Question 4
(a) Complete the table by grouping each of the following traits under the
appropriate heading.
Tongue rolling, hair colour, fingerprits, colour of seeds, ability to taste PTC, weight, size of seed, blood group, height of plant, sex of animal
Discontinuous variation Continuous variation
[10 marks]
(b) A non-colour blind couple gave birth to one boy and three girls but only the boy is colour blind. Explain briefly the inheritance of colour blindness in the boy. [5 marks]
(c) List three examples of sex-linked characters in humans. [3 marks]
(d) Mention two roles of mutation in evolution. [2 marks]
Observation
Some candidates were able to place the listed traits under the appropriate headings of types of variation.
Many candidates were unable to explain the reason why a boy born to non-colour blind couple is colour blind. Although some of them were able to list sex-linked characters.
Some candidates were able to state the roles of mutation in evolution.
The expected answers are:
- (a) Table
Discontinuous variation |
Continuous variation |
Tongue rolling |
Size of seed |
Blood group |
Hair coat colour |
Sex |
Colour of seeds |
Fingerprint |
Weight of students |
Ability to taste PTC |
Height of plant |
(b) Inheritance of colour blindness in the boy
- Colour blindness is a sex-linked character/trait;
- Controlled by a recessive gene;
- Located only on the X-chromosome;
- For the boy to be colour blind, the mother must have been a carrier of the trait;
- To pass on the X-chromosome on which the colour blind gene is located;
- While the father passed on the Y-chromosome;
- Thereby causing colour blindness to be manifested in the boy.
(c) Examples of sex-linked characters in humans
- Hairy ear lobe;
- Baldness;
- Colour blindness;
- Haemophilia;
- Fragile egg syndrome;
- Male infertility;
- Congenital night blindness;
- Ocular albinism;
- Duchenne muscular dystrophy.
(d) Roles of mutation in evolution
- Gene mutation occurring in the reproductive cells are inherited by all the cells of the offspring;
- Somatic mutations are transmitted in plants with vegetative reproduction, hence new varieties of plants are produced;
- Chromosomal mutations are as a result of loss/gain and it may lead to the formation of new species;
- They alter the anatomy, physiology or behavior in the offspring which can be more adapted to the environment.