Learning Center
Plans & pricing Sign in
Sign Out


VIEWS: 6,179 PAGES: 245

									Mendelelian Genetics
Dr. R. Siva VIT University, INDIA
More free powerpoints at

Gregor Mendel

Responsible for the Laws governing Inheritance of Traits

1822 - Brunn in Austria, now, Brno (Czecholovakia)
Financial and Health Problem

Unable to continue studies
Joined St.Augustinian Monastery

1847 – Priest 1851 – Higher studies at Vienna University 1854- Science teacher

Gregor Johann Mendel
Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents


Gregor Johann Mendel
Austrian monk Developed the laws of inheritance Presented his findings in the Natural History Society of Brunn – 1865 Paper entitled, “Experiments in Plant Hybridization” – 1866 German language.

 Mendel's work was not recognized until the turn of the 20th century  1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm

 Called the “Father of Genetics“

Site of Gregor Mendel’s experimental garden in the Czech Republic


Mendel’s workplace

Fig. 2.5


Particulate Inheritance
Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA


Genetic Terminology
 Trait - any characteristic that can be passed from parent to offspring  Heredity - passing of traits from parent to offspring  Genetics - study of heredity


Types of Genetic Crosses
 Monohybrid cross - cross involving a single trait e.g. flower color  Dihybrid cross - cross involving two traits e.g. flower color & plant height

Designer “Genes”
 Alleles - two forms of a gene (dominant & recessive)  Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)  Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

More Terminology
 Genotype - gene combination for a trait (e.g. RR, Rr, rr)  Phenotype - the physical feature resulting from a genotype (e.g. red, white)


 Homozygous genotype – When the two alleles are same (dominant or 2 recessive genes) e.g. TT or tt; also called pure  Heterozygous genotype – When the 2 alleles are different- one dominant & one recessive allele (e.g. Tt); also called hybrid


Punnett Square
Used to help solve genetics problems



The formula 2n can be used, where “n” = the number of heterozygous traits. Ex: TtRr, n=2 22 or 4 different kinds of gametes are possible. TR, tR, Tr, tr


Dihybrid Cross
TtRr X TtRr Each parent can produce 4 types of gametes. TR, Tr, tR, tr Cross is a 4 X 4 with 16 possible offspring.



9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered Or: 9:3:3:1


Genes and Environment Determine Characteristics


Mendel’s Pea Plant Experiments


Why peas, Pisum sativum?
Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially crosspollinated Bisexual. Many traits known. Above all, easy to grow

Reproduction in Flowering Plants
Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers

Mendel’s Experimental Methods
Mendel hand-pollinated flowers using a paintbrush He could snip (cut) the stamens to prevent self-pollination He traced traits through the several generations



How Mendel Began?
Mendel produced pure strains by allowing the plants to selfpollinate for several generations

Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p)



Mendel’s Experimental Results


Did the observed ratio match the theoretical ratio?
The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the theoretical ratio

Generation “Gap”
Parental P1 Generation = the parental generation in a breeding experiment. F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P1 generation F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation

Following the Generations

Cross 2 Pure Plants TT x tt

Results in all Hybrids Tt

Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

Monohybrid Crosses


Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr

P1 Monohybrid Cross

r R R Rr Rr

r Rr Rr

Genotype: Rr Phenotype: Round
Genotypic Ratio: All alike

Phenotypic Ratio: All alike

P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous recessive  Offspring all Heterozygous (hybrids)  Offspring called F1 generation  Genotypic & Phenotypic ratio is ALL ALIKE

Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr

F1 Monohybrid Cross

R R r RR Rr

r Rr rr

Genotype: RR, Rr, rr Phenotype: Round & wrinkled
G.Ratio: 1:2:1

P.Ratio: 3:1

F1 Monohybrid Cross Review
 Heterozygous x heterozygous  Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr  Offspring called F2 generation  Genotypic ratio is 1:2:1  Phenotypic Ratio is 3:1

What Do the Peas Look Like?


…And Now the Test Cross
Mendel then crossed a pure & a hybrid from his F2 generation This is known as an F2 or test cross There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid

Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr

F2 Monohybrid Cross

st) (1


r Rr Rr

Genotype: RR, Rr Phenotype: Round
Genotypic Ratio: 1:1

Phenotypic Ratio: All alike

Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr

F2 Monohybrid Cross (2nd)

R r r Rr Rr

r rr rr

Genotype: Rr, rr Phenotype: Round & Wrinkled

G. Ratio: 1:1 P.Ratio: 1:1

F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)  Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr  Phenotypic Ratio is 1:1  Called Test Cross because the offspring have SAME genotype as parents

Practice Your Crosses
Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits


Mendel’s Laws
1. Law of Dominance 2. Law of Segregation 3. Law of Independent assortment


Law of Dominance
States that on crossing homozygous organisms for single pair of contrasting characters, only one characters make its appearance in F1 generation and is name as Dominant character.


Results of Monohybrid Crosses
Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same

Law of Dominance
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds)

Law of Dominance


Character Drosophila Body Colour Eye colour Colour of hair Form of hair Colour of eye Lips Blood group

Dominant Gray Red Dark Curly Brown Broad and thicken A,B,AB

Recessive Black White Light Straight Blue Thin O




Law of Segregation
Two allele of a gene remain separate and do not contaminate each other in F1 or hybrid. OR The two alleles for each trait separate during gamete formation.


Law of Segregation
During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Law of segregation


Applying the Law of Segregation


Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.


Dihybrid Cross
Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green

RY Ry rY ry


RY Ry rY ry

All possible gamete combinations

Dihybrid Cross
RY RY Ry rY ry Ry rY ry


Dihybrid Cross
RY Ry rY ry

ry RrYy

RrYy Rryy

rrYY rrYy

RrYy Rryy
rrYy rryy

Round/Yellow: Round/green:

9 3

wrinkled/Yellow: 3 wrinkled/green: 1

9:3:3:1 phenotypic ratio

Dihybrid Cross
Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1



Question: How many gametes will be produced for the following allele arrangements? Remember: 2n (n = # of heterozygotes)
1. RrYy 2. AaBbCCDd 3. MmNnOoPPQQRrssTtQq

1. RrYy: 2n = 22 = 4 gametes RY Ry rY ry 2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd aBCD aBCd abCD abCD 3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

Summary of Mendel’s laws

PARENT CROSS TT x tt tall x short
Tt x Tt tall x tall



100% Tt tall
75% tall 25% short
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods



RrGg x RrGg round & green x round & green

Summary of Mendel’s Hypothesis
1. Genes can have alternate versions called alleles. 2. Each offspring inherits two alleles, one from each parent. 3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. 4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

The elegance of mendel’s experiments was partly due to the complete consistency between his observation and Hypotheses he developed. However, after mendel’s work was rediscovered, it became clear that simple medelian model did not adequately predict experimental observations in all situations.


