Chapter 3

HEREDITY AND VARIATION

3.1 Cell divisionAll living organisms grow and reproduce & are

made up of cellsCells reproduce by dividing and passing on their

genes to daughter cellsEach cell has its own nucleus which controls the

cell’s activities through the genetic material, DNA which acts as a set of instruction or code for life

What is genes?Genes are sections of DNA that code for the production

of protein and are arranged along the chromosomesCarry the inherited characteristics of a personGenes occurs in pairs, therefore each inherited

characteristics is carried by two genesOne gene from each pair comes from the father and

the motherExample of inherited characteristics carried by genes

are:- Colour of the eyes- Pattern of finger prints- Type of hair- Blood group- Intelligence

Deoxyribonucleic acids (DNA)Is a double helix, made up of a series of genes to

encode genetic informationThis determine the individual characteristics of an

organismAll the information in DNA is transmitted in the

form of a genetic codeShould any information not be followed, the baby

will be born with physical or mental defectsDNA molecule is very complicated and resembles

a twisted ladder or staircase

Chromosomes Is a thread-like structure found in the nucleus of a

cellChromosome and the genes on them are

composed of DNAIt exist in pair and are identicalhomologousA pair of chromosomes has genes arranged in the

same way. As a result the genes also exist in pairsHuman has two sets of chromosomes in the

body(one set from the father and another set from the mother)

MitosisOccur in somatic cells all cells body except

reproductive cellsIs the type of cell division that produces genetically

identical cellsDuring mitosis, DNA replicates in the parent cell, which

divides to produce two new cells, each containing an exact copy of the DNA as in the parent cell

This allows growth of multicellular organism from unicellular zygotes

Replacements of cells and repair of tissues is possible through mitosis followed by cell division

Cells are constantly dying and being replaced by identical cells

Is the basis of asexual reproductionthe production of new individuals of a species by one parent organism

MitosisMitosis is the type of cell division that

produces genetically identical cellsDuring mitosis, DNA replicates in the parent

cell, which divides to produce two new cells, each containing an exact copy of the DNA as in the parent cell

Cells produced are diploid (2n)Replacement of cells and repair of tissues is

possible through mitosis (Ex: cell replacement is rapid in the skin & in the lining of the gut

Is actively carried out at the ends of plant shoots and roots

Chromosomes thicken and shorten, and become clearly visible

Each chromosome duplicates itself and forms two chromatids attached at the centromere

The nuclear membrane dissapears

(a)

A fibrous spindle is formed between the chromosomesEach chromosome is attached to a spindle by its

centromereThe chromosome arrange themselves at the equator

of the spindle

(b)

The centromere of each chromosome splits into twoThe chromatids separate and move to opposite

poles.

(c)

A nuclear membrane is formed around the chromosomes at each pole

The cell begins to divide and form two daughter cellsEach daughter cell has the same number of

chromosomes as the parent cellIn a plant cell, the cytoplasm does not constrict.

Instead a new cell wall is developed across the cytoplasm

(d)

Importance of mitosisImportant for the following reasons:(a)Growth(b)Replacing old or damaged cellsEnables organisms to reproduce

themselves (Ex: by asexual reproduction; binary fission in amoeba)

Ensures that the new cells produced have the same number of chromosomes as the parent cells

MeiosisMeiosis is the process by which a nucleus divides

by two divisions into four nuclei, each containing half the number of chromosome of the mother cell

Resulting nuclei are haploid (n)The cells produced are genetically different to the

parent cell and to each otherOccurs in 2 stages that is meiosis 1 and meiosis 2Produces genetic variation (caused by crossing

over between homologous chromosome)In animals, meiosis occurs in testes and ovaries,

whereas in plants, it occurs in the anthers and ovaries

Importance of meiosis1. Production of gamete – the only process that

produces gametes2. Reduction in chromosome number – ensures that

each gamete has only half the number of chromosomes of the parent cell

3. Maintenance of chromosome number in somatic cells – the somatic cells have their number of chromosomes maintained

4. Production of variation – causes genetic variation in the descendants(can produce individuals who do not resemble their parents closely

* Variation occurs in TWO ways:1. Different combination of chromosomes2. Crossing over

Comparing and contrasting mitosis with meiosisSIMILARITIES of MITOSIS and MEIOSIS:- Both are processes of cell division- In each process, duplication of chromosomes

occurs once

MITOSIS DIFFERENCES

MEIOSIS

To produce new cells for growth and repair of damaged tissues

Aim of process

To produce gametes (reproductive cells)

Somatic cells (non reproductive cells)

