia
       There is an insufficient number of red blood cell in the blood to accomplish the transport
of oxygen in the quantities needed for the best performance of the cells of the body. This is
because RBC are not produced by the red bone narrow rapidly enough to supply the body
needs. The trouble lies in the deficiency of an anti- anaemic factor formed in the stomach and
involves and utilization of vitamin B12 and other food factors. The factors is stored in the liver
and extract can used to supply it. A recessive gene with variable expressivity can cause a low
uptake or vito B 12 and lead to anaemia.

b. Haemophilia
        It is a sex linked recessive trait and is a defect in blood coagulation (already described in
the previous booklet). The classical pedigree is that of Queen Victoria. It seems that the "h"
mutation arose in the germ line of Queen Victoria. Only males show the trait since they are
hemizygous 'hy'. Females would have to be 'hh' to have hemophilia and since it is a rare gene,
this is unlikely as it requires the pairing of a haemophilic male with a carrier (heterozygous)
female.

c. High blood pressure
        It is a rather common affliction among people beyond middle age, but may be present in
youth. Serious abnormalities are likely to accompany it like kidney trouble, heart trouble and
epileptic stroke. Hypertension or high blood pressure is greatly influenced by environment, but
its tendency to recur in families strongly suggests 'an heredity background. Lenz made an
extensive an heredity background. Lenz made an extensive study in Germany and concluded
that the predisposition for the development of high blood pressure is inherited as a dominant.
There are perhaps a number of genes which contribute to the nature and severity of symptoms.
12. Non. infectious diseases
      In these diseases the germs are not involved in their incidence.

a. Diabetes mellitus
        It is on of the common diseases which result from an endocrine substance. Persons with
this disease produce an insufficient amount of insulin in the pancreas and sugar metabolism is
defective as a consequence. Excess sugar accumulates in the system and is excreted in the
urine. Diabetic coma and death may result if proper treatment is not administered. A
predisposition to become diabetic seems to be inherited may be avoided if the intake of
carbohydrate foods is moderate. No doubt, other genes influence the degree of predisposition.

b. Gout
       It results from a perversion of purine metabolism resulting in excessive production of uric
acid. Persons with this disease have an abnormally high uric acid content in the blood, there are
attacks of severe arthritis and sometimes chalky deposits form in the cartilages of the joints.
However, many persons have 'hyperuricemia '(high uric content of the blood) who do not have
gout. A certain threshold must be reached before the uric acid is sufficient to cause the disease.
Hyperuricemia is inherited as a simple autosomal dominant, but only about 10% of those with
hyperuricemia reach the threshold necessary for the production of gout. More men develop gout
than woman because men normally have a higher uric acid content of the blood than women
and with defective purine metabolism it is more likely that they will reach or exceed the
threshold and develop gout.

c. Allergic diseases
        Hay fever, asthma, cyclic vomiting, migraine, hives (skin eruptions), eczema and colitis
are some of the diseases which may have an allergic basis. All allergic conditions result from a
response of the body to the presence of a foreign antigen. Antibodies are produced which react
rather violently when a sensitized person is exposed to the antigen. People very considerably,
however, in the ease with which they become sensitized to foreign antigens. Heredity seems to
be a primary factor in determining which people become sensitized and which do not, Thus, no
one inherits hay fever or antibodies to any particular foreign antigen (with the exception of the
AB blood type antigens), but rather inherits a capacity to become sensitized easily. A parent
may have hay fever, one child may have allergic asthma and another may develop an extensive
skin rash every time he eats eggs. All these could result for the same inherited tendency to
become easily sensitized.

d. Cancer
        In spite of the great amount of scientific research which has been done on this disease,
there is still much about it which is not known there is little doubt however, that heredity plays a
part in the predisposition to cancer. Not only is the tendency to develop cancer inherited, but
twin studies show that the age at which cancer develops, the type of cancer which develops,
and even the particular body organ which is affected are influenced by heredity. Multiple gene
inheritance seems to be indicated.

13. Infectious diseases
        These are the diseases which are caused by germ invasion of the body. Since the
germs are actual agents which cause these diseases, it is not possible to inherit an infectious
diseases appearing repeatedly in certain families, even though the member of the family may
have not contact with one another. this indicates an inherited susceptibility specific infectious
diseases.
a. Tuberculosis
        The germs of this disease are very prevalent, and few among our population escape
some degree of infection during their lives. In some persons however, the disease develops
rapidly, and quickly causes incapacitation and sometimes death. Many environmental agents
may influence susceptibility to infection, but studies of families living under similar conditions
show variations which are almost certainly due to heredity. Extreme susceptibility is believe by
some to be inherited as a recessive.

b. Poliomyelitis
        This disease can strike at certain families in a region although large number of persons
are unaffected. It is believed that an inherited susceptibility is a major factor in the incidence of
this disease. Persons who contract the disease are normally homozygous for the recessive
gene for susceptibility.

c. Diphtheria and scarlet fever
       It also indicate a recessive gene for the susceptibility to this serious disease of
childhood.

