CHAPTER 3

Heredity and Variation

Human Cells Chromosomes

DNAGenes Characteristics/ traits

Built up of containing

determiningcontaining

containing

3.1 CELL DIVISION

• Chromosomes, genes and DNA• Life cycle and cell division– Mitosis: growth– Meiosis: sexual reproduction

Chromosomes, genes and DNA Cik Emy ®

NUCLEUS

CHROMOSOMES-threadlike structure

-human beings have 46 (23 pairs-contain protein and DNA

DNA- Double helix structure

- -Made up of series of genes

GENE- Segment of DNA-Genetic codes which

determine an organism characteristics

Mitosis

• Definition: A cell division process in which a parent cell

(original cell) divides into two identical daughter cells (new cells)

• Occurs in:– Human and animal : somatic cells– Plants: tips of roots and shoots

Stages of mitosis

•Chromosomes in the nucleus shorten, thicken and more visible.

• Replication of chromosomes occur.• Each strand of chromosome is called

chromatid.

chromatid

centromere

nucleus membrane

• The nucleus membrane forms again.• Two daughter cells are formed.

• Each daughter cell has the same number of the chromosomes as the parent cell.

• Spindle fibres emerge.•Chromosomes align themselves in the centre

of the cell.• The nucleus membrane disappears.

• Chromatids divide and separate to the opposite ends of the cell.

• At the same time the cell starts to divide.

spindle fibres

The Importance of MitosisProduce new cells for

growth – unicellular zygote to multicellular organism

Asexual reproduction for simple organism like amoeba

Regenerate body parts such as lizard grows a

new tail

Vegetative reproduction such as rhizomes, tubers,

leaves

Produce new cells for replacing dead

cells/repairing tissues

Meiosis • Definition: A process in which a nucleus of a parent cell divides

into four daughter cells through two divisions.

• The daughter cells will have half the number of chromosomes in parent cell.

• Occurs in reproductive organs to produce gametes (ovum and sperms):– Human and animals: testes (male) and ovary (female)– Plants : anthers and ovaries

Stages Of Meiosis : Meiosis 1 And Meiosis 2

• Chromosomes in the nucleus shorten, thicken and more visible.

• Crossing over and replication process occur on chromatids.

Meosis 1

•Chromosomes align themselves in the centre of the cell.

• The nucleus membrane disappears.

•Chromatids separate and move to the opposite ends of the cell.

• The cell begins to divide into two.

• Two daughter cells are produced.• The nucleus membrane reforms.

•The nucleus membrane is formed again.• Four daughter cells are formed.• Each daughter cell has half of the

number of chromosomes in the parent cell.

• Spindle fibres emerge.• Chromosomes align themselves in the

centre of the cell.• The nucleus membrane disappears.

• Chromatids divide and separate to the opposite ends of the cell.• The cell starts to divide.

Meosis 2

The Importance of Meiosis

Crossing over process

Independent assortment of chromosomes

(characteristics are separated during formation of gamete and randomly

combined during fertilization)

1. To produce gametes

2. To allow variation through :

Conclusion Of Mitosis And Meiosis

Meiosis

Fertilisation

Mitosis

Male (46 chromosomes)

Female (46 chromosomes)

Sperm (23 chromosomes)

Ovum (23 chromosomes)

Zygote(46 chromosomes)

Baby(46 chromosomes)

Similarities And Differences Between Mitosis And Meiosis

SIMILARITIES

Replication of DNA

takes place

Involves division of

nucleus and cytoplasm

Cell division occurs

The Difference Between Mitosis And Meiosis

DIFFERENCESMITOSIS MEIOSIS

Replication process

Number of division

Number of daughter cells produced

Number of chromosomes in

daughter cell

Combination of gene

Occur

One only

Four

Same as in parent cell

Same as parent cell

Occur in the first division

Two times

Two

Half of parent cell

Different from parent cell

Crossing over processDoes not occur Occur

3.2 THE PRINCIPLES AND MECHANISM OF INHERITANCE

GeneExist in pair

Dominant gene-A gene which expresses the trait when paired with a dominant or

recessive gene-Denote as capital letter, eg

T (tall)

Dominant traitCharacteristics controlled by

dominant genes

Recessive gene-A gene which expresses the trait

when paired with a recessive gene only (in the absence of a dominant

gene)-Denote as small letter, eg t (short)

Recessive traitCharacteristics controlled by

recessive genes

Chromosomes are in pair. One from the father while the other one from the mother.

