3.0 Heredity and Variation

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3.0 HEREDITY AND VARIATION

3.1 CELL DIVISION

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3.1.1 Genes, Deoxyribonucleic acid 3.1.1 Genes, Deoxyribonucleic acid

(DNA) and Chromosomes(DNA) and Chromosomes

• Chromosomes are a long, fine, thread-like structures found in the nucleus

• It builds from molecules called deoxyribonucleic acid (DNA)

• Every species has its own number of chromosomes per cell to store their heredity information

• Example humans possess 46 chromosomes

• 2 types of chromosomes:

1. Autosomes – controls all somatic

traits (blood group, body weight &

skin colour)

2. Sex chromosomes – determine

gender

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• In human:

44 autosomes + 2 sex chromosomes = 46

chromosomes

• 2 types of sex chromosomes:

XY – chromosomes in males

XX – chromosomes in females

Male Chromosomes

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• Genes are basic heredity unit

• Genes control the development of traits (e.g. genes that control body height and skin texture)

• Each gene controls one trait that are inherit from parents

• Inherited characteristics are passed from parents to offspring through the genes in sperms and ova

• Different individual possesses different number and types of genes

Relationship between gene, chromosomes Relationship between gene, chromosomes

& DNA& DNA

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Genes in a

chromosomeMale Chromosomes

Exercise

1. What are chromosomes?

2. What are genes?

3. What is DNA stand for?

4. Name the two types of chromosomes.

5. Give the two types of sex chromosomes.

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Answers1. Chromosomes are a long, thread-like structure found in

the nucleus that store heredity information.

2. Genes are section of DNA that code for the production of protein and are a basic heredity unit.

3. DNA stand for deoxyribonucleic acid.

4. The two types of chromosomes are autosomes and sex chromosomes

5. The two types of sex chromosomes are XY for male chromosomes and XX for female chromosomes.

3.1.2 LIFE CYCLE AND CELL

DIVISION

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Life Cycle

• Life cycle started from a cell (a sperm from the

male & an egg from the female)

• Fusion of an egg and a sperm produce fertilize

egg.

• Fertilize egg undergoes cell division hundreds

of time to form embryo and undergoes cell

division hundreds of time again to form baby

• Baby grows by cell division into an adult

• Cell division continue to maintain the body

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Cell Division

• Has important roles in growth, repair &

reproduction in an organism

• Two types:

1. Mitosis – cells in different parts of human body,

root tips and shoot tips in plants (somatic cells)

2. Meiosis – cells in the sex organ to produce sex

cells/gametes

- in human & animals: ovary – female,

testes in male

- in plants: ovary – female, anther - male

1. Mitosis

• Cell division that produces genetically identical cells

• Produce 2 new cell, each containing an exact copy of

the DNA as in the parent cell

• The division continued, producing trillions of

genetically identical cells that make up your body

• The basis of asexual reproduction

• Replacement of cells and repairs of tissues

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Stages in Mitosis

• Stage 1

- Chromosomes

duplicated but remain

elongated

- Centrioles also

duplicate

• Stage 2

- Chromosomes begin

to condense and

become shorter,

thicken and visible

- Nuclear membrane

disappear

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• Stage 3

- Chromosomes arrange

themselves at the equatorial

plate

- Each chromosomes is

attached to a spindle fibre

through the centromere

• Stage 4

- Chromatids of each

chromosomes separates

become individual

chromosomes which

moves towards opposite

poles of the cell due to

contraction of the

spindle fibres

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• Stage 5

- Nuclear membrane reform

- Cytoplasm divided along

the equator

- Formation of 2 daughter

cells

Division of cytoplasm after completion of mitosis

in plant cell and animal cell

Plant cell Animal cell

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Division of cytoplasm after completion of

mitosis

in plant cell and animal cell

• Plant cell – cell wall will form between two

daughter nuclei to form two daughter cells

• Animal cell – cell membrane constricting

inward to form two daughter cells

Exercise

• Do activity 3.3 on page 29 in your practical

book

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Exercise

• Give a brief summary of life cycle.

• What are the importance of cell division?

• What are the types of cell division?

• What is mitosis?

• In which cell does the mitosis occur in plants?

Answers

• Life cycle started from a cell, the fertilize egg that

produce from the fusion of sperm and egg. This fertilize

egg will undergoes cell division to form embryo. The

embryo undergoes cell division again to form baby and

the baby will grow by cell division to become an adult.