D. Autosomal abnormalities
        Autosomes are the chromosomes bearing genes for the somatic characters. There are
44 autosomes in human beings arranged in twenty two pairs. A duplication or deficiency of a
part or whole of an autosomal chromosome results in a change in phenotype and is termed as
autosomal abnormality. Some important autosomal abnormalities related to man are discussed
here:

1. Down's syndrome
         The individuals with Down's syndrome symptoms have 47 chromosomes instead of 46.
In this syndrome the mental growth is retarded and the person has characteristic facial features
resembling Mongols. This condition may occur in all races. In this almost all organs are
defective in growth and development. Mongol females have underdeveloped sexual
characteristics though they are not sterile. Mongol males have undescended testes and their
semen contains a reduced number of sperm count. The life expectancy of Mongols is estimated
as 18 years. This syndrome is congenital, originating from the non - disjunction of chromosomes
of pair 21 during meiosis. The frequency of production to abnormal eggs is as under.


               1500 : 1 among mothers under 30
               750 : 1 among mothers at age 30 -34
               600 : 1 at maternal age 35 –39
               300 : 1 among mothers of 40 -45

2. Patau's syndrome
       Patau et.al. in 1960 described a trisomy of the chromosome number 13 (D-group),
hence this abnormality is called Patau's syndrome. It is characterized by multiple body
malformation as 3 well as profound mental deficiency. Hare lip and left palate is very common
among the sufferers. Polydactyly is most conspicuous. The internal organs are severely
malformed and the patients often suffer from disorders of kidney and heart Death usually occurs
soon after birth.

3. Edward's syndrome
        This syndrome, first described by Edward et.al. in 1960, is associated with the trisomy
of chromosome number 18 (E group). this produces receding chin, malformed ears and
defective nervous system. The hands are short and show little development of second phalanx.
The life expectancy in not more than a year or so.

E. Sex chromosomal abnormality
       An increase or decrease in the number of sex chromosome in the normal complement of
female (XX) and male (XY) results in syndrome of sex chromosome in human beings. The
common sex linked chromosomal abnormalities are given below.

1. Kleinfelter's syndrome
        About one male child out of every 5000 who are born expresses the symptoms
characterising this syndrome. Such children have typical male sex organs, but as they grow, the
testes do not grow proportionately to the rest of the body and in the adult are only about one
half normal size. Also, the fat deposits and growth of face and hair are somewhat feminine in
nature and some degree of breast enlargement may occur. Men who have kleinfelter's
syndrome are always sterile and about 25% have some degree of mental retardation. Their
blood is low in androgens (male hormones). An analysis of the individual chromosomes show
that there are two X -chromosome and one Y -chromosome in each cell in addition to the 44
autosomes

2. Turner's syndrome
       About one of every 3000 female births results in a child with turner's syndrome. There
are phenotypic females, but at adolescence the adults characteristics do not develop normally
and they never reach functional maturity. Persons afflicted with turners syndrome are dwarfed
physically and often show mental retardation. Many of them also have a characteristic
"webbing" of the skin on the side of the neck and wide spaced nipples of the mammary glands,
though these glands do not enlarge in normal women. Studies of the cells show only 45
chromosomes in these persons. There is only one X -chromosome and no Y -chromosome.

VI. Eugenics

          Eugenics deals with the application of the laws of genetics to the improvement of the
human race. We have accomplished remarkable improvements in domestic animals and
cultivated plants (mainly improvements which benefit mankind) through the application of these
laws. Can we not apply some of these principles to man and achieve a betterment of mankind?
Theoretically, it should be possible. But when such a plan is considered we run into many
difficulties.

        In spite of all these restrictions, however, it is possible that we may do something with
the factors which are under our control and which, even though they may not bring about any
improvement of the human species, may at least prevent its deterioration through dysgenic
forces.