Type of cell involved

Reproductive cells which produce gametes

Two daughter cells Product Four gametes

Nucleus of parent cells divides once

Division of nucleus

Nucleus of parent cell divides twice

Duplication of chromosomes takes place

Duplication of

chromosomes

Duplication of chromosomes takes place in the first cell division

Same as in parent cell,46 Number of chromosome

s in daughter

cells

Half the number of those in the parent cell,23

Does not occur Crossing over

Occurs when the chromosomes are at the equatorial plane

Same as in parent cell Combination of

chromosomes

Different from that of the parent cell

Does not cause genetic variation

Genetic variation

Causes genetic variation

DIFFERENCES OF MITOSIS AND MEIOSIS

Exercises : CELL DIVISION1. Which of the following is the simplest unit of

inheritance?A. NucleusB. ChromatidC. ChromosomeD. Gene

2. How many chromosomes are present in the nucleus of a human cheek cell?

A. 23B. 24C. 46D. 48

3. How many chromosomes can be found in the nucleus of a human ovum?

A.23B.24C.46D.48

4. What is the importance of mitosis in somatic cells?A.It maintains the number of chromosomes in all new

cells which are producedB.It occurs in all organs of the body so long as the

cells are actively growingC.It prevents the body from being reduced in size due

to the death of old cellsD.It produces new cells which will support the cellular

activities of the old cells.

5. What is the importance of meiosis in living organisms?

A.It maintains the number of chromosomes of zygotes produced from fusion of sex cells

B.It occurs only in parts of the living organism which are actively growing

C.It replaces old cells which are no longer able to carry out their cellular functions

D.It repairs damaged tissue in the body due to injury or disease

6. Where is DNA found in a living organism?A.Cell membraneB.CytoplasmC.NucleusD.Cell wall

7. What is mitosis and meiosis?

8. Describe the process of mitosis.

9. Describe the process of meiosis

10. What is the importance of mitosis and meiosis?

3.2 The principles and mechanism of inheritanceThe study of inherited characteristic is called

heredityGenes carry the inherited characteristics of a

personEach characteristic of a person is controlled by a

pair of genesA gene for a certain characteristic may be a

dominant gene or a recessive gene

Dominant genes and recessive genesA dominant gene is powerful and can mask the

effect of a less powerful or recessive gene in a pair

A recessive gene is weak and cannot show its effect if it is paired with a dominant gene

Each pair of genes is represented by two letters, a capital letter used for a dominant gene and a small letter for a recessive gene

Monohybrid inheritance Mendel chose true-breeding varieties of the

garden pea as the parentsMendel crossed a tall plant with a dwarf plant.Progeny from this cross, called the F1

generation (first filial generation) were all tallAll the F1 plants were then self-pollinated to

obtain the F2 generation (second filial generation)

The F2 generation included both tall and dwarf plants in the ratio of 3 tall to 1 dwarf(phenotype ratio)

It is shown that only the dominant(strong) trait appeared in the F1 generation.

The recessive (weak) trait is blocked by the strong trait from expressing itself and is therefore hidden in the F1 generation

The mechanism of trait inheritanceThe character of height is determined by two

contrasting traits, tall and dwarf.The gene for tall is dominant while the gene for

dwarf is recessiveIf the gene for tall is represented by “T” and the gene

for dwarf is represented by “t”, there will be three genetic combinations(genotype) for plant height

Genotype is the term used to describe the genetic make-up or constitution of a person. It includes the genes inherited from both parents,whether they are dominant or recessive

Phenotype is the physical traits such as colour of the eyes, height and ect..

3 possible combination are:

Phenotype Genotype

Tall (pure strain) TT (homozygous dominant)

Tall (monohybrid) Tt (heterozygous)

Dwarf (pure strain) tt (homozygous recessive)

Pure bred crossed with pure bred

Dominant traits and recessive traits in human

Dominant traits Recessive traits

Right handed Left handed

Tall Short

Black/Curly hair Brown/Straight hair

Black/Brown eyes Blue eyes

With ear lobe Without ear lobe

Can roll the tongue Cannot roll the tongue

Has normal vision Colour blind

Has dimples Has no dimple

Skin is pigmented Skin has no pigments(albino)

Long eye lashes Short eye lashes

The principles and mechanism of inheritanceFor questions 1 to 4 , use the information given below:

1. Which of the following crosses is likely to produce the ratio 3 brown eyed: 1 blue eyed?A. BB x bbB. Bb x bbC. BB x BbD.Bb x Bb

2. Which of the following crosses is likely to produce the ratio 1 brown eyed: 1 blue eyed?A. BB x bbB. Bb x bbC. BB x BbD. Bb x Bb

The brown eyed trait, B, is dominant over the blue eyed trait, b

3. Which of the following crosses is likely to produce ALL brown eyed individuals?

A. BB x bbB. Bb x bbC. bb x bbD Bb x Bb

4. Which of the following crosses is likely to produce all blue eyed individuals?

A. BB x bbB. Bb x bbC. bb x bbD. Bb x Bb

For questions no 5 to 8, refer to the information below:

5. Which of the traits are recessive?A.Straight hair, no dimplesB.Straight hair, dimplesC.Curly hair, dimplesD.Curly hair, no dimples

A girl has curly hair and dimples on her face. Her mother has straight hair and no dimples on her face. Her father has curly hair and has dimples on his face. Her mother is homozygous for both traits. Assume that the two traits are inherited independently of each other

6. If the curly hair traits is represented by the letter C and dimples by the letter D, what is the mother’s genotype for the two traits?