• Have roles in growth, repair and reproduction process.

• Mitosis and meiosis.

• Mitosis is cell division that produce two genetically

identical cells.

• At the root tips and shoot tips of the plant.

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Things we learn today

• About life cycle

• Importance of cell division

• Types of cell division

• The meaning mitosis

• The stages in mitosis

2. Meiosis

• Cell division that produce 4 cells, each containing half

the number of chromosomes of the mother cell

• The cell produced are genetically different to the parent

cell and to each other

• Nuclei are haploid (n)

• Take place in sex organ to produce sex cell/gametes

• Can be divided into 2 main stages:

- First stage: the separation of homologous

chromosomes

- Second stage: separation of chromatids

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First Meiotic Division

• Stage 1

- Chromosomes replicated,

visible as long and coiled

threads

- Gradually become shorter

and thicker

- Centriole also duplicate

• Stage 2

- Pair of homologous

chromosomes lie side by

side

- Nuclear membrane

disappears

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• Stage 3

- The paired of homologous

chromosomes line up the

middle of the cell

- The homologous

chromosome attached to

spindle fibre

• Stage 4

- Two members of each pair of

homologous chromosomes

separate

- Moves towards opposite ends

of cell due to contraction of

spindle fibre

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• Stage 5

- Nuclear membrane

reform

- Cell membrane

constrict inward

- Formation of two

daughter cells

Second meiotic division

• Stage 6

- Each chromosomes appears as a pair of chromatids

- Nuclear membrane disappear

- Chromosomes line up at the middle of the cell

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• Stage 7

- Each pair of chromatids

separate from each other

- Moves towards the

opposite end of the cell

due to contraction of

spindle fibre

• Stage 8

- Nuclear membrane reform

- Cell membrane constrict inward

- Each chromatid become a

daughter chromosomes

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• Stage 9

- Formation of four daughter

cell each containing the

haploid (n) number of

chromosomes

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Importance of Meiosis

• Produces haploid cells that contain half number of chromosomes of body cell to maintain a constant number of chromosomes in offspring produced by sexual reproduction

• Produce genetic variation due to crossing overbetween homologous chromosomes, independent assortment and random fusion of two gametes during fertilisation.

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Exercise

• Do the Activity 3.3 on page 34 in the Science

Process Skills

Exercise

• What is meiosis?

• Where does meiosis occur in

a) plant?

b) animal?

• State the importance of meiosis.

• What is the importance of crossing-over

during meiosis?

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Comparison Between Mitosis

and Meiosis

Mitosis Meiosis•Maintain the chromosome number

•Halves the chromosome number

•Has one division cycle •Has two division cycle

•Produce two daughter nuclei •Produce four daughter nuclei

•Does not involve crossing over and independent assortment

•Involve crossing over and independent assortment

•Produce two daughter nucleithat are genetically identicalto one another and to the parent nucleus

•Produces four daughter nuclei that are genetically different from one another and from the parent nucleus

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The Similarities and

Differences Between

Mitosis and Meiosis

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Similarities

• Both are a type of cell division

DifferencesMitosis Meiosis

To produce new cells for growth and to replace damaged cells

Function To form gametes

In somatic cells Place where it occurs

In sex cells

Occurs Duplication process

Duplicate in Meiosis 1 but not in Meiosis 2

One Number of cell division

Two

Two Number of daughter cells

produced

Four

Identical to one another and to the parent nucleus

Genetic content

Different from one another and from the parent nucleus

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3.2 The Principal and Mechanism

of Inheritance

• Inherited characteristics - characteristics

that are passed from parents to offspring

through the genes in sperms and ova

• During fertilisation of an egg, 23

chromosomes comes from the father in

the sperm and 23 chromosomes come

from the mother in the ova

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3.2.1 Dominant Genes and Recessive Genes

• Dominant gene – gene that can show its trait

or characteristic controlled by it although it

exist with a recessive gene (strong gene)

• Recessive gene – gene that only shows its trait

or characteristic when a dominant gene is

absent (weak gene)

• Dominant gene control dominant

characteristic/trait whereas recessive gene

control recessive characteristic/trait

What are the types of

characteristics that we can

inherited from our parents?