        The concept of eugenics was founded by Sir Francis Dalton, who defined it as "the study
of agencies under social control that may improve or impair the racial qualities of future
generations, either physically or mentally. "Most of the agencies concerned with the
improvement of mankind are dedicated to social welfare which is designed to im prove the
condition of the living generation. There are many medical, educational, social welfare and
religious institutions which function on government funds and through the donations of
philanthropic individuals who are interested in the betterment of man. Very few exist whose
primary concern is with the future welfare of mankind. Many persons, who do not understand
the stability of the genes and the principles of selection, believe that an improvement of the
environment of the present generation will be reflected in an improvement of future generations.
This is a carry -over of the views of Lamarck regarding the possibility of inheritance of acquired
characteristics. Unfortunately, we know that this theory is not in accord with the facts, the
physical and mental benefits which are acquired by a man during his lifetime are intearred with
his bones each generation must start afresh -and a good thing too, for the burden of acquired
disabilities and bad habits would far outweigh the value of acquired abilities of a desirable type.
Eugenics can be studied under two heads i.e. negative eugenics, and positive eugenics. Prior to
that, however, it is necessary to know the status of natural selection in man and its effects.

A. Status of natural selection in man
         The importance of natural selection as a purging agent in the species, weeding out the
less fit and perpetuating those best adapted, has already bee emphasized. So far, there is no
exception t this rule in all the forms of life which we have studied. We might will be concerned,
therefore with the status of natural selection in the human population of today. Among the many
human inhabitants of the earth, natural selection operates in the same fashion as with other
forms of life -it carries on the same cruel yet efficient elimination of the great majority of the
children which are born.

        In India, with a high population concentration there is practically unlimited reproduction
coupled with high rate of elimination. The standard of living of the majority of people in Indian
villages is pathetically low in contrast to the complex, industrialized societies of metropolis and
prominent cities of this country here factors are operating which tend to reduce the effective
action of natural selection some of these are described here.

1. Development of medical science
        There have been many phenomenal advances in medical science during the past
century, and every year new discoveries are made which increase man's chances of survival.
Many of the dreaded germ diseases which took such a heavy toll of life in the past are now
controlled through the use of antibiotics. Yet, we know that the susceptibility to many diseases is
inherited. So that by saving susceptible individuals we allow genes that would otherwise be
eliminated by natural selection to continue propagating.

       Preventive medicine also plays its part. Through vaccination, quarantine, sanitation etc.,
dangers of infection from serious germ diseases are greatly reduced. Similarly, advances in
surgery permit many inherited body deformities to survive and be propagated that otherwise
would be eliminated.

        All such medical advances have done incalculable good for the existing people. Infant
and child mortality have dropped greatly, out life span has been extended, and the physical
suffering and disabilities have been greatly diminished. But all this has at the same time
reduced the effects of natural selection, for many of those who would be eliminated through
natural selection in a more primitive environment, now live through a normal life span and
reproduce.

       The continued benefits of medical science are bought at a price, through their effects we
can expect a gradual increase in the number of those who could not exist without their aid. It is
conceivable that eventually a population could develop in which every member would have one
or more inherited defects which except for the availability of medical science would cause death.
By its very practice, medical science in increasing the load which it will be called on to bear in
the future.

2. A differential birth rate
        Most of the Indian population live in rural areas, where children are a definite asset as
potential helpers on the farm or house. Girls were married early in life, and bore children at a
rate of about one every two years for the entire fertile period of their lives. This way population
increases at a tremendous rate. In contrast, the birth rate in cities is low, because apartments
and city homes were not built to accommodate large families (b) cost of bearing and rearing
children soared (c) mothers received special medical attention during pregnancy and (d)
immunization shots were given the child at the proper time and special diets were provided.

        A declining birth rate is not a tragedy, however. Of far more serious concern, is the fact
that the decline in the birth rate has been differential among the social and economic classes.
Families in the lower socio-economic levels of our population tend to be larger than those in the
upper socio -economic levels. While of course, no one can say with assurance, that any
persons socio - economic level in society is an indication of the desirability or undesirability of
his genotype, yet there are many in the lower groups who are unable to compete in modern
society because of mental deficiencies due at least in part to heredity.

3. War
        Any large scale war is likely to be dysgenic in its effects on mankind. In the selection of
members of the armed services, those persons rejected who do not come up to certain physical
or mental standards. Large number of casualties, therefore tend to obliterate some of the best of
germ plasm and to leave the weak, the deformed and the mentally incompetent in full strength
in the population. There is a great likelihood that this outcome might be changed in any possible
future wars. With the weapons now at the disposal of the world powers, the deaths of the civilian
population would probably far outnumber military losses, should any country be so foolish to
unleash and atomic war. Death would be randomized.