Variations on Mendel 1. 2. 3. 4. 5. Incomplete Dominance Codominance Multiple Alleles Epistasis Polygenic Inheritance


Incomplete Dominance and Codominance


Incomplete Dominance
F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Carl Correns Crossed 4’O Clock Plant (flower) red (RR) x white (rr) R R RR = red flower rr = white flower
r r

Incomplete Dominance

R Rr Rr produces the F1 generation All Rr = pink (heterozygous pink)

Rr Rr


Incomplete Dominance


Another Example


Both the alleles can be expressed Eg. Red cows crossed with white will generate Roan cows. Might seem to support blending theory.
But F2 generation demonstrate Mendalian genetics (1:2:1)

Two alleles are expressed in heterozygous individuals. Example: blood type 1. 2. 3. 4. type type type type A B AB O = = = = IAIA or IAi IBIB or IBi IAIB ii

In humans, there are four blood types: A,B,AB and O Blood type is controlled by three alleles: A,B,O O is recessive, two O alleles must be present for the person to have type O blood A and B are Codominant. If a person receives an A allele and B allele, their blood type is AB type Crosses involving blood type often use an I to denote the alleles

Summary of Dominance Relationships


complete incomplete





In a plant species, if the B allele (blue flowers) and the b allele is (white flowers) are incomplete dominant (Bb is light blue). What offspring ratio is expected in a cross between blue flowered with light blue flowered plant? What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

Try these

A lethal gene causes death of individual in the appropriate genotype before they reach adulthood. Lethal are generally recessive, resulting in the death of the recessive homozygote. - Some lethals are dominant. In 1904 French geneticist Lucien Cuenot, discovered a recessive lethal affecting coat colour in mice.

He observed that yellow colour is dominant; But while crossing 2 yellow mice, he got only 2:1 ratio. While he crossed yellow with recessive wild type (grey) found that all are yellow. Concluded all yellow mice were heterozygotes.

Later it was suggested that homozygosity for yellow is lethal, and that these individuals died in utero, a fact observed by histological studies.


Recessive Lethal Alleles


Manx Cat
Manx Cat – Tailless cat is another trait caused by an allele that has dominant effect in heterozygous and is lethal in homozygotes. The Manx and normal alleles are denoted by M and m respectively Mm X Mm

Other example is achondroplasia, the most common form of dwarfism, with a normal length body trunk but shortened limbs.


Many genes have alleles that affect the rate of mortality but are not lethals. In general these are termed as deleterious or detrimental alleles.



- Impact of a single gene on more than one characteristic - Sickle-cell disease - Most common inherited illness among black people - RBCs are sickle-shaped Can cause many problems

RBCs: Sickle-cell disease

Normal RBCs


Sickle Cell Anemia
Under conditions of low oxygen tension, hemoglobin S will precipitate, causing cells to sickle
Mutations in same amino acid Some individuals die in childhood; Some individuals have mild symptoms HbA: V – H – L – T- P – G –G HbS: V – H – L – T- P – V–G


Sickle cell anemia may be the result of a genetic mutation that happened in malariaprone regions like Africa thousands of years ago. People with sickle cell trait may have been more likely to survive malaria epidemics and because they survived when others did not, this allowed the trait to be passed down through generations.




Gene Interaction and Epistasis
Mendel was lucky that in his studies each character was governed by a single gene Typical Dihybrid ratio: 9:3:3:1 The phenomenon of two or more genes governing the development of single character is known as Gene interaction. When one gene affects in anyway the expression of another gene, the phenomenon is called Epistasis.

Types of Gene Interaction
1. Typical Dihybrid ratio for a single trait 2. Duplicate gene action 3. Complementary gene action 4. Supplementary gene action 5. Inhibitory gene action 6. Masking gene action 7. Polymeric gene action And 8. Additive gene action.

1.Typical Dihybrid ratio for a single trait






Pea (PPrr)


Ross (RRpp)

Walnut (PpRr)


A typical dihybrid ratio in the case of single character, Viz Comb shape in poultry. Genotype Ratio : 9:3:3:1 Where, 9: Walnut 3: Pea 3:Rose 1: Single


2. Duplicate Gene Action:
The non-floating habit in rice is controlled by two genes A1/A1; A2/A2 Like that floating habit is controlled by a1/a1; a2/a2


Here, the presence of a single dominant allele of any of the two genes governing the trait produces the dominant phenotype (non-floating). The recessive genotype (floating habit) is produced only when the genes are in the homozygous recessive state. The genes that show duplicate interaction is called Duplicate gene action.


Duplicate Dominant Eptistasis (15:1)
A;B A;b a;B a;b

A;B A/A;B/B A/A;B/b A/a;B/B A;b a;B

A/A;B/b A/A;b/b A/a;B/b A/a;b/b

A/a;B/B A/a;B/b a/a;B/B a/a;B/b

A/a;B/b A/a;b/b a/a;B/b a/a;b/b

Non Floating Floating



3. Complementary gene action
In a sweet pea the development of purple flowers requires the presence of 2 dominant genes, C and R, e.g., CCRR.
When either C or R alone present purple flowers cannot be produced; as a result white flowers are obtained e.g., ccRR Or CCrr Or ccrr


Parents: CCRR (Purple) Gametes: CR X ccrr (White) cr


CcRr (Purple)





A;B A/A;B/B A/A;B/b A/a;B/B A;b a;B

A/A;B/b A/A;b/b A/a;B/b A/a;b/b

A/a;B/B A/a;B/b a/a;B/B a/a;B/b

A/a;B/b A/a;b/b a/a;B/b a/a;b/b



4.Supplementary Gene Action In this gene interaction, the dominant allele of one gene produces a phenotypic effect. The dominant allele of the other gene does not produce any phenotypic effect on its own; But when it is present with the dominant allele of the first gene, it modifies the phenotypic effect produced by the first gene.

