97436282-CHAPTER-3：-HEREDITY-AND-VARIATION.ppt

CHAPTER 3 ： HEREDITY AND VARIATION

3.1 CELL DIVISION

GENES ， DEOXYRIBONUCLEIC ACIDS （ DNA ） AND CHROMOSOMES

• Genes – units of inheritance that determine the characteristics of an individual

• Example ： colour and type of hair ， types of ear lobes

• Genes are found in long molecules called deoxyribonucleic acids （ DNA ）

• There are thousands of genes in a DNA molecule

，

• DNA molecules are found in chromosomes

• Chromosomes are thread-like structure in the nucleus of cell

• Chromosomes exist in pairs ， so do genes 。 Each pair of genes controls one characteristics of an individuals

CELL DIVISION

• There are two types of cell division ：a) Mitosis

b) Meiosis

• Mitosis – a process whereby a cell divides into two identical cells ， each having the same number and kind of chromosomes as the parent cell

• Meiosis – a process whereby a cell divide into four daughter cells ， each containing only half the chromosomes number of the parent cell

SIMILARITY AND DIFFERENCES BETWEEN MITOSIS AND MEIOSIS

THE IMPORTANCE OF MITOSIS AND MEIOSIS

• Mitosis

a) produces new cells for growing and replacing dead cells

b) Maintain the genetic materials in the daughter cells

c) Used in the techniques of cloning and tissue culture to produce crops with desireable traits

• Meiosis

a) Produce gametes （ half the number of chromosomes ）

b) Will result in differences among individuals of the same species when crossing over and random distribution of chromosomes occur

Exercise 3.11. State the meanings of

a) Mitosis b )Meiosis 2. State two differences between mitosis

and meiosis.3. In which parts of a plant does mitosis

occur?4. In which parts of a flowering plant does

meiosis occur?5. A somatic cell in corn plants contains 20

chromosomes. How many chromosomes does each of its male gamete have?

3.2 THE PRINCIPLES AND MECHANISM OF INHERITANCE

• There two types of genes ：a) Dominant genesb) Recessive genes• Dominant genes – genes that show the

characteristics they control when paired with a dominant gene or recessive gene

• Recessive genes – genes that only show the characteristics they control when paired with another recessive genes

MECHANISM OF TRAIT INHERITANCE

6 。 HOMOZYGOUS – A person who has two identical genes 。 Example ： CC or cc

7 。 HETEROZYGOUS – A person wiyh two different genes 。 Example ： Cc

8 。 PHENOTYPE – Observable characteristics of an organism 。 - Example ： curly hair ， straight hair ， being right-handed or left-handed

Exercise 3.21. Explain the following terms :

( a ) Dominant genes (b) Recessive genes

2. The figure below shows a monohybrid cross between two plants. R : gene for red flowers ( dominant)r : gene for white flowers ( recessive )

( a ) ( i) Complete the schematic diagram to show the inheritance of the colour of flowers in the plants. (ii) Determine the phenotype ratio of the F1 generation

3.3 SEX DETERMINATION AND THE OCCURRENCE OF TWINS IN HUMAN BEINGS

SIMILARITY AND THE DIFFERENCES BETWEEN IDENTICAL TWINS AND NON-IDENTICAL TWINS

Exercise 3.31.( a ) Name the two types of sex chromosomes.

( b )What is the function of sex chromosomes?

2. ( a ) State the number of autosomes and the type of sex chromosomes in the zygote produced from the fertilisation of an ovum by

i. A sperm with ( 22 + X) chromosomesii. A sperm with ( 22 + Y) chromosomes

( b ) Determine the sexes of babies in 2 (a)(i) and (a) (ii). ( c ) Is the sex of an offspring determined by an ovum or a sperm? ( d ) Explain your answer to 2 ( c )

3. ( a ) Why is it possible for non-identical twins to have different sexes? ( b ) Do siamese twins have exactly the same genetic content? ( c ) Explain your answer to 3 a(b )

3.4 MUTATION

• Mutation is the spontaneous change to the structure of genes or chromosomes in somatic cells or in gametes of an organism

• Two types of mutation are

a) CHROMOSOME MUTATION ： change in the structure of number of chromosomes

b) GENE MUTATION ： due to chemical changes in individual genes

a) CHROMOSOME MUTATION

1) Down‘s syndrome • Has 47 chromosomes in somatic cell

（ Caused by the possession of an extra chromosomes 21 ）

• Characteristics ： flat cheeks and short necks ， small and narrow eyes ， short broad hands and weak muscles ， low resistance to disease ， mentally ang physically retarded

2. Klinefelter’s syndrome

• Will affecting males （ caused by the presence of three sex chromosomes （ XXY ） instead of usual two （ XY ）

• Characteristics ： posses some female characteristics

3) Turner‘s syndrome

• Female who has 45 chromosomes （ has only one x chromosomes ）

• Characteristics ： no ovaries ， fails to develop normal secondary sexual characteristics ， short and has folds of skin on her neck

b) GENE MUTATION

1) Colour blindness• The inability of a person to differentiate

between colours• The gene for colour blindness is recessive and

carried in the X chromosome• Colour blindness more common among males

than females 。 This is because females have two X chromosomes 。 A female will only be colour blindness if she receives the recessive genes from both parents

2) Albinism

• Occurs when a gene mutation interferes with melanin （ body pigment ）

• Characteristics ： pinkish skin and eyes ， white hair

3) Haemophilia • Is a blood clotting disorder （ caused by

a recessive gene carried by X chromosome ）

• Common among males than females （ carried by X chromosome ）

• Haemophilia patients face severe bleeding problems when they are wounded 。 This is because their blood takes a long time to coagulate around the wounds

The causes of mutation

• Mutagens ： external factors to increase the chances of mutation

• Examples ：i. Radioactive rays ： enter body cells and

change the structures of chromosomes or genes in the cells

ii. Toxic chemicals ： such as pesticides may react with and damage DNA

iii. Ultraviolet radiation ： cause mutation and skin cancer

3.6 VARIATION

• Variation ： the differences among individuals of the same species

• Two types of variations are ：• Continuos variation

• Discontinuos variation

The difference between continous variation and discontinuos variation

Terms of differences

Continous variation Discontinuos variation

Explanations There are intermediate categories between the two extreme

No intermediate category between the distinct categories

Example of traits

weight ， skin colour ， hair colour ， intelligence ， height ，

the ability to roll the tongue ， blood group ，sex ， type of hair （ curly or straight ），being right-handed or left-handed

Terms of differences

Continous variation

Discontinuos variation

Factors influence genetic and environmental factors

genetic factors

Shape of the graph

Factors which cause variation

• Variation can caused by ：a) Genetic factors （ during meiosis ）• Chromosomes crossing-over and

random distribution during meiosis

• During crossing-over ： chromatids of a pair of chromosomes exchange genetic material

b) Environmental factors

• Include climatic changes ， nutrition and physical activities 。

• Example ： frequent exposure to the sun causes construction workers to have darker skin compared to office workers

The importance of variation

• Enables us to differentiate individuals of the same species

• For the survival of a species when environmental conditions change

• In agriculture ， variation in crops and farm animals allows selective breeding to be carried out

Exercise 3.6

1. What is variation? Give two examples of variation among human beings.

2. Differentiate between continuous variation and discontinuous variation.

3. State two factors that can cause variation.

4. What is the importance of variation?