This document provides information on inheritance and variation. It discusses two types of variation: continuous variation which is influenced by both genes and environment, and discontinuous variation which is solely due to genes. Examples like height and skin color are used to illustrate continuous variation, while blood type demonstrates discontinuous variation. The role of chromosomes, DNA, genes, and mutations in inheritance and causing variation are also explained. Sickle cell anemia is presented as an example of a condition resulting from a gene mutation.

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INHERITANCE
Variation
Observable differences (different characteristics) within a species that causes
as a result of sexual reproduction is known as variation. Sexual reproduction is
the main cause of variation but there is an exception occurs when the offspring
develop from the same ovum and sperm, in which case they are ‘identical twins’
What are observable differences within a species?
 Skin colour, height, mass, size, coat color, eye colour, length of fur etc.
There are two types of variation
 continuous and discontinuous variation
Continuous variation
Continuous variation is the result of the interaction of two factors. They are:
i. The genes that are inherited by an individual.
ii. The effect of environment on the individual.
Environmental factors
i. Availability and the type of food (in animals)
ii. Disease
iii. climate
 amount of sunlight
 temperature
 amount of water availability.
iv. the ions present in the soil (in plants)
v. Competition from other organisms in the environment.
In continuous variation, individual show a range between the two extremes. Every
possible form (intermediates) between the two extremes will exist.

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Examples of continuous variation
i. body mass
ii. height
iii. foot size
Height in metres Percentage of people in population at each height
1.5 (lower extreme) 1
1.7 (intermediate) 6
1.9 (intermediate) 10
2.1 (intermediate) 12
2.3 (intermediate) 6
2.5 (higher extreme) 1
Figure 1.1
Continuous variation
Figure 2.2
1
6
10
12
6
1
0
1.5 1.7 1.9 2.1 2.3 2.5
% of people in population at each
height
height in metres

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Example of variation caused by gene and the effect of environment
A fair skinned person may be able to change the colour of his or her skin by
exposing it to the sun. These people have extra inherited gene for producing brown
color. This gene has interaction with the environment. A fair skinned person with the
genes for producing brown pigment will only go brown if he exposes himself to
sunlight. This is the reason that our colour changes when we are exposed to the sun
during hot days. So your tan (brown colour) is caused by both, inherited gene and
the effect of environment.
Discontinuous variation
This is only the result of the gene that had been inherited by an individual. There is
no effect of environment on the gene, so the environmental condition does not affect
the phenotype (appearance) of the individual. For example you cannot change you
blood group by altering your diet. A genetic dwarf cannot grow taller by eating more
food. There are few types with no intermediates. In sex, of human there is no
intermediate form in between male and female. A part from a small number of
abnormalities, sex is inherited in a discontinuous way.
Examples of discontinuous variation
i. blood group
ii. the ability to roll tongue into U shape
Example of variation caused by inherited gene only
Some fair skinned people never go brown in the sun, they only become sun burned.
They have no inherited genes for producing extra brown pigment in their skin.

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Discontinuous variation
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3
A AB B O
Difference between continuous and discontinuous variation
Continuous variation Discontinuous variation
 Continuous Variation is the result
of the interaction of two factors
1. genes (inheritance)
2. Environment
42
 Discontinuous variation is the
result of inheritance(genes)
 In continuous variation, individuals
show a range between the two
extremes
 No intermediates
(an organism has the characteristics
or it does not have it)
 Examples- Body mass (very
heavy and very light) and a range
of values in between. Most
individuals are about average
 Height,(very tall average - very
short)
 Foot size ( Large, medium, small)
 Blood groups(A, B, AB, O)
 Male or female
 The ability to roll the tongue into
U shape
 Fixed ear lobes or free ear lobes
 Combined effect of many genes  By one or few genes
 Not easily distinguished  Easily distinguished
Advantages of variation
 Variation allows the survival of the fittest.
 New varieties of organisms may arise due to genetic variation.
 Competition occurs among the different varieties of organisms and nature
selects those varieties that are more competitive, more resistant to disease
and better adapted to changes in the environment to survive and reproduce.
No: of people in a population with
each blood group (percentage)
Blood group