Parents: RRPrPr (Purple) Gametes: RPr X rrprpr (White) rpr


RrPrPr (Purple)

Supplementary gene action In maize, the development of colour is governed by two completely dominant genes R and Pr. The dominant allele R is essential for (red) colour production Homozygous state of the recessive allele r (rr) prevent the production of red colour. Phenotypic ratio: 9:3:4 (Purple: Red: White)


RPr RPr




• Modified ratio • Still 1/16ths

9:Purple 3:Red 4: White

9 :4 :3


5. Inhibitory gene action

One dominant inhibitory gene prevents the expression of other dominant gene. E.g. , There are two varieties of white comb which are genetically different. One variety is called Leghorn represented by IICC, which carries a colour gene C, and other gene I, that inhibits the expression of the colour gene C The other white var. called Wyandotte is recessive to the two genes viz., iicc and so it is white or colourless

When cross made btn 2 white var. Leghorn and Wyandotte, F1 hybrid also white, When this F1 allowed to self fertilize there was a segregation of 13:3 ratio of white and coloured respectively.
Parents: IICC X White (Leghorn) Gametes: IC F1 IiCc (White)

iicc White (Wyandotte) ic

IC IC Ic iC






6.Masking gene action
In barely, seed colour is governed by two dominant genes B and Y. The allele B produces black colour, while Allele b produces white colour The dominant allele Y produces Yellow While, allele y produces white colour


Parents: BByy (Black) Gametes: By X bbYY (Yellow) bY


BbYy (Black)





A;B A/A;B/B A/A;B/b

A/a;B/B A/a;B/b

A;b a;B a;b

A/A;B/b A/A;b/b A/a;B/b A/a;b/b

A/a;B/B A/a;B/b a/a;B/B a/a;B/b

A/a;B/b A/a;b/b a/a;B/b a/a;b/b




7. Polymeric Gene Action In Polymeric gene action, two completely dominant genes controlling a character produce the same phenotype when their dominant alleles are alone. But when dominant alleles of both the genes are present together, their phenotypic effect is enhanced as if the effect of the two genes were cumulative or additive.

Polymeric Gene Action

In barley, two completely dominant genes A and B affect the length of awns (the needle like extensions of lemma). Dominant allele of the gene A or B alone (e.g., Aa bb, AA bb, BB aa, Bb aa) give rise to awns of medium length . Thus the phenotype of A is same as that of B. But when A and B together they produce long awn. Where as aa bb are awnless.

Parents: AABB (Long Awn) Gametes: AB X aabb (Awnless) ab


AaBb (Long Awn)





9: Long Awn 6: Medium Awn 1: Awnless



8. Additive Gene action
In additive gene action, each positive allele of the two genes governing a trait produces equal and identical effect on the character. Therefore the genes showing additive gene action are called multiple factors or more commonly Polygenes


Each polygene has two alleles: one allele of each polygene produces a +ve effect in the character governed by the gene. – Positive allele. The other allele of each gene has no effect on the character and is negative allele


Seed Colour in tetraploid wheat is governed by two polygenes R1 and R2, and determining the colour, where as r1 and r2 do not produce any colour. Each R1 and R2, allele produces a small amount of colour. Therefore the total intensity of colour depends on the total number of positive alleles of the two genes present. Thus,R1R1R2R2 produces dark red colour; R1R1R2r2 generate medium dark red and R1r1R2r2 yield medium red and so on.

Parents: R1R1R2R2 (Red) Gametes: R 1 R2 r 1r 2 X r 1 r 1 r 2r 2 (White)


R1 r 1 R2r 2 (Medium red)

1: Dark red 4: Medium dark red 6: Medium red 4: Light red 1: White


Summary of Epistasis


Multiple Allele
It is generally accepted that gene has two alternative forms called Allele. Usually, one of the two alleles of a gene is dominant over the other, which is recessive e.g, Tall (TT) But in many cases, several alleles of a single gene governing the concerned trait and is known as Multiple alleles.

Both prokaryotes and Eukaryotes show multiple alleles. Examples: 1. Human ABO 2. Rh and other blood group 3.Fur colour in rabbits and other mammals. 4. Drosophila eye colour


What is blood made up of?
An adult human has about 4–6 liters of blood circulating in the body. Among other things, blood transports oxygen to various parts of the body.
The red blood cells contain hemoglobin, a protein that binds oxygen. Red blood cells transport oxygen to, and remove carbon dioxide from, the body tissues.

The white blood cells fight infection. The platelets help the blood to clot, if you get a wound for example. The plasma contains salts and various kinds of proteins.

What are the different blood groups?
The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies.
The antigens are located on the surface of the red blood cells and the antibodies are in the blood plasma.

Individuals have different types and combinations of these molecules. The blood group you belong to depends on what you have inherited from your parents.

There are more than 20 genetically determined blood group systems known today, but the AB0 and Rh systems are the most important ones used for blood transfusions. Not all blood groups are compatible with each other. Mixing incompatible blood groups leads to blood clumping or agglutination, which is dangerous for individuals.
Nobel Laureate Karl Landsteiner was involved in the discovery of both the AB0 and Rh blood groups.

Blood group A If you belong to the blood group A, you have A antigens on the surface of your red blood cells and B antibodies in your blood plasma Blood group B If you belong to the blood group B, you have B antigens on the surface of your red blood cells and A antibodies in your blood plasma

Blood group AB If you belong to the blood group AB, you have both A and B antigens on the surface of your red blood cells and no A or B antibodies at all in your blood plasma. Blood group 0 If you belong to the blood group 0 (null), you have neither A or B antigens on the surface of your red blood cells but you have both A and B antibodies in your blood plasma.


Multiple Allele


Rh factor blood grouping system
Many people also have a so called Rh factor on the red blood cell's surface. This is also an antigen and those who have it are called Rh+. Those who haven't are called Rh-. A person with Rh- blood does not have Rh antibodies naturally in the blood plasma (as one can have A or B antibodies, for instance). But a person with Rh- blood can develop Rh antibodies in the blood plasma if he or she receives blood from a person with Rh+ blood, whose Rh antigens can trigger the production of Rh antibodies. A person with Rh+ blood can receive blood from a person with Rh- blood without any problems.

Blood is frequently exchanged between mother and the fetus during childbirth. Thus, Rh-negative mothers often immunized by blood from Rh-positive fetuses (which may result when father is Rh-positive).
Usually no ill effects are associated with exposure of the mother to the Rh-positive antigen during the first child birth. Subsequent Rh-positive children carried by the same mother, however, exposed to the antibodies produced by the mother against the Rh antigen.