        Thus we find that our modern highly civilized society has created conditions which tend
to run counter to the ruthless force of natural selection which is nature's way of keeping the
species fit. Some feel concerned lest these conditions bring about a gradual deterioration of the
genetic qualities of our future generations. Much thought is, thus, given to the question is it
possible to substitute an intelligent program of human selection for the' more rigorous program
of natural selection which we have thwarted to some extent?

       In general, the proposals which have been made as partial solutions to this problem are
roughly classified into negative and positive eugenics.

B. Negative eugenics
        Negative eugenics has as its goal the prevention of the deterioration of the human race
through a reduction of the birth rate among the defectives so that they do not produce more
children than are produced by the more normal members of society.

1. Segregation
       Segregation is one method of negative eugenics which has been practiced for many
years. The inmates of mental institutions are not allowed to mingle, marry and bear children.
This prevents the propagation of genes for defects from those whose condition is due to
heredity. It is a fact, however, that the majority of persons suffering from various forms of mental
defects are not confirmed to institutions. Of course many of these defects are due to
environmental agents, such as brain injury, and hence would not be transmitted to future
generations. The majority however, have some hereditary background for their defects, and
certainly none of them would be desirable as prospective parents. Segregation would not be
feasible for such persons who are not dangerous to themselves or to society when living
unconfirmed. Aside from the great financial burden it would entail, it would not be in conformity
with our principles of human consideration.

2. Controlling immigration
        Many nation are very selective in permitting foreigners to enter their lands and mingle
with their population. They believe in encouraging only the 'cream' of other nations, i.e. people
with high intellect. This an attempt to shut out germ plasm which may be undesirable.

       It is compulsory to get vaccinated and injected as an immunity measure, against certain
contagious and epidemic diseases before traveling to a foreign country.

       But these measures are not sufficient to check undesirable germplasm because a
person may have high intellect and may possess sound health, but in his genome he may be
carrying an undetected recessive gene which which homozygous may be fatal.

3. Marriage restrictions
        Every country or society has the customs or laws which tend to restrict marriages.
According to biological aspects marriage is an experiment in breeding, not human courtship as
described by various novels. In several countries the marriages of mental defectives, habitual
drunkards, idiots, feeble minded, insane persons, epileptic, alcoholics and person having
veneral diseases are prohibited. As a result of these regulation, the genes necessary for these
defective traits do not have chance to mix in the generation. Only marriages between
undefective persons should occur. Even the cousin marriages are extremely useful if they are
without sex-linked diseases.

4. Sterilization
        It is a drastic step of restricting defective germ plasm from meeting with the other germ
plasm. By this method, ducts carrying sperms or eggs from gonads are surgically thus blocking
the way of undesirable germplasm to the external world. This is of two types.

a. Vasectomy
       It is a minor operation in males in which the sperm ducts are surgically removed or
rendered ineffective by some means.

b. Salpingeotomy
       In this type oviducts are gutted or removed surgically in females and thus avoid the
chance of fertilization due to non - availability of ovum.

        Operations may be temporary or permanent. In the temporary sterilization sperm ducts
or fallopian tubes are tied- off with the help of surgical thread. In the latter method, these ducts
are removed permanently.

C. Negative eugenics (Genetic variables)
       In practice the effectiveness of marriage restrictions, segregation, control on immigration
and sterilization as eugenic measures depends on several genetic variables like.

1. The mode in which undesirable trait is inherited;
2. The rate of frequency of the gene or genes concerned in the populations;
3. The age at which the defective trait makes its appearance;
4. The extent to which environment may check or inhibit the expression of the gene. Regarding
to mode of inheritance of these trails the following possibilities may exist.
a. The trait may be due to dominant gene;
b. It may be due to a gene without dominance (i.e. heterozygote intermediate);
c. Sex linked recessive gene may bring it about;
d. It may be due to autosomal recessive gene; and
e. Finally, a trait may be produced as a result of cumulative genes.

D. Positive eugenics
       The effectiveness of positive eugenic measures depends on several genetic variables.
They are mentioned here.

1. Subsidizing the fit and sperm banks
          Because the highly endowed persons lead a well planned life and to avoid unnecessary
difficulties in nursing the children they often prefer to have smaller number of children. Therefore
the selected young men and women of best eugenic value should be encouraged to increase
their birth rate.