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Chromosomes
 Thread –like structures present in the nucleus.
 Chromosomes are situated in the nuclei of all living cells (except bacteria and
RBC)
 Chromosomes are made of DNA ( Deoxyribonucleic acid)
 There is a fixed number of chromosomes in each species ( e.g. Human- 46)
 The number of chromosomes in a species is the same in all of its body cells
 The chromosomes have different shapes and sizes.
 The chromosomes are always in pairs, eg. Two long ones, two short ones,
two medium etc. In human, chromosomes consist of 23 from father and 23
from mother.
Nucleic acid
DNA (deoxyribonucleic acid)
 DNA carries the genetic code which determines how all cells will work and
the characteristics organisms will develop.
 DNA determines the whole chemistry of the cell.
 Nucleic acids are made up of long chains of subunits called nucleotides.
Each nucleotide is made up of a base, sugar and a phosphate group.
 In DNA, there are four different nucleotides, each containing a different base.
 Four bases are; A (Adenine)
C (Cytosine)
G (Guanine)
T (Thymine)
 These bases link with one another in the following ways
A always with T
C always with G
The DNA molecule, looking rather like a very long, twisted rope ladder, is made
up of two strands. Notice that adenine (A) on one strands is always placed

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opposite thymine (T) on the other strand. Cytosine (C) is always placed opposite
guanine (G).
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This is called the base pairing rule.
The unit of inheritance
All living organisms manufacture proteins in their cells.
Uses of protein
 Structural and chemical purposes
e.g. growth, repair, muscle formation etc
 To make enzymes, hormones, haemoglobin etc.
How proteins are made?
Amino acids linking to form a protein molecule (there are 22 different amino asids).
The sequence of bases of DNA first split into triplets. e.g . CAT, GCT, AGC, CTA etc.
Each triplet is then responsible for lining up of one particular amino acid. Each of the
22 amino acids has its own triplet.
Since the sequence of bases on DNA molecules is different for each individual
(sexually produced), it follows that no two individuals will make a protein molecules
with exactly the same sequence of amino acids.
Each chromosome is divided into short sections of DNA called genes. The length of
chromosomes which contains the bases necessary to make one protein molecule is
known as gene.
A gene is defined as a unit of inheritance, forming part of chromosome. It is passed
on from parents to offspring through chromosomes in the nuclei of the parents’
gametes.

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Genetic Inheritance
Chromosomes exist in matching pairs. For example, human beings have 23
matching or homologous pairs of chromosomes, a total number of 46. Of each
pair of matching chromosomes, one is inherited from a person’s mother and one is
inherited from their father.
23 pairs of chromosomes of normal
human male (XY)
23 pairs of chromosomes of normal
human female (XX)
Variation as a result of mutation
Genes and chromosomes are subjected to change (mutation) as a result of
environmental forces acting upon them. These forces are known as mutagens, and
include X rays, atomic radiation, Ultra violet and some chemicals. Exposure to higher
doses of any of these mutagens will lead to a greater rate of mutation.
Mutation
Mutation is a spontaneous (permanent) change in the structure of gene or
chromosome. There are mainly two types of mutation.
i. Gene mutation
ii. Chromosome mutation
Gene mutation
Gene is a section of chromosome that code to make a particular protein which
controls a specific characteristic of an organism. If there is a permanent change
in the structure of a gene, it is considered as gene mutation. In gene mutation part of

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the DNA on a chromosome is changed and results to produce defective protein
(imperfect protein) or no protein at all. This can lead to a considerable change in a
characteristic. For example sickle cell anaemia.
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Sickle cell anaemia
Sickle cell anaemia is an example of condition caused by gene mutation. Both parent
pass mutated (recessive) alleles for making haemoglobin in red blood cells. The
homozygous recessive offspring cannot make effective haemoglobin, and cannot
carry sufficient oxygen in the blood. Their red blood cell takes on a distorted shape
(sickle shape). A person with this condition is likely to die at an early age.
Note:
 Malaria is a life threatening disease caused by protozoan which invades red
blood cell.
 A heterozygote person having the gene for sickle cell anaemia (HNHn) is
protected for malaria, because the protozoan is unable to invade the sickle
cells.
 A person homozygous for sickle cell (HnHn) also has protection.
 A person with normal haemoglobin (HNHN) is at high risk of transmitting
malaria because they are not protected by sickle cell.
Chromosome mutation
Chromosome mutations occur when cell division fails to work with complete
accuracy. The possible causes are
i. section of DNA turned around (inversion)
ii. section of DNA move on to a different chromosome (translocation)
iii. section of DNA cut out and lost (deletion)
iv. Extra DNA or chromosome added (insertion)