Such children may develop symptoms of hemolytic jaundice or anemia, a condition referred to as erythroblastosis fetalis. . The symptom may be mild or severe, even resulting in the death of the fetus.

The number of different genotype is possible to find out in multiple alleles If n is the number of alleles of a gene, the number of different genotypes possible is n(n+1)/2, thus 2,3,4 or 5 alleles there are 3,6,10 and 15 possible genotype respectively.
Note that although a large number of different alleles of a given gene may be present in a population or sp., only two of those alleles can be present in any diploid organism.

Alleles 1 2 3 4 5 6 n

Homozygous 1 2 3 4 5 6 n

Heterozygous 0 1 3 6 10 15 n(n-1)/2

Genotype 1 3 6 10 15 21 n(n+1)/2


Coat colour in Rabbit Multiple alleles involves coat colour in rabbits. Four alleles of rabbit coat colour (c) gene have been studied: C+ wild type or full colour Cb “himalayan” white with black tips on the extremities Cch “chinchilla” mixed colour & white hairs C albino These alleles show a gradation in dominance of C+ > Cb > Cch >C

Multiple allele in Drosophila eye colour
Wild type Drosophila have red eyes; But a vast array of eye-colour mutants have been studied extensively. Mutant alleles of one gene (white) result in flies with eye colour ranging from pure white through a series of intermediate colours up to nearly wild type red when present in homozygous condition. For e.g., the allele that produces the most extreme mutant phenotype was named white because the eyes of the flies homozygous for this allele are completely white (no coloration)

Other alleles of the gene were designated wa (white apricot)

we (white eosin), wch (white cherry), wco (white coral), ww (white wine), wb (white blood), wcrr (white carrot), wcf (white coffee) and so on,

to indicate that different levels of coloration of the eyes occurred in flies homozygous for each particular allele.


Codominance Problem
Example: homozygous male Type B (IBIB) x heterozygous female Type A (IAi)

1/2 = IAIB 1/2 = IBi




Another Codominance Problem
Example: male Type O (ii) x female type AB (IAIB)
IA i i IAi IAi IB I Bi I Bi
1/2 = IAi 1/2 = IBi


Try These
1. If a male has blood type B and a female has blood type A, what are the possible blood types in the offspring? 2. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not? 3. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

Discovery of Linkage:
In 1900, Mendel’s work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible.
William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).

observe deviations from 9 : 3 : 3 : 1 ratios derived from dihybrid crosses 1 : 1 : 1 : 1 ratios derived from test crosses

too many too few

It is universally accepted that genes are located in chromosomes During cell divisions each chromosome behaves as a unit Therefore, genes located in same chromosome would move together to the same pool during cell division. Linkage: The tendency of two or more genes to stay together during inheritance is known as Linkage. Because they are located on same chromosome.

Recombination During Meiosis

Recombinant gametes

Parental gametes

Linkage in Drosophila
In Drosophila Grey colour is dominant (G or b+) over black colour (g or b) and long wing (L or Vg+) over vestigial wing (l or Vg). The genes of these characters are linked together in same chromosome.

ebony body, vestigial wild-type female winged male




Coupling and Repulsion In Maize, Dominant gene C produces coloured seeds (C) Recessive gene produces colourless seeds (c) Another dominant gene governs full seeds (Sh) While recessive produce shrunken seeds (sh) Parents Gamets



ccsh c sh


Test cross: When F1 are test crossed with recessive strain, out of 8,368 seeds 4,032 (48.2%) were coloured full; 4,035 (48.3%) were colourless shrunken; 149 (1.7%) were coloured shrunken; 152 (1.8 %) were colourless full. Here, the four phenotypic classes are not in the expected ratio of 1:1:1:1. The phenotypic classes coloured full and colourless shrunken have a much higher frequency than the expected 25%. These two character combinations are called Parental phenotype or parental types or Parental combinations.

In the above e.g., it appears as if the 2 dominant genes C and Sh have a strong affinity for each other so that the phenotypes are greater than expected. This situation is referred to as Coupling phase. It is due to the presence of genes C and Sh in the same chromosome.


Similarly when coloured shrunken seeds (CC shsh) were crossed with colourless full (cc ShSh), F1 were coloured full (CcShsh). But when F1 test crossed with recessive strain (cc shsh) 47.9% were coloured Shrunken; 49.1% colourless full; 1.4% coloured full; 1.5% colourless shrunken In this case also the parental types were more frequent. It appears as if in this cross the dominant genes c and Sh dislike each other. This situation is referred to as Repulsion phase. It is due to the presence of dominant allele of one gene, e.g., C with the recessive allele of the other gene, e,g, sh in the same chromosome.

Complete and Incomplete linkage
Gene show a linkage because they are located on a chromosome. For e.g., C and Sh are present in one chromosome, while their recessive alleles c and sh are situated in the homologous chromosome.
C sh c Sh






Each chromosome behaves as a unit during cell division. Therefore, C and Sh would move to one pole to while c and sh move to opposite pool. If this always happened, F1 generation (CcShsh) would produce 2 gametes viz., C Sh and c sh When only parental character combinations are recovered in test progeny , it is called complete linkage. However, sometime, allele recombine to produce recombinant types like C sh, cSh Such a type are called Incomplete linkage.

Linkage is never 100%. No matter how tightly two genes are linked, if you observe enough individuals, you will find some recombinants.


Crossing Over
Recombinant phenotypes are produced by recombinant gametes. These gametes, in turn, are produced due to crossing over. The exchange of homologous segments between nonsister chromatids of homologous chromosomes is known as Crossing over.
Crossing over takes place during pachytene .


Centromere sister chromatids
sister chromatids





Crossing over occurs during meiosis of gametogenesis. The homologous chromosomes move towards each other and come to lie side by side. This phenomenon of pairing of homologous chromosomes is called synapsis. The paired homologous chromosomes are called bivalents. The homologous chromosome split into two chromotids



Chiasmata mark the sites of recombination


Chiasmata mark the sites of recombination


chiasmata: visible manifestations of crossing over
Proposed by Janssens

Prior to crossing over, the chromosomes of each bivalent get duplicated to form a tetrad.
Crossing over occurs only between the nonsister chromatids of a tetrad.