A.CC and DDB.cc and DdC.cc and ddD.Cc and Dd

7. If the curly hair is represented by the letter C and dimples by the letter D, which of the following is the possible genotype of the girls’s father?

A.cc and ddB.CC and DdC.cc and DDD.Cc and dd

8. What is the genotype of the girl?A.cc and ddB.CC and DdC.Cc and DDD.Cc and Dd

3.3 Sex Determination and the occurrence of twins in human beings

Sex Chromosome• Autosomes are chromosomes which are not sex chromosomes•The characteristic features of a person are determined by the autosomes•The sex of a person is determined by the sex chromosome•In a woman, the sex chromosomes are a pair of identical chromosomes called X chromosomes•In a man, the sex chromosomes consists of one X chromosome and a smaller Y chromosome

Therefore, chromosomes in female can be written as 44+XX while male as 44+XY

Mechanism of Sex DeterminationFather’s sperm determine the sex of the babyIf the baby inherits Y chromosome from its father

boyIf the baby inherits X chromosome from its

fathergirlTHUS, the sex of the baby depends on which

sperm fertilizes the egg cell(ovum).

Parent Phenotype: Male Female

Parent Genotype: 44+XY 44+XX

Parent’s gametes:

Offspring genotype : 44+XX 44+XX 44+XY 44+XY

Offspring phenotype: Female Female Male Male

Ratio: (1 girl : 1 boy)

22+X

22+Y 22+X 22+X

Identical and non-identical twins(1) Identical twins When an ovum is fertilized by a sperm, a

zygote is formed The zygote then divides into 2 separate cells in

the womb producing 2 embryos identical twins

Identical twins are two normal, genetically identical individuals of the same sex and share a common placenta during developmental stage

(2) Non-Identical twinsSome women may produce two ova at the same

time.When the two different ova are fertilized at the

same time by two different sperms, two zygotes are formed non-identical twins(fraternal twins)

Non-identical twins are genetically different individuals, can may be of the same sex or different sex and each have their own placenta during developmental stage

Comparing and contrasting identical twins with non-identical twins1. SIMILARITIES: Two foetuses develop in the uterus at the same

time Two babies are born at the same time

2. Differences

Identical twins Differences Non-Identical twins

One ovum and one sperm No of gametes involved

Two ova and two sperms

One ovum is fertilized and one zygote is formed

Fertilization Two ova are fertilized and two zygotes are formed

The single zygote divides into two zygotes

Division of zygote formed

The two zygotes formed do not divide

The two foetuses share one placenta

Number of placentas

The two foetuses have two separate placentas

Have identical chromosomes and genes

Chromosomes and genes

Have different chromosomes and genes

Have the same sex-both boys or both girls

Sex of twins The twins may have the same sex or opposite sexes

The twins have similar characteristic feature

Characteristic features

The twins have different characteristic features

Siamese twinsSometimes a zygote can not divides into two

completely, so the two zygotes are still linked to each other

The linked zygote develop into foetuses and are born as twins joined together in some parts.

Siamese twins are identical twins that are formed when the two embryos do not separate completely during the developmental stage and are attached to one another

Exercises :Sex determination and the occurrence of twins in human beings1. What are autosomes?2. Name the chromosome which determines

that zygote will develop into a male3. Why is the Y-chromosome smaller than the X-

chromosome?4. Why do identical twins have identical

features?5. State the possible sex of non-identical twins.

3.4 MutationIs a change in the amount or the chemical

structure of DNACan occur spontaneously or induced by

substances called mutagensThe new genetic material produced are called

mutantsMutation occur in gamete cell can be inheritedMutation occur in somatic cell can only be

inherited by daughter cells produced by mitosis

Types of mutation

1.Chromosomal mutation2.Gene mutation

Chromosomal mutationIs the result of changes in the number or

structure of chromosomesCan cause changes in the characteristics of the

organismChanges in the no of chromosomes are usually

the result of errors occurring during meiosis or mitosis

These changes may involve the loss of a pair or a single chromosome, or the increase in an entire haploid set of chromosomespolyploidy