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Characteristics Dominant

characteristics

Recessive

characteristic

Tongue rolling Ability to roll the

tongue

Inability to roll the

tongue

Ear-lobe Free earlobe Attached earlobe

Eye colour Brown eye Blue eye

Skin pigment Normal skin colour Absence of skin

pigment (albino)

Hair colour Black hair Brown hair

Hair type Curly hair Straight hair

Chin Projecting chin Receding chin

Nose Straight nose Upturned nose

Activity

• In pairs, list down your friends

characteristics.

• Determine whether your friend have

dominant characteristics or recessive

characteristics.

• Present your findings to the class.

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Conclusion

• More students have the dominant

characteristics than the students with

recessive characteristics.

Exercise

1. What is the meaning of inherited

characteristics?

2. What is dominant gene?

3. What is recessive gene?

4. Give two types of dominant characteristics.

5. Give two types of recessive characteristics.

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Answers1. Inherited characteristics are characteristics that are

passed from parents to offspring through the genes in sperms and ova

2. Dominant gene is gene that can show its trait or characteristic controlled by it although it exist with a recessive gene.

3. Recessive gene is gene that only shows its trait or characteristic when a dominant gene is absent.

4. Ability to roll the tongue, free ear-lobe, brown eyes, normal skin pigment etc.

5. Inability to roll the tongue, attached ear-lobe, blue eyes, absence of skin pigment (albino) etc

3.2.2 The Mechanism of Trait

Inheritance

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• Heredity - The study of inherited

characteristics

• Gregor Mendel – the first person who studied

heredity in a scientific way

• The body characteristics of organism are

inherited in different ways

• Monohybrid inheritance (simplest pattern of

inheritance) – inheritance controlled by a

single pair of genes.

• Character is determine by two contrasting traits (dominant and recessive)

• Example: Height – tall (dominant) and dwarf (recessive)

• Dominant trait represent by capital letter (T –tall) and recessive trait represent by small letter (t – dwarf)

• Gene is made of allele

• Homozygous – same allele (TT and tt)

• Heterozygous – different allele (Tt)

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• Genotype – genetic composition of particular

characteristic

• Phenotype – actual appearance of a

particular characteristic

Phenotype Genotype

Tall (pure strain) TT (homozygous dominant)

Tall (monohybrid)

Tt (heterozygous)

Dwarf (pure strain)

tt (homozygous)

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Activity

• Discuss Activity 3.3 on page 62 and 63 in your

text book.

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3.3. Sex Determination and Occurrence

of Twins in Human Beings

Sex Determination in Human

Being

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3.3.1 Sex Chromosomes

• Determine the sex of an individual

• Two types:

1. XX – in female

2. XY - male

• In female an ovum contain 22 autosomes and an X chromosome

• In male there are two types of sperm produced, each contain either 22 autosomes and an X chromosome or 22 autosomes and a Y chromosome

Chromosomes In Human

Male Female

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• The sex of a baby is determine by the type of

sperm that fuses with the ovum during

fertilisation:

a) When an ovum is fertilised by a sperm

containing an x chromosomes, the resulting

zygote has a genotype of XX and will develop

into a girl

b) When an ovum is fertilize by a sperm with a Y

chromosomes, the zygote has a genotype of

XY and will develop into a boy

Parents: Male 44+XY Female 44+XX

Gametes: 22+X 22+Y 22+X 22+X

Offspring

genotype: 44+XX 44+XY 44+XY44+XX

Offspring

phenotype: Girl BoyGirl Boy

Genotype ratio – 2XX : 2XY

Phenotype ratio – 2 Girl : 2 Boy

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Occurrence of Twins in Human

What are twins?

• A type of multiple birth.

• Two foetus develop in the womb at the

same time

• Two types of twins:

1. Identical twins

2. Non-identical twins

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1. Identical Twins

• Formed when one egg is fertilised by one sperm

• After fertilisation, the zygote splits into two

• Two embryos are formed from one zygote, and each develops as a separate baby

• Both children inherit the same genes

• Alike in appearance

• Same sex

• Share a common placenta

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2. Non-identical Twins

• Also known as fraternal twins

• Form when two separate eggs are fertilised by two sperms

• Two embryos are formed from two separate zygote, and each develop as a separate baby

• Both children inherit different genes

• Not alike

• Sex maybe the same or may be different

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Siamese Twins

• A type of identical twins

• Form when two embryos do not separate

completely during the development stage

• Attached to each other at the stomach, head,

or even share the same internal organs (liver,

heart or brain)

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Which picture show the identical twin and

non-identical twins?

