Inheritance

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.

Dhiffushi School Page 1


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




2.5 (higher

extreme)

1

Figure 1.1



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



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



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



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



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



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



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)



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



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



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.



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


B B b b

Gamete

s

B B

bBb

(brown)

Bb (brown)

bBb

(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

F2 generation

Possible genotypes: all

Bb

Phenotypes:

all brown

Ratio: 3 : 1


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)


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



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.



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



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



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



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



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.

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.



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 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)


Science

Inheritance