Chapter 3: Heredity and Variation

Chapter 3: Heredity and Variation

AnswerExercise 11. (i) Two daughter cells are formed in mitosis. ( )

(ii) In mitosis, cytoplasm divides once. ( )

(iii) Mitosis is important for producing new cells to replace the old or damaged cells ( )

(iv) Mitosis cause genetic variation. ( X )

(v) The number of chromosomes in the daughter cell is half the number of chromosomes

in the parent cell. ( X )

(vi) The parent cell and the daughter cell have the same genetic content. ( )

(vii) Crossing-over does not take place in mitosis. ( )

(viii)Mitosis occurs in somatic cells. ( )

2. (i) Four daughter cells are formed in meiosis. ( )

(ii) In meiosis, the number of chromosomes in each daughter cell is the same as in the

parent cell. ( X )

(iii) Meiosis is important to organisms that reproduce sexually because it produces

gametes. ( )

(iv) Meiosis cause genetic variation. ( )

3.

Exercise 21.TERMSDEFINITIONS

Phenotype

Describes the characteristics traits of an organism, including the physical and the physiological.

Homozygote

Organism which has two identical genes. Example: TT or tt.

Genotype

Composition of gene in an organism

Chromosomes

Thread-like structure present in the nucleus of a cell

Recessive gene

A gene which is weak and its effect can be masked by a dominant gene

Hybrid

An organism which has one dominant gene and one recessive gene for a certain characteristic. Example: Bb ( B = dominant gene for black fur , b = recessive gene for white fur )

Gene

Unit carrying a characteristic trait which exists in pairs in a chromosome.

Heterozygote

Organism which has two different members of the same gene. Example: Tt.

Dominant gene

Exchange of genes when two chromatids cross giving rise to genetic variation.

Crossing over

A gene which masks the effect of a weaker gene

2.

Father

Mother

Parent

Meiosis

Gamete

Fertilisation

Offspring genotype

Offspring phenotype

50 % black hair : 50% blonde hair.

MEIOSIS

MITOSIS

SIMILARITIES

DIFFERENCES

Twice

Number of

divisions

Only once

Four

Number of daughter cells produced

Two

Half

Number of chromosomes in daughter cell compared to parent cell

Same

Different

Combination of chromosomes in daughter cell compared to parent cell

Identical

Occurs

Crossing

over

Does not occur

Variation

occurs

Genetic

variation

None

Involves cell division

Occurs in the nucleus

Chromosome replication takes place

bb

Bb

b

B

b

b

bb

bb

Bb

Bb

Blonde

Blonde

Black

Black