HSC – Biology – Blueprint of Life
3. Chromosomal structure provides the key to inheritance
Outline the roles of Sutton and Boveri in identifying the importance of chromosomes
Boveri and Sea Urchins
Theodor Boveri carried out experiments on sea urchin eggs, studying the behaviour of the cell nucleus and
chromosomes during meiosis and fertilisation. Sea urchin eggs were ideally suited for stuffy because they
could be easily fertilised in a laboratory and have a quick (48 hour) time frame for larval development.
Boveri’s studies were advanced for his time, as he did not simply rely on observations made with a
microscope, but also solved problems by experimenting with the larvae of sea urchins.
It was already known at the time that each species of living organism has a set number of chromosomes
and that, during fertilisation, an egg cell and a sperm cell fuse. Boveri’s experimental work with sea urchins
showed that the nucleus of the egg and sperm each contribute the same amount (50%) of chromosomes to
the zygote (fertilised egg), making a connection between chromosomes and heredity.
His experiments showed:
■ when a normal egg and sperm fused, the resulting offspring showed characteristics of both
■ if the nucleus of only one parent was present, the larvae resembled that parent, but showed
abnormalities. When an egg, whose nucleus had been removed, was fertilised with a sperm, the
resulting sea urchin larvae showed characteristics similar to the male parent. However, they were
smaller, had only half the normal number of chromosomes and showed some abnormalities. From
this he deduced that:
— a complete set of chromosomes (chromosome pairs) is needed for normal development
— the inheritance ‘factors’ are found on chromosomes within the nucleus—that is,
chromosomes are the carriers of heredity.
From his ongoing studies, he realised that there are more hereditary ‘factors’ than chromosomes and so
deduced that there must be many factors (today known as genes) on one chromosome.
Together with independent studies by Walter Sutton, Boveri’s studies contributed to the chromosome
theory of inheritance.
Sutton and grasshoppers
Walter Sutton studied meiosis in cells of Grasshoppers. As a result of his observations, he made the
connection between the behaviour of chromosomes during meiosis and Mendel’s laws of heredity. His
independent findings together with those of Boveri, formed the basis of the chromosome theory of
Sutton’s observations of meiosis in grasshoppers revealed that:
■ chromosomes occur in distinct pairs, visible during meiosis in grasshopper cells; one chromosome
of each pair is paternal and the other maternal and the chromosomes in each pair have the same
size and shape
■ during meiosis (reduction division), the chromosome number of a cell is halved: the chromosomes
in each pair of chromosomes separate and each gamete receives one chromosome from each pair
■ fertilisation restores the full number of chromosomes in the zygote. He concluded that
chromosomes were the carriers of heredity units and behaved in the same manner as Mendel’s
‘factors of inheritance’
In addition, Sutton stated that:
■ Chromosomes arrange themselves independently along the middle of the cell just before it
divides—that is, they assort independently of each other during segregation, like Mendel’s factors
■ Chromosomes are units involved in inheritance. Sutton, like Boveri, also believed that several
Mendelian ‘factors’ must be present in one chromosome and could therefore be inherited as a unit.
Parallels between the behaviour of genes and the behaviour of chromosomes
Genetic Behaviour Chromosomal Behaviour
Segregation of the members of each pairs of Separation (disjunction) of the members of each pair of matching
alleles into gametes chromosomes into different gametes
Random assortment of the alleles of different Random orientation of different pairs of chromosome across the cell
genes into gametes equator prior to their separation during gamete formation.
Explain the relationship between the structure and behaviour of chromosomes during
meiosis and the inheritance of genes
During meiosis, genetic variation arises as a result of the behaviour of chromosomes at two stages:
1. during crossing over
2. when chromosomes randomly segregate and paternal and maternal chromosomes assort
independently of each other.
During meiosis I:
1. chromosomes line up in homologous pairs (one maternal and one paternal chromosome in each
pair) during prophase I
2. crossing over occurs—arms of homologous chromosomes exchange genetic material (during
metaphase). This introduces genetic variation— genes that occur on the same chromosome are
said to be linked. Crossing over (synapsis) ensures that linked genes on a chromosome can be
inherited independently of each other. The exchange of genetic material between homologous
chromosomes during crossing over causes the mixing of paternal and maternal genes and the result
is an increased number of combinations of genes that may be transmitted by gametes to offspring,
thereby increasing genetic variation.