In this stage, the nonsister chromatid overlap with one another and form chiasma or point of contact

Recombination Frequency
RF = recombinant progeny X 100% total progeny


Salient features of Crossing over 1. Crossing over is the interchange of chromosomal segments between the homologous chromosome 2. It occurs during meiosis or gametogenesis 3. The crossing over occurs only between nonsister chromatids of the homologous chromosome 4. The number of crossing over is depends upon the length of the chromosome. The longer the length, higher percentage of crossing over 5. When the genes are located apart from one another the chances of crossing over is higher when the genes are closely located the chances for crossing over is lesser.

products of meiotic recombination


Detection of recombination using a test cross


Kinds of crossing over: 1. Single crossing over In this type, only one chiasma is formed. Only one chromatid of each chromosome is involved in single crossing over. 2. Double crossing over Here two chiasma are formed. The chiasmata may be formed between the same chromatids or between different chromatids. Thus 2 or 3 or all the four chromatid may be involved. 3.Multipel crossing over: More than two chiasma are formed

Crossing over occurs in the Tetrad Stage Neurospora crassa highly suited certain types

of genetic studies. 1. After two haploid nuclei from cells of two different matting types fuse to form a diploid nucleus (similar like fertilization in higher org). 2. The haploid products of meiosis, called ascospores, are maintained in linear order within an elongate tube like structure called ascus (plural asci). Thus each ascus contains all four products of a single meiotic event. Moreover all of the ascospore in each ascus can be analyzed genetically.

3. These haploid ascospores germinate and grow to produce multicellular mycelia (Singular mycelium) and all of these cells are also haploid. The genotype of each product can thus be determined without carrying out the test cross or other genetic manipulations. Because of the haploid state of the mycelium, the presence of recessive marker is never masked by dominant alleles. 4. Neurospora can be grown on a simple synthetic media

Neurospora crassa life cycle


Cytological basis of Crossing over
Normally, the two chromosomes of any homologous pair are morphologically indistinguishable. Stern, Creighton and B. McClintock, however identified homologous that were morphologically distinguishable, that is they were not entirely homologous, Stern(1931)obtained a type of female Drosophila in which two X chromosomes are different from each other and also from other sets of chromosomes. He found that X chromosome in which a piece of Y chromosome is attached. The second X chromosome has been broken into two unequal segments and is shorten than the unbroken X chromosome

Both the chromosomes can be distinguished by microscopic examination. The broken C chromosomes contain a recessive gene (c) for carnation eye colour and the other chromosome contains dominant allele (C) red eyes. In addition, the broken X chromosome contains a dominant gene (B) for narrow bar eyes while, homologous contain recessive allele (b) for normal round eyes. Stern crossed this female having red bar eyes with a double recessive male having carnation round eyes.

He selected only females for his experiment. In the absence of crossing over, only two types of female gametes are produced: One type having broken X containing c and B genes; Other type having X with piece of Y attached containing C and b. But if crossing over occurred two More types of gametes are produced: One type having c and b in a normal size X and the other type having C and B on a broken X and with a piece of Y Chromosome.


Stern’s: crossing over in Drosophila
F1 female car B car+ B+

Abnormality at another locus of X chromosome
car B

Abnormality at one locus of X chromosome

No crossing over

car B

car+ B+ Crossing over during meiosis in F1 female

car B
car B+ Fertilization by sperm from carnation F1 male

car+ B+

car B+

car+ B
car B+ Fertilization by sperm from carnation F1 male

car B+

car car B B+

car+car B+ B +

car car B+ B+

car+ B

carnation, normal bar



Parental combinations of both genetic traits and chromosome abnormalities

Recombinant combinations of both genetic traits and chromosome abnormalities

Factors affecting crossing over
1. High temperature increases the frequency of crossing over 2. X-ray increases the frequency. 3. The frequency crossing over decreases with increasing age in female Drosophila 4. Crossing over is less frequent near centromeres and the tips of the chromosomes. 5. Nutritional and chemical effect:


Linkage Mapping
Each gene is found at a fixed position on a particular chromosome. Making a map of their locations allows us to identify and study them better. In modern times, we can use the locations to clone the genes so we can better understand what they do and why they cause genetic diseases when mutated. Thus, the percentage of gametes that had a crossover between two genes is a measure of how far apart those two genes are.


Alfred Sturtevant, a student of Morgan’s was assigned a project of consolidating the early information on D. melonogaster. He conceived an approach in which recombination data would be used to describe the physical relationship of genes on a chromosome in a linear arrangement called linkage map or genetic map.


For example, here 11.26% of the offspring from the cross in Table were recombinant between genes pp and ll, making the map distance between them 11.26 map unit.

As pointed out by T. H. Morgan and Alfred Sturtevant, who produced the first Drosophila gene map in 1913. Morgan was the founder of Drosophila genetics, and in his honor a recombination map unit is called a centiMorgan (cM). A map unit, or centiMorgan, is equal to crossing over between 2 genes in 1% of the gametes.


Morgan and his students discovered a number of genes in D.melonogaster and examined their progeny, they found substantial variation in the frequency of recombination for different gene pair. Genes
Yellow (Y) – White (W) Yellow (Y) – Vermilion (V) White (W) – Vermilion (V) Vermilion (V)- Miniature(M) White (W) –Miniature(M) White (W) – rudimentary (r) Vermilion (V)– rudimentary (r)

Recombination frequency
214/21.736 1,464/4551 471/1,584 17/573 2,062/6116 406/898 109/405 = 0.010 = 0.322 = 0.297 = 0.030 = 0.337 = 0.452 = 0.269

Sturtevant constructed a map beginning with Y at 0.0 on left side and using the frequencies of recombination between adjacent genes.

Y W * * 0.0 1.0

V * 30.7

M * 33.7

r * 57.6


Three Point Crossing over


Three – Point Crossing over
Assuming that 3 genes involved are arranged in a linear fashion; there are 3 possible order. Each having a different gene in the center. With 3 linked genes, there are 3 different recombination frequencies between pair of loci: between v and ct; between ct and cv; between v and cv.

Genotype v cv+ ct+ v+ cv ct v cv ct+ v+ cv+ ct v cv ct v+ cv+ ct+ v cv+ ct v+ cv ct+ total

# 580 calculate distances: 592 2 genes at a time; 45 e.g.: v  ct = 40 [ ( 89 +3) / 1448 ] x 100 89 94 3 5 1448

Drosophila ...