       The artificial insemination is already widely practiced to permit those women whose
husbands are sterile or have some "serious hereditary afflictions to bear children. The sperm
and eggs of outstanding persons can be stored for future use by quick freezing and storing them
in deep freeze. These germ cells thus can be stored for 100 or more years.

2. Promotion of genetic research.
        The mechanism of inheritance of many specific traits can be clearly understood provided
the basic principles of heredity and distribution of genes are known. The physiological action of
genes is another problem and with its complete knowledge, many disease can be cured. The
nature of mutation, their effects and possible control may bring about the better offspring having
high learning capacity.

        Besides, genetic research on plants and animals is receiving much support from
different state governments. By this encouragement, good varieties of plants and animals highly
beneficial to man have been produced.

3. Early marriage of those having desirable traits
        It is most commonly observed that the highly placed persons of the society often have
great ambitions for the future life. In achieving their ambitious goals, they often devote the best
part of their youth and they arc able to marry in their mature age (i.e.30 to 35 years). The
biological and psychological investigation have revealed that the germ plasm of the aged
persons loses its vigor. Therefore, the young persons having best hereditary traits should be
encouraged for early marriages.

4. Genetic counseling
        To produce healthy progeny should be the endeavor of man, as this alone can ensure a
better future for humanity. Genetic counseling can make a significant contribution in this
direction. For families with history of genetic diseases, genetic counseling can provide the much
needed relief.
        By careful examination the genetic counselor can detect genetic abnormalities like
Down's syndrome and even recessive genes for diseases like sickle-cell anaemia. Knowledge
of the possibilities of transmitting these diseases to offspring can help a person in choosing a
marriage partner.

       Genetic counseling is equally useful after marriage. An Rh- woman with an Rh+ partner,
when aware of the implications in advance, can go in for suitable medical aid well in time. The
study of amniotic fluid (amino-centesis) can reveal the sex of the foetus as well as congenital
diseases (like down's syndrome) and metabolic disorders that it may posers. This information
can help the couple to decide whether or not to retain the pregnancy, or at least to be mentally
prepared for the inevitable.

5. Protection against mutagens
       Mutagens can induce mutations, some of which may be deleterious and even lethal.
Harmful mutations can result in deformed offspring and increase human misery. It is therefore,
essential to check exposure of human germ cells to mutagens like high energy irradiations,
chemicals, etc. Medical X-ray should be advised and carried-out with utmost care. Chemical
mutagens should be experimented with arid handled with all possible precautions. The
December 3, 1984 gas tragedy in Bhopal, Madhya Pradesh, (leakage, of MIC) and the
subsequent discovery of 'dangerous' experiments carried out in Bhopal should be a sufficient
warning against possible hazards of chemical warfare in which chemical mutagens may be
used.

6. Improvement of environmental conditions.
       Environment plays a dominant role in the heredity of man. Better facilities of training,
schooling living and studying should be given. These all are eugenic programmes for the
benevolence of human race.

E. Euphenics
        Euphenics deals with the treatment of genetic diseases of man, more specifically it deals
with the control of several inherited human diseases, especially in errors of metabolism in which
the missing or defective enzyme has been identified. The excellent example of is the
phenylketonuria or PKU. In this case, the patients are unable to metabolize an amino acid -the
phenylalanine properly, the resulting product causes severe mental retardation. Such patients
are advised to take phenylalanine free diet.

       Although a number of inherited diseases can be treated in a similar euphenic manner,
but these constitute only a small fraction: of known inherited diseases. For the most part,
biochemical geneticists could not identify the biochemical errors of many genetic diseases. In
other cases, such as albinism, even though the metabolic block leading to an abnormality is
known, but it is not possible to correct it.

VII. Conclusion

       One of the most important subdivisions of the science of biology is genetics, the study of
heredity. Genetics deals not only with the way in which characteristics are transmitted from one
generation to the next but also with the action of the units of heredity as they bring about the
characteristics which they control.

      As human beings, it is quite natural that we should be particularly interested in the
method of inheritance in man. Man is not the best form of life for studies of genetics for the
given reasons, yet our knowledge of human genetics has increased greatly in recent years as
techniques for study have been developed.

       To the untrained person, genetics may appear to be a vague, indefinite subjects. But
now we know that heredity plays a very important role in development of physical and mental
characteristics, aptitudes, temperament and even the minor body mannerisms.

        In addition to the theoretical interest in genetics as a means of extending our knowledge
of the unknown, genetics has many practical applications which are of great value to man,
including better breeds of domestic animals and hybrids of cultivated crops and other plants of
economic use, medicine and genetic counseling and treatment.

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