Consequences of chromosomal mutation1. Down’s syndrome Individuals suffering from Down’s syndrome

have 47 chromosomes Additional chromosome is autosome no 21 Symptoms: mental retardation, reduced

resistance to disease, congenital heart abnormalities, a short stocky body, a thick neck and the characteristics folds of skin over the inner corner of the eye

Termed as mongolism

2. Klinefelter’s syndromeHave an extra X chromosome, so that the usual

karyotype of XY is replaced by one of XXYMale with underdeveloped testis, infertility and

some female characteristic such as breast enlargement

3. Turner’s syndromeLack one X chromosomeSuch women are XO, rather than the normal XX. Have ovaries and breast that do not develop, do

not experience the menstrual cycle and do not ovulate

** Down’s syndrome and other related chromosomal abnormalities occur more frequently in children born to older women related to the age of the mother’s egg cells

2. Gene Mutation/point mutationIs a change in the structure of the DNA at a

single locusDifficult to detect because physical changes on

specific genes are not visible and cannot be observed under a microscope

Often have undesirable effects but are sometimes beneficial to the mutant organism

If gene mutation produces a beneficial phenotype, the frequency of the mutation in the population will increase from generation to generation as a result of natural selection

Consequences of gene mutation

1. Colour Blindness Is a sex-linked recessive trait Recessive gene for colour blindness is found on

the human X chromosome Most common form is red-green colour

blindness(cannot distinguishes red & green) Females have pair of genes that control color

blindness but male have only one If a male inherits one gene for CB,he has this

disorder Female can only has this disorder if she inherits

two recessive genes,one from each parent

2. AlbinismOccurs when the gene which is responsible for

normal pigmentation undergoes mutationThe gene responsible for albinism is recessiveAlbino human beings are unable to produce

the black pigment (melanin)So, they have no distinct colour in their skin,

hair or eyes

Causes of mutation

1. Radioactive substances- Radioactive radiation such as alpha, beta

and gamma radiation- X-ray also can cause mutation- Can leads to cancer- Can change the structure of functions of

gene and chromosome

2. High Intensity Ultraviolet Light- Ultraviolet from the sun can destroy the cells in the

skin and cause mutation- Can leads to skin cancer3. Chemical substances- Exposure of certain chemical substances like

formaldehyde, pesticides, benzene,nicotine- Can cause cancer and foetal deformity

4. Changes in environment- After a long period of time, changes in the

environment can cause organisms to adapt themselves and mutate into new species

- These new species with their adapted or mutated features are most suitable for living in the changing environment

Advantages and disadvantages of mutationAdvantages Disadvantages

1. Mutation produces genetic variation. This can give rise to new species.

1. Can cause genetic disorder such as albinism and haemophilia which can be passed on future generations

2. Plants and animals mutate in response to adverse changes in the environment. Mutation gives them a better chance to survive

2. Can cause abnormalities in children

Exercise: Mutation1. What is mutation2. What pigment gives our skin colour?3. Why is haemophilia known as a sex-linked

disease?4. State one disorder in a woman with

Turner’s syndrome.5. Illustrate what will happen if a normal

female (carrier) haemophilia married to a normal male.

VariationEach person has unique features which

enable you to differentiate him or her from other friends

No two people are exactly the sameThese differences among individuals in a

population or a species are called variation

The differences may be the result of genetic differences, the influences of the environment, or a combination of genetic and environmental influences.

In human usually the character referred to are the physical characters such as height, weight, skin colour, etc..

The differences in physiological processes such as our blood glucose level and immunity level are also considered

There are two types of variation:(1)Continuous variation(2)Discontinuous variation

Continuous VariationThe characteristics can be measured or graded on a

scale such as human heights, weight,skin colour, etc..Cannot be grouped into distinct and discrete phenotypesUsually quantitative (they can be measured) and are

determined by genes and influenced by environmental factors.

In *CV, a normal distribution curve is obtainedFrom the distribution curve, it can be seen that

individuals show gradual difference from one extreme to another extreme

There are very few individuals who are very short or very tall

Most of them are of average height

Discontinuous variationThe characters are discrete and cannot be

measured or graded on a scale such as ABO blood group, color blindness, presence or absence of earlobe,etc..

Are qualitative(they cannot be measured or graded)

usually determined by a single gene with distinct effect.

Discrete distribution is obtained

The differences of continuous variation and discontinuous variation

Factors that cause variation

Importance of variationCauses natural differences in appearance

among individuals of the same species enables us to identify different individuals

Improves the ability to survive in a changing environment

Gives rise to a large variety of plants and animals.

Exercises: Variation1. What is variation2. What is continuous variation. Give an

example.3. State two important factors which cause

variation4. Why is the ability to roll one’s tongue

considered as a discontinuous variation?5. State one important use of variation in

daily life.