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Down’s syndrome
Down’s syndrome is an example of a condition caused by chromosome mutation.
There are 46 numbers of chromosomes in every normal cell of human body; there is
23 pair of chromosome in each gamete. Forty six is known as diploid number and
23 as haploid number.
In the production of gametes one extra chromosome enters on one of the gametes
and changes the number of chromosomes in the gametes to 24 (instead of 23). If
this gamete involved in the process of fertilization, there will be 47 (instead of 46)
chromosomes in the zygote. In older parents, there is a greater tendency for
chromosome number 21 not to separate properly as gametes are being made.
Features of a child who has Down’s syndrome
 Their physical and mental development will be slow
 They will have a distinctive facial appearance. .g. broad forehead, short nose,
short neck, protruding tongue, fold eyelid,
 Mental retardation
Genetic diagrams
Genetic diagrams are way of looking at the combinations of alleles produced by two
parents. In constructing genetic diagrams, the letters of the alphabet (rather than
beads) are used to represent alleles. A dominant allele is represented by a capital
letter (like A, B, C) and its recessive allele is represented by simple letters (like a, b,)
Monohybrid inheritance
Monohybrid inheritance refers only on pair of contrasting characters, such as curly or
straight hair controlled in the individual by a single pair of alleles. There are two
types of monohybrid inheritance.

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i. With complete dominance
ii. With codominance
With complete dominance
In complete dominance the appearance (phenotype) of an individual is determined
by the presence of a single dominant allele of alleles
Phenotype
Genotype BB Bb bb
Example: coat colour in mice
In mice black coat colour is dominant over white coat colour. In an experiment a
homozygous dominant (pure breeding) brown male mouse mated with a
homozygous recessive (pure breeding) white female mouse. All the offspring of F1
(first filial generation) generation were found to be black. The offspring of F1
generation were than allowed to freely interbreed. It was found that their offspring
(F2 generation) were brown to grey in a 3:1 ratio. This can be explained in a genetic
diagram as shown below.
Example: cystic fibrosis in human
Cystic fibrosis is an inherited condition that affects the type of mucus found in
people’s lung. Most people produce normal protein in the mucus of their lungs. They
possess at least one dominant allele, which may be called ‘F’. The homozygous
recessive person, suffering from cystic fibrosis, has the genotype ‘f’. Their lungs
contain particularly thick and sticky mucus, which makes gaseous exchange difficult.
Genetic diagram: both parents heterozygous for cystic fibrosis
The diagram below, there are two parents who are both heterozygous for cystic
fibrosis (their genotype is ‘Ff’). If they have a child, the probability of this child having
the genotype ‘ff’ and therefore suffering from cystic fibrosis, is 25%

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Gametes F f
F FF Ff
f Ff ff
Punnett square
Punnett square allows you to work out the results from a genetic cross. Write the
genotypes of one set of sex cells across the top of the square and those of the other
sex cells down the side. Then combine the alleles in the two sets of gametes; the
squares represent the possible fertilization
Test cross (back cross)
It is a breeding experiment between an organism showing a dominant feature,
whose genotype is unknown, and one showing the recessive feature.
For example, in pea plants the allele for tallness is dominant to that of dwarfness,
so a tall plant could be either homozygous or heterozygous. If we use the symbols
‘T’ for the tall allele and ‘t’ for the dwarf allele, then it could have the genotype ‘TT’
or ‘T t’ .
There is no way of telling from their phenotype which type they are. Therefore, a test
(or back) cross is performed.
In a test cross, the individual is mated with a homozygous recessive (t t) partner
If the unknown tall plant was homozygous I f t h e u n k n o w n t a l l plant was heterozygous
Parent genotypes: T T x t t
Gametes: T T t t
Offspring genotypes: Tt Tt Tt Tt
Phenotype: all tall
Ratio: all tall
Parent genotypes: T t x t t
Gametes: T t t t
Offspring genotypes: Tt Tt tt tt
Phenotype: 2 tall and 2 dwarf
Ratio: 1:1
Heterozygous parents F1 genetion 1:1 ratio Homozygous parents All dominant in F1

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Cross between homozygous brown - coated mouse and grey- coated
mouse
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Key to alleles
 ‘B’ represents the dominant allele for brown coated colour in mice
 ‘b’ represents the recessive allele for grey coated colour in mice
Parents: male x female
Genotype: BB x bb
Phenotype: brown x grey
Alleles found in gametes
F1 generation
Gametes B B
b Bb (brown) Bb (brown)
b Bb (brown) Bb (brown)
Possible genotypes: all Bb
Phenotypes: all brown
Ratio: 3 : 1
(F1 self allowed to interbreed)
Parents: male x female
Genotype: Bb x Bb
Phenotype: brown x brown
Alleles found in gametes
B B b b
B b B b