# gene isolation ...
# mapping 1st step… # tomato…

chromosome numbering

map (ca. 1952)

Interference RF is defined as the proportion of exchange between two linked genes. As we seen more than one recombinational exchange may occur on a chromosome. A standard way of describing the difference between the observed and expected numbers of double crossing over was first used by H.J.Muller. Interference is I= Observed frequency of double recombinants 1Expected frequency of double recombinants


Positive value indicated interference between recombination recombination events. While, values near zero suggesting no interference, that is that different recombinant events are independent to each other. For closely linked genes, the measure of interference may often be near one, while for widely separated genes on the same chromosome, I is often near zero.


Try this


Mapping: Locating genes along a chromosome


What determines Maleness and femaleness? Nature encompasses a vast array of diverse mechanisms of sex determination in different species. Generally in any organism, individual normally exhibit one of the two sex phenotype: female or male. In such sp, females produce female gametes (egg, ovules or macrospores) and males produce male gametes (sperm, pollen or microspores) Dioecious : Sp with separation of the sexes in different individuals are called dioecious – all the higher animals and some higher plants are e.g. Monoecious: Sp in which both male and female gametes are produced by each individual are monoecious,


In lower animals, the production of both eggs and sperms by the same organisms is more commonly called hermaphroditism, and individual organism producing both types of gametes are termed hermaphrodites. Although the two sex phenotypes are usually quite distinguishable in human and fruit flies, this is not universally the case. In lower eukaryotes, the two genetically distinct types of gametes are sometimes morphologically indistinguishable; this is called isogamy. E.g., green alga Chlamydomonas

Sex Chromosomes
At the cellular level the sex of an individual is determined genetically by the sex chromosomes.
XX = female, XY = male All other chromosomes are called “autosomes” Thus, humans have 46 chromosomes, (44+2)


Human males are the heterogametic sex with two different sex chromosomes, (XY).

Human females are the homogametic sex (XX). In other species sex determination differs: male birds ZZ female birds ZW


What is so different between the X and Y chromosomes?
X- over 1000 genes identified Y- 330 genes identified, many are inactive One gene on the Y is very important: SRY. The SRY gene is the primary determinant of sex.
If SRY is present, testes develop in the early embryo. The testes secrete the hormone testosterone, which causes development as a male.

If SRY is absent (no Y chromosome), ovaries develop instead of testes, and the embryo develops into a female.

Genes on the Y chromosome
There are three classes of genes on the Y.

Genes shared with X chromosome define the pseudoautosomal regions (PAR) Genes similar to X chromosome genes are X-Y homologs
Genes unique to the Y including SRY gene


What is SRY? SRY – Sex determining Region of Y
TDF – testis determining factor SRY – starts male development by - turning on testis-determining genes - turning off ovary-determining genes


Embryology of internal reproductive organs


1.Initiates the process that directs the indifferent gonads toward testis development

2. Activates Sertoli cells to produce Mullerian inhibiting hormone, causing Mullerian duct degeneration


Stimulates Leydig cells to secrete testosterone, which then directs development of the Wolffian ducts towards epidiymides, vas deferens and seminal vesicles
- Testosterone conversion to dihydrotestosterone (DHT) - directs development of the urethra, prostate gland and Male sex organ.


Y chromosome ( SRY region;TDF gene) is not present. - no TDF to tell it to form testis
- gonadal tissue develops towards ovary formation

- In the absence of testosterone – Wolffian duct system degenerates - In absence of MIH – Mullerian ducts continue to develop towards fallopian tubes, uterus, and upper vagina.

The different mechanisms of chromosomal sex determination may be grouped into five classes: 1. XX female, XO male 2. XO female , XX male 3. XX female, XY male 4. XX male, XY female 5. Diploid (2n) female, haploid (n) male.


1. XX female, XO male
In grasshoppers, protenor and many other insects belonging to Orthoptera, females have two X chromosomes, while males have only one X. Consequently, somatic cells of female have one chromosomes more than those of males. Here females are homogametic sex i.e., the sex producing only one type of gametes with respect to the sex chromosomes. In males, the single X chromosome remains unpaired and half of the sperms produced by males have one chromosome, while the other have none.

2. XO female; XX male
This system of sex determination is known in some insects species only, e.g., Fumea. In such sp., females have only one X chromosome. As a result they produce two types of eggs, half having an X chromosome and other half having none. The males, on the other hand, have two X chromosomes; all the sperms, therefore have one X chromosomes. Here male – homogametic sex Female – heterogametic sex.

3. XX- female; XY- male
This system is most common among animals. It is found in humans, mice, most other mammals, etc. Male- XY Female – XX Y chromosome differs in morphology from the X chromosome, and generally smaller than X.


4. XY- female, XX- Male
This system of sex determination operates in birds, reptiles, some insects (Silk warm). The scheme is essentially opposite to mammals.
The bird sex chromosomes are called Z and W.

In birds, a ZZ individual is male, and a ZW individual is female.


Diploid (2n) female, Haploid (n) male This system of sex determination is found mainly in Hymenoptera – honey bees, ants, termites, etc. In these sp, the somatic chromosome number of female is diploid, while that of males is only haploid. The unfertilized haploids develop male and called drones. These drones carry only half the number (16) of chromosomes of the female (32)


Egg size: In the case of sea worm, Dinophilus, the size of eggs appears to determine the sex of animals developing them. Animal developed from eggs of relatively larger size are females, while those obtained from comparatively smaller sizes are males.



Association with females: The larvae of the sea worm Bonelia are sexually undifferentiated. These larvae that attach to the proboscis of female worms develops into males. In contrast, those larvae that do not attach to female remains free living female


Temperature: Some species have environmentally determined sex. Among reptiles, the temperature at which the eggs develop determines the sex. For example, in the turtles, eggs incubated at 30oC become female, while those incubated at lower temperatures become male.


Sex index or Genic balance theory
Formulated by Bridges. According to this theory, sex is determined by the relative number of X chromosomes and autosomes. Its actually the ratio between X chromosome and autosomes. Thus, Number of X chromosome Sex Index = Number of haploid sets of autosomes If the sex index is 1 = female 0.5 = male intermediate = intersex Above 1 = meta female less than 0.5 = meta male

In Drosophila, some individuals show male characteristics in a part of their body, while their remaining parts show the female phenotype. Such a individuals are known as Gynandromorphs. Gynandromorphs are believed to arise from XX zygotes . During embryonic development, in one or more cells one of the two X chromosomes does not pass to any pole at anaphase and as a result, is lost.