Same genes:Dominant gene and recessive gene are placedopposite to each other

Homologous chromosomes

t T

Examples of dominant and recessive traitsTrait Dominant Recessive

Earlobes Free earlobes Attached earlobes

Tounge Ability to roll Inability to roll

Dimples Presence Absence

Hair Curly Straight

Iris colour Brown Blue

Height Tall Dwarf

The Mechanism Of Trait Inheritance

• Gregory Mendel was the first person who discovered the principles of heredity.

Genotype

Phenotype

The genetic information of an organism

The physical characteristics of an organism

Monohybrid Inheritance By Gregory Mendel

TT ttTall (pure breed) Dwarf (pure breed)

T T tt

T t tT

Tall Tall Tall Tall

Tall Tall Tall Dwarf

Phenotype :Genotype :

Phenotype :

Genotype :

Phenotype :Genotype :

F2 Generation

F1 Generation

Gametes

Gametes

Tt Tt Tt Tt

TtTtTT t t

Key: Homozygous: both genes are

same Eg: TT, tt

Heterozygous: genes are different

Eg: Tt

Exercise 1

• In the pea plant, the gene for round seed(R) is dominant to the gene for wrinkled seed(r). a) If a homozygous round-seeded pea plant is

crossed with a homozygous wrinkled-seeded plant, what will be the phenotype of F1 plants?

b) If the F1 plants are self-pollinated, what will be the genotype and phenotype ratios of the F2

plants?

RR rrRound seed Wrinkled seed

R R rr

Round Round Round Round

Parental Phenotype :Parental Genotype :

F1 Generation

Gametes

Rr Rr Rr Rr

Hence all the F1 plants have round seeds.

a) Key: R: round seededr : wrinkled seeded

b)

R r rR

Round Round Round

Wrinkled

F2 Phenotype :F2 Genotype :

F1 Generation

Gametes

Rr Rr

RrRrRR r r

Genotype ratio is 1 RR:2 Rr: 1rr

Phenotype ratio is 3 plants with round seeds : 1 plant with wrinkled seed

Exercise 2

• B represents the dominant gene for black cat, and b represents the recessive gene for a white cat.

What is the phenotype ratio of the number of black cats to the number of white cats if black fur cat (Bb) is crossed with another black fur cat (Bb)?

B b bB

Black Black Black White

F1 Phenotype :F1 Genotype :

Parents

Gametes

Bb Rb

BbBbBB bb

Phenotype ratio is 3 black fur cats: 1 white fur cat

Key: R: black catr: white cat

Answer:

Exersice 3

• In the pea plant, the gene for purple flowers (P) is dominant to the gene for white flowers (p). A cross between a heterozygous pea plant with purple flowers and a homozygous pea plant with white flowers is done. What are the genotypes of the offsprings?

P p pp

Purple White Purple

White

F1 Phenotype :F1 Genotype :

Parents

Gametes

Pp pp

ppPpPp pp

Genotype ratio is 2 Pp : 2 pp

Key: Pp: heterozygous , purple flowerpp: homozygous , white flower

Answer:

3.3 SEX DETERMINATION AND THE OCCURRENCE OF TWINS IN HUMAN BEINGS

• Sex chromosomes: the last pair in human chromosomes

Male: 22 pairs of autosomes and a pair of sex chromosomes XY.

Female: 22 pairs of autosomes and a pair of sex chromosomes XX.

• Sex determination : the sex of baby is determined by the father’s sperm (XY)

44 + XY 44 + XX

Y XGamete cells: orX

Fertilisation

Baby girl 44+XXBaby boy44+ XY

44 + XX 44 + XY

22 + X 22 + X

44 + XX 44 + XY

22 + Y22 + X

44 + XX 44 + XY

Mother Father

Gametes :

Parents :

Offsprings:

Ratio of daughter : son is 1:1

Therefore , the chance to get daughter or son is 50:50.

Daughter Daughter Son Son

The Formation Of Identical And Non-identical Twins

An ovum is fertilised by a sperm to form a zygote.