3. the chromosomes in each pair of chromosomes separate (during anaphase I), so that one entire
chromosome of each pair moves into a daughter cell. This separation of chromosomes, also referred
to as random segregation, ensures the chromosome number in the resulting gametes will be half
that of the original cell. The manner in which these chromosomes separate is termed independent
assortment— the paternal and maternal chromosomes sort themselves independently of each
other. For example, the maternal chromosomes do not all move into one gamete and the paternal
into another. Which chromosome of each pair ends up in a gamete is random and determined
completely independently of the separation of any other gene pair. As a result, mixing of maternal
and paternal chromosomes occurs and so independent assortment leads to further genetic
During meiosis II: The two daughter cells that result from meiosis I each undergo meiosis II, which is
similar to mitosis, and the behaviour of chromosomes in the second meiotic division does not further affect
The genetic consequences of meiosis are:
■ one cell undergoes two meiotic divisions to generate four haploid cells
■ the genes in each haploid cell are a new combination of the parental genes
■ the new combination results from both crossing over and random segregation, allowing the
individual alleles of maternally and paternally derived chromosomes to assort independently.
How meiosis accounts for the separation of Mendel’s ‘factors’
Mendel developed two laws: dominance and segregation, and independent assortment. Mendel did not know
about genes and chromosomes; Sutton and Boveri noted the similarity between the behaviour of
chromosomes and Mendel’s Laws. Units of heredity on chromosomes were later termed genes (1909) and
their inheritance patterns are now used to explain the ratios derived in Mendel’s laws.
construct a model that demonstrates meiosis and the processes of crossing over, segregation of
chromosomes and the production of haploid gametes
Explain the role of gamete formation and sexual reproduction in variability of offspring
The term variability means something different to variation. Variability in genetics relates to the different
forms of a gene within a population (that is, the total of all alleles present in the gene pool of a population).
Both variation and variability may have one of three origins:
■ a combination of both genes and the environment.
Both variation and variability are of evolutionary advantage only if they have a genetic basis.
Genetic variation in individuals (and therefore variability in a population) arises as a result of sexual
reproduction. This involves gamete formation (by meiosis), followed by fertilisation (fusion of male and
female gametes). The resulting zygote then grows and develops into a new individual.
Gamete formation and variability
Gametes form by meiosis, where recombination of genetic material takes place as a result of crossing over
and random segregation:
■ in crossing over, homologous chromosomes exchange genes and so the resulting combinations of
alleles on chromatids differ from those originally on the parent chromosomes
■ in random segregation and independent assortment, genes on different chromosomes sort
independently of each other, giving different gene combinations in gametes from those of the
parents. Gametes that arise from genetically dissimilar parents (cross-fertilisation as opposed to
self-fertilisation) are likely to differ from each other more than those produced by self-fertilisation.
Cross-fertilisation produces a greater variety of gametes, increasing variability.
Fertilisation and variability
Gametes contain different, recombined genetic material and the possibility of many different combinations
of gametes fusing increases variation. New combinations of genes occurring in the offspring lead to greater
variability within a population.
Example of variability introduced by sexual reproduction
One parent may have blue eyes and fair hair, the other dark eyes and dark hair. If the traits for hair colour
and eye colour assort independently from each other, their gametes may combine to produce offspring that
have blue eyes and dark hair or brown eyes and fair hair. In this case, the offspring have a different
combination of the parents’ genes, increasing variation.
Further variability arises in a population if a greater number of alleles are present for each gene. If within
the population there are individuals with red hair and green eyes, there is greater variability and an even
greater opportunity for more gene combinations to arise in gametes produced by individuals. Therefore,
variability may be increased as a result of:
■ a recombination of genetic material
■ an increased number of alleles for a particular gene.
Both meiosis (gamete formation) and fertilisation play a significant role in causing variability. When gametes
form, crossing over and random segregation in meiosis result in genetic recombination of paternal and
maternal genes within each gamete. When two different gametes unite during sexual reproduction
(fertilisation), a further mixing of genetic material occurs and this results in offspring with many new
combinations of genes.
Importance of sexual reproduction and variability
Sexual reproduction is important in increasing genetic variability— the amount by which individuals in a
population vary from each other genetically.
Mechanisms of sexual reproduction, such as gamete formation (meiosis) and fertilisation, increase gene
recombination and therefore variability in a population. Evolutionary studies show that greater variability
improves the ability of a population to adapt to changes in the environment, resulting in an increased
chance of survival. If there is little or no variability within a population, the result is a static or unchanging
population that is less likely to be able to adapt to sudden changes in the environment and is more likely to
be wiped out.
Describe the inheritance of sex-linked genes and alleles that exhibit co-dominance and
explain why these do not produce simple Mendelian ratios.