Sex determination in Plants
The mechanism of sex determination in plants are similar to animals

Environmental sex determination:

In some plants, either sex determination is due to envt. Or it is greatly affected by envt. Equisetum plants grown under optimum conditions became female and while those grown under adverse conditions became male. In many other plants like, melons, cucumber, Cannabis etc, the sex of the flower is affected by many environmental factors such as temperature, day length and ethylene, GA3, etc. In general, a treatment with GA3 or ethylene promotes production of female flowers.(e.g, Cannabis)

Chromosomal sex determination in plants
Many dioecious plants do not show distinct sex chromosomes. But some of the sp, do show clear cut chromosome. (i) XX female, XY male: Asparagus, Spinach (ii) XY female, XX male: Fragaria elateria In Melandrium the Y chromosome determines maleness , while X chromosome specifies femaleness. A single Y chromosome is able to produce male flowers even in the presence of four X chromosomes (XXXXY), but such plants produce some bisexual flowers.

The Y chromosome of Melandrium is the largest chromosome of its genome. It has four functionally distinct segments. The first segment contains female suppressor and is located at one end of the Y chromosome. A deletion for this segment produce hermaphrodite flowers on XY chromosome. The second region, lying next to first, has the genes that promote maleness, i.e., initiate the development of the anthers. The third segment contains genes governing male fertility and anther maturation. A deletion of this segment produce male sterile plants with abortive anthers. The last segment located end of the Y chromosome, is homologous to a segment of X chromosome.

Sex Linked Inheritance
Chromosomes are the carriers of genes. Genes are located on both the autosomes and sex chromosomes. Genes on Autosomes – somatic characters Sex chromsomes- sex characters However, certain genes present in the sex chromosomes control the somatic characters. The characters which are controlled by such genes are called sex linked character and transmission of such characters from one generation to next is called sex linked inheritance

Discovery and types of Sex linked inheritance

Was first discovered by Thomas H. Morgan in 1910 on Drosophila melanogaster. It can be classified into three types depending upon the chromosomes (X or Y) having sex linked genes.
1. X – Linked inheritance 2. Y - Linked inheritance 3. XY - Linked inheritance


1. X- Linked inheritance
Certain sex linked genes are located only on X chromosomes and their alleles are absent in Y chromosomes. These genes are called X – linked genes; Their mode of inheritance is called X-linked inheritance This pertains to the inheritance of those characters which are controlled by genes located in the non-homologous part of X-chromosomes

Examples: Colour blindness, Hemophilia – human Eye colour in Drosophila

Y – Linked inheritance
The sex linked genes are located on Y chromosomes only are called Y linked genes. The Y linked genes are confined only male (human). Hence they are also called Holandric genes (holo – whole; andros –male) These genes are transmitted directly from the father to the son Example: Hairy ear rims in Men


XY Linked inheritance
Certain sex linked genes are located on both X and Y chromosomes They are called XY linked genes and their mode of inheritance is called XY linked inheritance.


General patterns of Sex linked Inheritance X – Linked Recessive: 1. Usually more males than females are affected 2. No offspring of an affected male are affected, making the trait skip generations in pedigree. But an exception to this pattern occurs in the rare instance when the affected male mate with a female carrier, producing a affected female offspring. X – Linked Dominant: 1. Affected male produce all affected female offspring and no affected male offspring 2. Approximately half the offspring of affected females are affected, regardless of their sex.

Y – Linked Inheritance: 1. Traits are always passed from father to son 2. Only males are affected. Kinds of Sex Linkage: 1. Diagenic: when the characters pass from father to grandson through his daughter Father → Daughter → Grandson 2. Diandric: When the characters pass from mother to grand-daughter through her son Mother → Son→ Granddaughter

3. Hologenic: when characters go directly from female to female, i.e., from mother to daughter and then to granddaughter. Mother → Daughter → Granddaughter 4. Holandric: When the characters are pass from male to male, i.e, from father to son and then to his grandson. Father → Son → Grandson


Sex linkage in Drosophila:
Normal wild type of fruit fly has red eye (RR). In the population of red eyed flies, white eyed (recessive) appeared as mutation (rr). When white eyed male was mated with red eyed female F1 flies had red eyes
In F2 on an average of 3 flies had red eyes and 1 had white eyes


This results reveal white eye is due to recessive gene. But if F2 individuals are classified on the basis of their sex as well as eye colour, a peculiar picture is emerged. All the F2 female had red eyes, but half of the males had red eye and the other half had white eyes. It appears that eye colour of a fly in F2 depended on its sex

But when white eyed female flies were mated with red eyed males, half the flies were red-eyed and the remaining half had white eyed. In F2 generation, both male and female flies showed the ratio of 1 red: 1 white. Morgan correctly reasoned that white eye gene was located on X chromosome


We have 3 color receptors in the retinas of our eyes. They respond best to red, green, and blue light. Each receptor is controlled by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sexlinked).

Colour blindness


Features of Colour blindness
Colour blindness is a sex linked character discovered by Wilson It is a hereditary disease and the affected person cannot distinguish green and red colour. The red blindness is called Protonopia, these persons cannot see red colour While, green blindness is called deuteronopia, such a persons cannot see green colour. Colour blindness is a recessive character, represented by cc The genes are for colour blindness is located on X chromosome. It is common in male but rare female. Colour blindness follows criss- cross inheritance as transmitted from father to grandson through daughter. It is never transmitted from father to son

XCXC - Normal female
X CX c

Carrier female


- Normal Male - Affected male - Affected female



Hemophilia is another sex-linked trait. Discovered by John Cotto. Hemophilia is characterized by delayed blood clotting. People with hemophilia can easily bleed to death from very minor wounds. Otherwise called as “Bleeders disease” Hemophilia is sex linked recessive character (hh) and the genes are located on X chromosomes It is common in men and rare in women


This disease is appeared as a mutant in Queen Victoria and from her it was transmitted to her descendants.
“Royal disease”
XH XH XH Xh XH Y Xh Y Xh Xh - Normal female - Carrier female - Normal Male - Affected male - Affected female

Sex linked in Cat
Sex linkage has been noticed in Tortoise shell cats. This involves the inheritance of coat colour. Tortoiseshell cats have patches of black and orange fur. Almost all tortoiseshells are female. Heterozygous for the X-linked ♀ ♂ coat color gene, one allele black and the other allele orange. XB XB X Xb Y When a black coloured female is (Yellow) matted with yellow coloured male, (Black) tortoise shell female is obtained. XBXb Even reciprocal also tortoise are (Tortoise shell) female

Incomplete Sex Linkage
The genes located on homologous regions of sex chromosomes do not inherit together because crossing over occur may occur in these region. So these genes are incompletely sex linked genes and their mode of inheritance are incomplete sex inheritance. Xeroderma pigmentosum is an abnormal condition of skin in man, in which the skin becomes extremely sensitive to light. Even low light can produce pigmentation. In extreme cases, develops cancerous growth in body. Retinitis pigmentosa is another case of incomplete sex linkage caused by the interaction of a number of dominant and recessive genes. In this case, the affected person develops night blindness leading to the formation of pigmentation in retinal wall.