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Note
The results are given in statistical ratio in large sample. The smaller the sample, the
less likely the ratios will be the same as shown
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F2 generation
Gametes B b
B Bb (brown) Bb (brown)
b Bb (brown)
Possible genotypes: all Bb
Phenotypes: all brown
bb (grey)
Ratio: 3 : 1
In humans where only one offspring is likely to be produced at a time, the probability
of that offspring inheriting a particular feature is often given. Probability is usually
expressed as a percentage.
with Co dominance
In the previous examples, we have stated that an allele is either dominant or
recessive. Sometimes both alleles have an equal effect on the phenotype of an
individual, then the alleles are said to be co dominant. We have also assumed that
a gene only ever has two alleles. This is also not the case; sometimes there are
more than two alleles of a gene controlling a single characteristic. These are referred
to as multiple alleles. ABO blood group is a good example to demonstrate both
these concepts.
 If a characteristics is the result of two alleles which are equally dominant, the
phenotype is an intermediate nature
 In humans, the IA and IB alleles are codominant in the AB blood group.
 These types of alleles are termed codominant.

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Inheritance of human blood groups
 The gene that controls the ABO blood group in humans has three different
alleles.
 They are IA , IB and IO.
 IAand IB are codominant, while IO is recessive to both IAand IB.
 For the blood group, there can only be 2 alleles in any one genotype.
Blood group
(phenotype)
Genotype
A I A I A or IA IO
B I B I B or IB IO
AB IA IB
O IOIO
A woman is heterozygous for group B and her husband is heterozygous for group A.
We can represent the inheritance of ABO blood groups among their children using
the following genetic diagram.
Parental Father Mother
Phenotypes Blood group A x Blood group B
Genotypes I A IO I B I O
Gametes I A IO I B I O
Possible genotypes I A I B I A IO I B IO IOIO
Phenotype (blood group) A B A B O
Probability % 25% 25% 25% 25%
Co dominance complete dominance

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Human pedigree
A pedigree is a diagram of family relationships that uses
symbols to represent people and lines to represent genetic
relationships. These diagrams make it easier to visualize
relationship within families, particularly large extended families.
Pedigrees are often used to determine the mode of inheritance
(dominant, recessive, etc) of genetic diseases.
In a pedigree, squares represent males and circles represent females. Horizontal
lines connecting a male and female represent mating. Vertical lines extending
downward from a couple represent their children. Subsequent generations are
therefore written underneath the parental generations and the oldest individuals are
found at the top of the pedigree.
How a man and a woman can have children with two different blood groups, in
a probable ratio of 3 : 1 .
Answer: When both parents have heterogeneous genotype for the same blood
group, it is possible to have children with two different blood groups in a probable
ratio of 3:1.
i) I A IO X I A IO
ii) I B IO X I B IO
 Parental Father x Mother
Phenotype blood group BB
Genotype I B IO I B IO
Gametes IB IO IB IO

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Fertilization
IB IO
IB IBIB IB IO
IO IB IO IO IO
Offspring Genotype IBIBIBIO IB IOIOIO
Phenotype B BB O
Percentage 75% Group B 25 % Group O
Ratio 3: 1 (3 blood groups B: 1 blood group O)
The inheritance of sex
 Whether a child is born male or female is determined at the moment of
fertilization.
 Of the 23 pairs of chromosomes in a human nucleus, one pair is known as the
sex chromosomes.
 In the female, the sex chromosomes are identical and are called XX
chromosomes.
 In the male, they are nor identical. One of them is an X chromosome, exactly
those in the female, but the other is (shorter) Y chromosome and is called XY
chromosomes.
 The gametes contain 23 single chromosomes.
 In female, all gametes contain an X chromosome.
 In males, 50% of the gametes contain an X chromosome and 50%
contain a Y chromosome.
* fusing an X carrying sperm with ovum to produce a daughter, or
* fusing a Y carrying sperm with ovum to produce a son.

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Parents Father x mother
Sex chromosomes
in body cells X y XX
in gametes X y X X ( only
At fertilization:
Gametes X y
X XX XY
Offspring
Genotype XX XY
Phenotype Female male
Probability 50% 50%
Selective breeding:
Allowing breeding between only those individuals of a species which would produce
offsprings with specific, desirable characteristics.
Natural selection
It is the environment which ‘decides’ which organisms survives.
e.g. 1. Some mosquitoes that are not killed by the insecticide may have
undergone mutation to become resistant to the harmful effects of the insecticide.
The theory of Natural selection was put forward by Charles Darwin.
His observations are:
 There will be a struggle for existence
 Some will be better adapted to their environment
 Those best adapted will survive and reproduce in greater numbers than those
less well adapted.(Survival of fittest)