A B

Identical twins Non-identical twins

Activity

• In your group, discuss to complete the graphic

organizer shown on page 66 in your text book.

• Copy the complete graphic organizer into your

note book.

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Answers

Similarities

A type of multiple birth

Two fetus develop in the womb at the

same time

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Differences

Identical Twins Non-identical

Twins

One Number of ova involved Two

One Number of sperms

involved

Two

One Number of zygotes

produced

Two

One Number of placenta

formed

Two

Same Sex of twins Same or

different

Same Genotype of twins Different

Exercise

1. What are twins?

2. Explain in brief how identical twins occur in

human.

3. Give a similarity between identical twins and

non-identical twins.

4. Give two differences between identical twins

and non-identical twins.

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3.4 MUTATION

What is mutation?

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• A sudden change of chromosomes or genes that can change human traits

• Change in the amount or the chemical structure of DNA

• Occur in somatic or gamete cells

• Mutation in somatic cells cannot be passed but mutation in gamete cells are inheritable

• A permanent process

• Occur spontaneously under natural condition or induced by substances called mutagens

• Organism that have undergone mutation are called mutants.

Types of Mutation

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• Two types:

1. Chromosome mutation

� occurs when changes take place to the

structure of the chromosomes or to the

number of chromosomes

� changes in the structure may caused by

deletion (lost of genes), inversion,

translocation and duplication

� changes in the number of chromosome may

caused by additional or deletion of one or more

extra sets of chromosomes

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2. Gene mutation

� change to the chemical structure of a gene

� causes some inherited diseases such as

albinism, sickle-cell anaemia, haemophilia

and colour blindness

Examples of Mutation

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1. Male with genotype 44+XXY (Klinefelter’ssyndrome) is sterile, mentally handicapped and has female characteristics due to the extra X chromosome

2. Male with genotype 44+XYY usually has high level of testosterone, have severe acne and tall in height due to the extra Y chromosomes

3. Female with genotype 44+XO (Turner’s syndrome) is sterile because her reproductive organs are not functional due to the fact that she has only one X chromosome

4. Female with 44+XXX tendency to be tall and a higher incidence of below- normal intelligence due to the extra X chromosome

5. Down’s Syndrome (Mongolism)

• extra chromosomes to the 21st

• have a short and stocky body, slanting eye,

broad face and stubby nose

• mentally retarded and have heart defects

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6. Haemophilia – the blood fails to clot therefore

suffers prolonged bleeding from cuts caused by

gene mutation at the X chromosome

7. Colour blindness – cannot identify colours due

to the mutation at the X chromosome which

cause defects in one or more of the three types

of cones in the retina

8. Sickle-cell anaemia – mutation that changes

the shape of the red blood cell to become

sickle-shaped which have low efficiency in

transporting oxygen and leading to anaemia

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Albinism in human Albinism in animal

9.9.Albinism Albinism –– mutation that cause the gene mutation that cause the gene

which controls the skin pigment fails to which controls the skin pigment fails to

produce pigment melanin therefore will cause produce pigment melanin therefore will cause

a person have pale skin, light hair and pink a person have pale skin, light hair and pink

eyeseyes

Exercise

• Do the exercise in the Process Skill book page

42.

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Things that we’ve learned today

• The meaning of mutation.

• The types of mutation.

• Examples of mutation.

Causes of mutation

• Spontaneously or induced by mutagens

• Types of mutagens

1. Industrial chemical – benzene, formaldehyde, mustard gas, nitrous acid, diphenylamine and bromouracil

2. Radiation – ultraviolet light, X-rays and nuclear waste (alpha, beta and gamma)

• The higher the dosage of radiation, the higher rate of mutation

3. Increase of temperature outside the normal temperature range

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Advantages and Disadvantages of

Mutation

Advantages Disadvantages

1. 1.

2. 2.

3.

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Advantages Disadvantages

1. Lead to variation 1. Physical deformities

- polydactyl traits →

extra fingers or toes

2. Creation of new species 2. Genetic disease

- gene mutation→

colour blindness and

albinism

- chromosome mutation

→ Down’s Syndrome

and Turner’s

Syndrome

3. Precursor to the

evolution process

Exercise

1. What is mutation?

2. Give types of mutation.

3. Give two examples of mutation.

4. What cause mutation?

5. Give one advantage and disadvantage of

mutation.