Some genes fail to produce their phenotypic effects in the presence of certain hormones and are described as sex limited. In human being several characters are restricted with particular sex. For e.g, facial hair in male only (beard )

Sex Limited Traits


Sex-Influenced Traits
In contrast to Sex limited gene where one expression of a trait is limited to only one sex only; sex influenced genes are those whose dominance is influenced by the sex of the bearer. Such characters are called Sex influenced traits. Good examples: male pattern baldness in humans and horns in sheep. Pattern baldness is found in both sexes, but is rarer in females, but it is dominant in males and recessive in females. Thus, male heterozygotes are bald but female heterozygotes have normal hair.

The Adams family and Baldness


Dominant sex influenced trait
Autosomal genes responsible for horns in some breeds of sheep may behave differently in the presence of male and female sex hormones. In suffolk sheep neither sex is horned, and the genotype is “hh”. Among the heterozygous F1 progeny crosses between these breeds, horned males and hornless female are formed. Because both the sex are genotypically alike (h+h), the genes behave as dominance in male and recessive in females, i.e., only one allele is required for male but the allele must be homozygous expression in female. Genotype Males Females h+h+ Horned Horned h+h Horned Hornless hh Hornless Hornless

Deleterious recessive Sex Linked Genes in Human

1. Lesch –Nyhan Syndrome:
Characterized by excess of uric acid, is inherited through sex linked recessive gene. This means that mother contributes the X chromosome with defective gene to a male zygote. Half of the male children of carrier mothers may be expected to inherit this disease. These are deficient for the enzyme hypoxanthine-guanine phosphoribiosyltransferase.(HPRT) Infant who receive this disease appears normal at birth and for several months. By about 10 months of age, they become abnormal and mental retardation, and patient may literally bite of his own fingers and lips.

2. Duchene type Muscular Dystrophy: Is also depends on sex linked recessive gene. If mother is carrier, about half of the male children are expected to be affected. Can be identified by chromosome study. It affects male before they reach teens, with muscular deterioration . Muscles of leg and shoulders become stiff and the children usually become paralyzed and crippled during their middle or late teens. Virtually all die before age of 21.

3. Hunter syndrome: It is characterized by mental retardation, coarse features, hirsutism (abnormal hairiness), and a characteristic broad nose and a large protruding tongue. Symptoms appear in childhood, and a chemical procedure for diagnosis has been developed. Mucopolysaccharides (Containing an amino sugars as well as uronic acid) accumulate in skin cells of these patients. Stained with O-toluidine blue, will results in pink in colour.

Genes controlling the phenotypes of eukaryotes are contained in nucleus. This was certainly the case for the genes studied by Mendel, and for most, if not all. “However, the nucleus is not the whole story” Several important phenotypes of eukaryotes are controlled by genes in the cytoplasm. The genes that are present on the cytoplasm are called Plasmagenes. Plasmagenes also transmit character from one generation to next This is known as cytoplasmic or extrachromosal inhreitance

The first good example of cytoplasmic inheritance emerged from Carl Corren’s studies of the four –O’ Clock plant and from Erwin Baur’s work on Pelargonium in 1909. Both Mirabilis and Pelargonium frequently have variegated leaf colour, showing patches of green and white.

However, certain strains of both the sp can be either all green or all white (though the white strains do not survive very long).

Non-Mendelian inheritance In Mendel expt., it made no difference which parent carried which genotype. For eg., yellow seed (male) X green (female) Will give the same result as reverse. But with leaf colour with Mirabilis it made a great difference: When Correns crossed any male (white or green or variegated) with green female shows all progeny were green. Similar with white female. This pattern of inheritance is called Maternal inheritance


This behavior violates Mendel’s law in 2 ways: 1. Usual Mendelian ratio of phenotypes were not observed 2. There was a difference between reciprocal cross

How do we explain this phenomenon?


The situation become much clearer when we realize the colour in the plant is due to chlorophyll in the chloroplast. Genes are governing in the chloroplast and not in the nuclear gene. But you can ask why such maternal parent dependence?
This is because the maternal parent contributes most of the cytoplasm


Kappa particles in Paramecium
Sonnerborn found that there are two strains of paramecium They are killer and sensitive Killer strain produces a toxic substances “Paramecin” that kills the other type. The production of paramecin in killer is type is controlled by certain cytoplasmic particles is known as “Kappa” particle (KK). The sensitive strains lack these kappa (kk). Kappa particles can pass from one generation to next. They transmitted through the cytoplasm.


When killer KK conjugate with non-killer kk, the ex-conjugants are Kk. But the development of a particular type depends on the duration of cytoplasmic exchange. In a normal case of conjugation the nuclear material alone is exchanged and as a result each conjugate gives rise to the organism of its own type. i.e., killer exconjugant produce killer while, nonkiller will produce non-killer type.


But sometime the conjugation period is prolonged In such case apart from nuclear material, cytoplasmic material are also exchanged.
During this cytoplasmic exchange the kappa particles present in the cytoplasm of the killer type will enter into the non killer type and convert it into a killer type.


Shell Coiling in Lymnea
The snail Limnaea shows two types of coiling namely dextral (towards right side) and sinistral (towards left side). The dextral coiling is due to dominant alleles “DD” while the sinistral coiling due to recessive allele “dd” The experiments of Boycott indicate that the coiling is determined by the gene of the mother and not by the individuals own gene. If a dextral female crossed with sinistral male, all F1 offspring develop dextral shell like mother

To top