Mendelian ratios of inheritance apply only in situations where conditions are similar to those studied by
Mendel. However, if genes do not assort independently or do not show dominance, Mendel’s ratios are not
obtained. Two examples where these deviations from Mendel’s ratios have been seen are in sex-linked
inheritance and co-dominance.
Sex-linkage Sex chromosomes
Every cell in the human body contains 23 pairs of chromosomes: 22 pairs of autosomes (chromosomes that
code for general traits within the body) and 1 pair of sex chromosomes. Sex chromosomes carry genes that
determine the sexual characteristics of a person and therefore influence whether they are male or female.
Sex chromosomes in an individual may differ from each other in size and shape (in contrast to autosomes,
where homologous chromosomes all have the same appearance). In humans, females have a pair of similar
sex chromosomes— their genotype is represented as XX (described as homogametic), but the sex
chromosomes of males appear to be different (heterogametic) and their genotype is represented as XY. In
human males, one sex chromosome is a typical X chromosome, but the other is shorter and seems to be
missing part of its ‘arms’, resulting in a ‘Y’ shape
The offspring of most animals have an equal (50%) chance of being male or female.
This is determined by the following mechanisms during the life cycle:
■ the segregation of sex chromosomes during meiosis
■ the transfer of one sex chromosome to each gamete
■ the fusion of gametes during fertilisation.
During meiosis, the sex chromosomes segregate, just like any other homologous pair of chromosomes, and
only one of each chromosome pair passes into a gamete. Since a female has 44 autosomes + XX, when the
chromosome number is halved during gamete formation, each female gamete (egg cell) receives 22
autosomes + X. In males, half the gametes (sperm cells) receive 22 autosomes + X and the other half
receive 22 autosomes + Y. It is therefore the male gamete in humans that determines the sex of the
offspring. Fertilisation follows and the combination of sex chromosomes in the zygote dictates the sex of the
child: a zygote that inherits an X chromosome from both the mother and father will be female (XX). A
zygote that receives an X chromosome from the mother and a Y chromosome from the father will be male
(XY). In humans, Y carries the testis-determining gene and therefore, if present, the Y assures the child will
be male. In some cases, non-disjunction of chromosomes occurs during meiosis (the chromatids do not
separate), resulting in the incorrect number of chromosomes in the offspring (e.g. 45 or 47). If this occurs
in the sex chromosomes and only one X chromosome is present, the child will be female. If the individual is
XXY, it will be male
The larger sex chromosome (X in humans) may also carry a few genes that code for non-sexual body
characteristics. These genes are termed sex-linked genes, since they are physically linked to the sex
chromosome and are inherited together with the sexual traits. Sex-linked genes in males and females tend
to differ in their inheritance patterns, since males lack one X chromosome and therefore have only one allele
for each sex-linked gene, rather than a pair of alleles (as is present in females). Mendel’s experiments did
not show sex-specific effects and so sex-linked inheritance shows a deviation from Mendel’s expected ratios.
Note: A complete set of chromosome images is organised to form a karyotype.
Describe the work of Morgan that led to the understanding of sex linkage
The importance of Morgan’s work
Morgan was initially skeptical about aspects of both Mendelian inheritance and the chromosome theory and
so it is ironic that it was his experiments on the fruit fly that provided the significant evidence needed by
scientists before they could finally accept Sutton and Boveri’s chromosome theory of inheritance.
Morgan’s experiments showed without any doubt that:
■ the gene for eye colour in fruit flies is located on the X chromosome, and
■ hereditary factors can be exchanged between the X chromosomes of an individual.
The results of Morgan’s studies led to a greater understanding of how genes are arranged on chromosomes
and how genetic material can be exchanged (during crossing over) in meiosis.
Details of Morgan’s work
Morgan was working with fruit flies, which normally have red eyes. He discovered a mutant male fruit fl y
that had white eyes.
Morgan began with a series of crosses in the typical Mendelian sequence to determine if the gene for white
eyes would show a Mendelian pattern of inheritance:
Cross 1: He cross-bred pure-breeding parents to obtain F1 hybrid offspring. Morgan crossed a white-eyed
male and a pure-bred (homozygous) red eyed female.
Cross 2: He then crossed the F1 hybrid offspring to obtain the F2 generation. Morgan was expecting a
Mendelian 3:1 ratio but, surprising to him, his results showed something different—more than 80% of the fl
ies had red eyes and less than 20% had white eyes; in addition, most fl ies with white eyes were male. At
first, he thought that perhaps female flies could not have white eyes.
Cross 3: He performed a typical ‘test cross’ to investigate this hypothesis. He crossed a white-eyed male
with a hybrid red-eyed female. His results showed in the F2 that both females and males could have white
He would have to come up with another hypothesis to explain the unexpected ratios in his second cross.