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Artificial selection
Man deliberately selects and breeds individual plants or animals for his own
preference or profit. e.g. 1 A farmer saves the best seeds from his maize crops to
sow for next year’s crop. e.g. 2 Farmers crossed two breeds of cattle, the Jersey
from Europe and the Sahiwal from Africa to produce highest milk yielding offsprings.
Genetic engineering
Genetic engineering involves artificially inserting genes from one species to another.
Production of hormone insulin by Genetic Engineering
 Identification of the human DNA which codes for hormone insulin from
pancreas.
 The desirable gene is cut from chromosome with specific restriction
endonuclease enzymes.
 Cutting of a bacterial plasmid using restriction endonuclease enzymes.
 Fixing human gene and bacterial plasmid using ligase to join them together.
 Using the plasmid as a vector is now reinserted into the host bacterial cell.
 The bacterium is cloned
 Many identical plasmid, complete with human gene, are produced inside the
bacterium.
 Selected bacteria are cultured in fermenter where they breed and secrete the
hormone.
Important products of genetic engineering
 Insulin ( required for treatment of diabetes)
 Human growth hormone
 Factor VIII (blood clotting factor for haemophilia)
 BST an important animal hormone to speed up the growth of beef cattle

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Steps involved in genetic engineering
Advantages of genetic engineering
 Engineered organism can offer higher yields.
 Genetic engineering gives much more predictable results than selective
breeding.
 Genetic engineered crops can cope with extreme environmental conditions.

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 The product is very pure and chances of body rejection is less
 The product can be made in large quantities, making it less expensive.
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Public concern over genetic engineering or disadvantages
 Engineered bacteria may escape from the laboratory with unpredictable
consequences.
 Plants engineered for pesticide resistance could pollinate with wild relatives,
creating “super weeds”
Other hereditary diseases
Albinism
An albino lacks gene for producing the pigment melanin. As a result skin is easily
damaged by sunlight. The albinism allele is recessive to the pigment producing
allele.
Hemophilia
It is a genetic disease in which blood clots very slowly as lack of a plasma protein
called factor VIII which plays a part in clotting. Quite minor cuts tend to bleed for a
long time and internal bleeding may occur which may be fatal
Huntington’s disease
Huntington’s disease is an inherited disorder that affects the nervous system. It is
caused by a dominant allele. This means it can be passed on by just one parent if
they have the disorder.

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Key terms used in genetics and inheritance
TERMINOLOGY
EXPLANATION
Variation Observable differences (different characteristics) within a
species that causes as a result of sexual reproduction
Continuous variation Both inherited and environmental factors determine the
characteristics of an individual. ( eg: body mass, height)
Discontinuous variation Inheritance of gene alone determines the characteristics
of an individual.
Chromosome Collection of genes that code for proteins necessary to
control all the characteristics of an organism
Gene Gene is a section of chromosome that code to make a
particular protein which controls a specific characteristic of
an organism. It is known as the unit of inheritance.
Gamete Male or female sex cell (sperm or egg)
Alleles A gene controlling character may sometimes have two or
more alternative (different) form. Each form of agene is
called allele.
(alternative form of a gene)
Dominant allele The allele that dominate over a recessive allele. In the
presence of at least a dominant allele always determines
the phenotype of an organism (appearance/
characteristic). Dominant allele is represented by capital
letters (A, B, C etc.)
Recessive allele The allele that cannot be expressed itself in the presence
of a dominant allele unless two recessive alleles are
present. The recessive allele is represented by simple
letters (a, b, c, etc)
Genotype The genetic make up of an individual (TT, Tt, tt)
Homozygous An organism whose genotype for a particular character
contains identical alleles (eg: TT, tt)
Heterozygous An organism whose genotype for a particular character

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contains two different alleles (eg: Tt)
Homozygous dominant An organism whose genotype for a particular character
contains two dominant alleles (eg: TT)
Homozygous recessive An organism whose genotype for a particular character
contains two recessive alleles (eg: tt)
Phenotype The expression or appearance of a character of an
organism
Eg: Tall or Dwarf / white or black
Mutation Change in gene or chromosome through environmental
forces or mutagens (eg: X rays, UV radiation)
Monohybrid inheritance One pair of contrasting character is controlled by only one
pair alleles. eg: coat color in mice.[one pair (two alleles)
Bb]
Complete dominance The presence of a single dominant allele or identical pair
of dominant alleles will have the same effect of the
phenotype of an organism. Eg: in coat colour of mice the
presence of a single dominant allele (Bb) or two dominant
alleles (BB) have the same effect. Both the cases the
organisms are phenotypically brown.
Codominance Both alleles have equal effect on the phenotype of an
offspring or organism. Eg: allele AB are codominant both
can be expressed without masking any one. (AB blood
group)