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Grade 10 / Biology notes Inheritance
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.
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In continuous variation, individual show a range between the two
extremes. Every possible form (intermediates) between the two extremes
will exist.
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
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1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.50
2
4
6
8
10
12
14
1
6
10
12
6
1
Continuous variation
height in metres
% o
f people
in p
opula
tion a
t each
heig
ht
Figure 2.2
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
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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.
A AB B O05
101520253035404550 46
93
42
Discontinuous variation
Blood group
No:
of
peop
le in
a p
op
ula
tion
w
ith
each
blo
od
gro
up
(p
er-
cen
tag
e)
Difference between continuous and discontinuous variation
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Continuous variation Discontinuous variation Continuous Variation is the
result of the interaction of two factors
1. genes (inheritance) 2. Environment
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.
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
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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 opposite thymine (T) on the other strand. Cytosine (C) is
always placed opposite guanine (G).
This is called the base pairing rule.
The unit of inheritance
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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.
Genetic Inheritance
Chromosomes exist in matching pairs. For example, human beings have
23 matching or homologous pairs of chromosomes, a total number
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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 the DNA on a chromosome is
changed and results to produce defective protein (imperfect protein) or no
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protein at all. This can lead to a considerable change in a characteristic.
For example sickle cell anaemia.
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 diagramsGenetic 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
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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.
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
Phenotyp
e
Genotyp
e
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
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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%
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.
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In a test cross, the individual is mated with a homozygous recessive (t t)
partner
If the unknown tall plant was
homozygous
If the unknown tall 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
Cross between homozygous brown - coated mouse and grey-
coated mouse
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
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B B b b
Gamete
s
B B
bBb
(brown)
Bb (brown)
bBb
(brown)
Bb (brown)
Grade 10 / Biology notes Inheritance
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
F2 generation
Possible genotypes: all
Bb
Phenotypes:
all brown
Ratio: 3 : 1
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B b B b
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
Gamete
s
B b
BBb
(brown)
Bb (brown)
bBb
(brown)
bb (grey)
Grade 10 / Biology notes Inheritance
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.
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
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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
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.
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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
Fertilization
IB IO
IB IBIB IB IO
IO IB IO IO IO
Offspring Genotype IBIBIBIO IB IOIOIO
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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.
Parents Father x mother
Sex chromosomes
in body cells X y XX
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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)
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
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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.
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Genetic engineering gives much more predictable results than
selective breeding.
Genetic engineered crops can cope with extreme environmental
conditions.
The product is very pure and chances of body rejection is less
The product can be made in large quantities, making it less
expensive.
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
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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 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
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masking any one. (AB blood group)
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