Zygote undergoes mitosis and separates into two to develop into two embryos.

Genetically identical.

Share the same placenta.

Same sex.

IDENTICAL TWINS

Two different ova fertilised by two different sperms at the same time.

Two different zygotes are formed and develop into two embryos.

Genetically different.

Each has own placenta.

Same sex or different sex.

Known as fraternal twins.

NON-IDENTICAL TWINS

IDENTICAL TWINS NON-IDENTICAL TWINS

Zygote undergoes mitosis

Two embryos

Foetus share the same placenta

Two ova are fertilised

Two different zygotes are formed

Two separate embryos

Each foetus has his own placenta

• Siamese twins– Identical twins that are joined at some parts of the

bodies.– Formed when zygote fails to divide completely in

the course of formation.

Similarities And Differences Between Identical And Non-identical Twins

IDENTICAL TWINS NON-IDENTICAL TWINS

Similarities:The twins are formed in the uterus.

Two babies are formed.

One

One

Same sex

Yes

Same / identical

Yes / sharing placenta

Two

Two

Same or different sex

No

Different

No / separate placenta

Number of sperms

Fertilised ovum splits into two

Genetic make up

Sharing of placenta

Sex

Number of ova

Differences

3.4 MUTATION

• Mutation: the spontaneous change to the structure of genes or chromosomes

Occurs in

Somatic/body cells(can be passed down

through mitosis)

Gametes/reproductive cells

(inherited)

TYPES OF MUTATION

Gene mutation

Change in the structure of DNA

Chromosome mutation

A change in the number of chromosomes or in

the structure

Examples Of Chromosome Mutation

• Down’s syndrome: an extra chromosome in autosome 21 (47 chromosomes)

• Klinefelter’s syndrome: a male with extra X chromosome (XXY)

• Turner’s syndrome: a female with one less X chromosome (XO)

• Polyploidy: having one or more chromosomes in a set of chromosome

Examples Of Gene Mutation

• Colour blindness: a mutated gene in the X chromosome

• Sickle cell anemia: mutation in gene that produces haemoglobin; inefficient in transporting oxygen in the blood

• Albinism: the gene for skin colour is mutated; unable to produce black pigment (melanin)

• Haemophilia: mutation in gene that is important for blood-clotting

Examples Of Chromosome Mutation

Down’s syndromeAdditional chromosome at autosome 21.

•Physical retardation•Thick neck•Slanted eyes•Protuding tounge•Reduced resistance to diseases

Klinefelter’s syndromeA male with extra X chromosomes (having XXY)

•Infertile•Underdeveloped testes•Narrow shoulder •Enlarged breasts

Turner’s syndromeA female having one less X chromosome (XO)

•Infertile•Short•Mentally retarded•Has no ovaries•Small breasts•Folded skin on neck•No menstrual cycle

Polyploidy– The number of chromosomes is increased by one

or more sets.– More common in plants.

Examples Of Gene MutationColour blindness

Mutant gene (recessive gene) on the X chromosome.

•Not able to differentiate colours especially green and red•More common in males because male has only one X chromosome

Albinism

The gene for skin colour is mutated (recessive gene)

•Unable to produce melanin (black pigment)•White hair•Pink eyes and pinkish skin•Skin is sensitive to sunlight

Sickle-cell anemiaMutation in gene that produce haemoglobin

•Sickle-cell shaped of red blood cells•Not efficient in transporting oxygen•Facing painful crises such as chest pain, fatigue, bone pain,•Yellowing of eyes •Inherited from both parents

HaemophiliaMutation in chromosome X (for blood clotting)

• Have problem in blood clotting• May bleed continuously or for a longer period• A woman carrier may inherit the disease to a son

Spontaneous changes during cell division

Exposure to physical mutagens: UV radiation,

X-rays, radioactive radiation

Environmental change: the change of temperature for a long period causes genetic

evolution in organism

Exposure to chemical mutagens: Benzene,

formaldehyde, nicotine, sodium nitrite, pesticides

Causes Of Mutation

Advantages And Disadvantages Of Mutation

Give rise to variation which leads to differences in traits

Better chance of survival when new species are more resistant to diseases

Advantages

Cause physical defects

Causes genetic diseases

Some chromosome mutations and gene mutation are lethal (fatal)