He arrived at his next hypothesis that the white eye characteristic is ‘sex limited’ and is carried on the X
chromosome. He followed this with subsequent genetic crosses that proved that red eyes were in fact sex-
Note: Crosses with parental phenotypes reversed are known as reciprocal crosses.
Extra: X-Linked Dominant pattern
Extra X-Linked Recessive pattern.
Explain the relationship between homozygous and heterozygous genotypes and the
resulting phenotypes in examples of co-dominance.
Co-dominance is another example of inheritance that does not show a Mendelian pattern. This is because in
genes of some organisms, pairs of alleles do not show dominance of one over the other – that is, they are
exceptions to Mendel’s Laws of prominence. In a heterozygote where two different alleles for the same gene
are present, both alleles are expressed as separate, unblended phenotypes and so they are termed co-
dominant – that is both appear dominant – because both are expressed.
Pure-breeding (homozygous) cattle may have a red or white coat colour. Hybrid individuals (heterozygotes),
which have one allele for red and one for white coat colour, have a roan appearance—both red and white
hairs are present, not in patches but interspersed (that is, both types of hairs are present, indicating that
both alleles are expressed
Another example occurs in chickens. If a homozygous black fowl is crossed with a homozygous white fowl,
the heterozygous offspring in the F1 generation appear ‘blue’. At first this was thought to be a ‘blending’ of
characteristics (a concept called incomplete dominance), but closer examination revealed that the blue fowls
have both black and white feathers present—a typical example of co-dominance.
Blood cells have proteins on their surfaces called antigens and these play an important role in allowing a
person’s own body cells to be recognised by the immune system as ‘self’. The antigens can be described as
the ‘uniform’ that a cell wears so that the immune system will recognise it as ‘belonging’. Invading cells
have on their surfaces antigens that are different, so the body recognises them as ‘foreign’ and produces
antibodies to attack and destroy the foreign invaders. If the wrong blood group is given in a transfusion, the
antigens on the surface of these blood cells are recognised as ‘foreign’ and the recipient’s body will produce
antibodies to attack and fight these cells. The clumping reaction can block blood vessels and prove fatal.
The inheritance pattern of the ABO system of blood antigens shows co-dominance. There are three alleles in
the population for this particular gene—alleles A and B code for the presence antigens A and B respectively,
but allele O codes for no antigen. Any one individual will have only have two alleles for a particular trait such
as ABO blood group. Therefore the possible genotypes are AA, BB, AB, AO, BO and OO.
If the allele A is present, the blood cells produce a surface antigen known as A. If allele B is present, B
antigen is produced. If both A and B alleles are present, blood cells have both A and B surface antigens—
both alleles are expressed in each other’s presence. A and B are said to be co-dominant. If neither antigen
is present, the blood cells produce no antigen and are said to be group O. Studies of the inheritance of the
ABO blood system do not produce Mendel’s ratios. The gene does not follow his law of dominance—there is
equal expression of the A and B alleles in the heterozygous form, as these alleles exhibit co-dominance
(they are both expressed when present in a heterozygote).
Outline ways in which the environment may affect the expression of a gene in an
A phenotype is not always produced exclusively by the genotype. In some cases the phenotype is a result f
the interaction of the genotype and the environmental factors:
GENOTYPE + ENVIRONMENT ==> PHENOTPYE
Cats and Temperature
Siamese kittens are all white at birth. A few weeks later, the kittens begin to develop pigmentation first
along the edges of their ears. Gradually the pigmentation spreads until the kittens show the characteristics
on the face, ears, feet and tail. The pattern of colour change is due to an interaction between the cats’
genotypes and their environment.
Siamese cats have a particular form of a gene that is involved in the pigment production. This gene codes
for the production of a protein enzyme, known as tyrosinase. This enzyme catalyses one step in the
production of the pigment
In Siamese cats, the particular form (allele) of this gene is heat sensitive. This enzyme can only catalyse the
step in the production of the pigment if temperature is lower than the core. Thus depending on the
environment, the extent of the pigmentation will be observed.
Plants and Soil pH
Hydrangea plants produce blooms with colours that vary depending on the acidity or alkalinity of the soil in
which they are growing. The colour is die to pigments known as anthocyanins which are located in
membrane-bound sacs within the petal cells. In soil with an acidic pH, these pigments are a bright blue,
while at alkaline pH, they are a pale pink or off white.
The inherited disorder, phenylketonuria (PKU) results from the action of genes that controls production of an
enzyme known as phe hydroxylase. Babies that inherit the homozygous recessive genotype pp from their
parents cannot produce this enzyme. If these babies are fed diets that include proteins that contain normal
quantities the amino acid, phe, the babies will suffer brain damage and become mentally retarded, so early
diagnosis is critical. However, if these babies are fed a special diet that include proteins and very low levels
of phe, the babies will not suffer brain damage and will show a normal phenotype.
A child with the missing enzyme cannot have a diet that has no phe, because this is an essential amino acid
required for growth and development.
Describe the chemical nature of chromosomes and genes
Chromosomes are compact coils of thread-like molecules called DNA (deoxyribonucleic acid), organised
around proteins called histones.
DNA (deoxyribonucleic acid) is the molecule that meets all the requirements of the hereditary material:
■ It can carry, in coded form, all the instructions for the formation and functioning of cells, despite the fact
that its ‘alphabet’ consists of only four nitrogenous bases.
■ Its structure allows self-replication.
■ It can be transferred (packaged in the form of chromosomes) by gametes from one generation to the
We now know that each chromosome is made up of two chemicals:
1. DNA, a long, thin thread-like macromolecule, which is the information-carrying part of the chromosome
2. proteins around which the DNA is coiled, to keep it neatly ‘packaged’.
Identify that DNA is a double-stranded molecule twisted into a helix.
General structure of DNA
DNA is a double helix or ‘twisted ladder’. A DNA molecule is made up of two chains or strands of small
building blocks or monomers called nucleotides. Each nucleotide consists of three parts—a phosphate, a
sugar (deoxyribose sugar) and a nitrogenous base. There are four types of bases and each nucleotide is
named after the base that it carries—adenine, thymine, guanine or cytosine. These are often simply referred
to by their first letters. The bases are arranged in a sequence along each strand of DNA— and so each DNA
molecule is thousands of bases long. The whole ‘ladder’ molecule, instead of being fl at, spirals and is
therefore known as the ‘double helix’.
This conformed with the research:
■ X-ray crystallography suggested a helix measuring 3.4 nm for every turn and this fitted the model
where exactly 10 base pairs would measure 3.4 nm in length and make up one twist of the helix.
■ The ratio of adenine to guanine and cytosine to thymine could be explained by their
complementary base pairing.
The two complementary chains of DNA could ‘unzip’ or open up along the line of hydrogen bonds between
the base pairs, allowing them to replicate.
The chemical structure of DNA
The DNA molecule is a long chain molecule consisting of two complementary strands. Each strand is made
up of a sequence of many nucleotides and the strands are held together by weak hydrogen bonds in the
centre. The two strands in the double-helix model have a ‘antiparallel’ arrangement
The vertical sides of the ladder are made up of alternating sugar and phosphate molecules (a ‘sugar–
phosphate backbone’) and the ‘rungs’ of the ladder are pairs of nitrogenous bases (adenine, thymine,
guanine and cytosine, or A, T, C and G respectively). Each nucleotide is made up of a phosphate group, a
sugar (deoxyribose sugar) and a nitrogenous base attached to the sugar.
There are four types of nitrogenous bases: adenine, guanine, cytosine and thymine. Chemically, these bases
have to pair in a particular manner: adenine with thymine and guanine with cytosine (that is, A-T and G-C),
held together in the centre by hydrogen bonds, forming two complementary strands
Genes and chromosomes
A gene is considered to be the smallest unit of heredity. Chemically, each gene is a portion of DNA with a
specific sequence of bases that encodes for a particular trait that can be passed from parent to offspring.
A locus is the position of a gene on a chromosome. The coded information within genes determines how
living things look, behave and function—that is, it infl uences particular characteristics (phenotypes).
A chromosome can therefore be described as a linear sequence of genes. The total amount of genetic
material that an organism has in each of its cells is called its genome.
Specific staining techniques are used to show up banding patterns on chromosomes and these bands
correspond on homologous pairs of chromosomes. The banding patterns can also be used to identify the
positions of particular genes on chromosomes. With modern technology, particular genes can be marked
with fluorescent tags that show up on the chromosome, assisting gene mapping.
Extra: Incomplete dominance (NSW Biology).
A snapdragon is like pea plant, but with red, white and pink flowers. However, the inheritance in this
case does not follow Mendel’s laws. Instead of getting red or white flowers, the flowers are pink due to
the combined or blending effect of the red and white alleles. This is called incomplete dominance. If two
pink flowers are cross-fertilised, 50% will be pink, 25% will be red and 25% will be white. This results
supports Mendel’s conclusion that alleles are retained in offspring and assort independently.
When incomplete dominance occurs, neither the allele for red or the white flower is dominant.
Heterozygotes have and intermediate phenotype.