Causes mental defects

Disadvantages

3.5 THE EFFECTS OF GENETIC RESEARCH ON HUMAN LIFE

Gene therapy: to replace damaged/faulty genes during

the foetal stage

Genetic screening: to predict diseases likely to develop

Gene test: to screen unborn children for diseases

Genetic engineering: to transfer DNA from one

organism to anotherHuman genome project: maps all human genes to determine genes which

causes diseases

Field of Medicine

To produce plants which are resistant to diseases and

infections

To produce engineered plants which are able to live in unfavourable conditions

To produce plants which grow and mature fast

To produce higher yields

To produce high quality of crops and livestock

Field of Agriculture

Selective BreedingA process which involves cross

breeding and cloning of plants or animals

Advantages New varieties with

desired characteristics can be produced

To produce new and high quality crops

More resistant to diseases

Disadvantages May produce new

varieties with undesirable

characteristics

Example 1: Cow

is cross bred with

Friesian Cow Sahiwal Cow

Mafriwal Cow-Grow faster -Bigger in size-More milk -High resistant to disease

Example 2: Palm oil

is cross bred with

Dura Pisifera

Tenera-Thick mesocarp and large kennel :much oil-Thin shell: easier to extract

Thinmesocarp

Large kennel

Thickmesocarp

Smallkennel

Tissue CultureA process of cloning a plant by using a plant part or single cell in a test tube

under a controlled condition

AdvantagesTo produce many

copies of the same plants in a short time

To produce plants anytime we want

Shorter maturation time

No variation

Disadvantages

Complete wipe-out of crops if there is spread

of diseases

Steps of Tissue Culturea) Explant:

b) Multiplication:

c) Transplanting:

Cloning The process of producing an organism that is an exact genetic copy of another

Process:

Genetically Modified FoodFoods from plants and animals which

have had their genetic information changed by adding genes from

another organisms

Advantages:

For crops: enhanced taste and quality, increased yields,

matures faster, better resistance to diseases,

remain fresh longer

For livestock: increased resistance

to infections and diseases, higher yield

of milk, meat and eggs

Cause allergies and side effects to human beings

Disadvantages of Genetically Modified Food and Organisms:

Milk of genetically modified animals may be unsafe

Antibiotic-resistant genes in GM organisms may be pass on to bacteria;

difficult to kill them

Genetic research is expensive, poor farmers cannot afford

3.6 VARIATION AMONG LIVING THINGSVariation:

The differences among individuals of the same species

Continuous variation

Discontinuous variation

-Characteristics among individuals which do not show distinct differences

-Quantitative and can be measured

-Determined by genes and can be influenced by environmental factors

-Characteristics among individuals which show distinct differences

-Qualitative and cannot be measured

-Determined by a single gene and is not influenced by environmental factors

Continuous variation

Discontinuous variation

Height

Skin colour

Length of foot

Intelligence

Weight

Earlobes

Iris colourColour blindness

Tongue rolling

Blood group

Factors That Cause Variation

Genetic factors

Crossing over during meiosis: genetic material is exchanged

Random fusion of male and female gametes during fertilisations

Chromosome and gene mutation: creates new phenotypes

Independent assortment of chromosomes: different gametes

Environmental factors

Sunlight exposure: will have darker complexion

Water climate, temperature

pH: the Hydrangea flower is blue in acidic soil and pink in alkaline soil

Diet: a person who eats a lot will be fatter

Importance Of Variation

IMPORTANCE

Ensures the survival of a species under changing

environment

Enable any species to occupy a wider range of habitat

Enable natural selection: species with favourable characteristics will be

selected by nature

Enable us to distinguish

individuals of the same species

CONTINUOUS VARIATION

DISCONTINUOUS VARIATION

Similarity:Show differences in characteristics among

individuals of the same species

Yes

Not distinct

If it is influenced by environmental factors cannot be inherited

Two or more genes for the same character

Quantitative/ can be measured

No

Distinct

Can be inherited

Single gene

Qualitative/ cannot be measured

Influenced by environmental factors?

Inheritance

No of gene

Type of data

Characteristic

Differences

Similarities And Differences Between Continuous And Discontinuous Variation