Question 2
The diagrams below are illustrations of cross breeding between two animals of the same species. Study them and answer questions 2(a) to 2(h).
(a)
(i) Identify the sex of each of organisms A and B. [2 marks]
(ii) State one reason each for the answers in 2(a)(i). [2 marks]
(b) What does the letter X between diagrams A and B represent? [1 mark]
(c)
(i) Identify the stages labelled I to IV. [4 marks]
(ii) Give one reason each for the answers in 2(c)(i). [4 marks]
(d) Name the process which took place in C and D to produce E and F. [1 mark]
(e)
(i) Name the organ of the animal A where C can be found. [1 mark]
(ii) Name the organ of the animal B where D can be found. [1 mark]
(f)
(i) Which of the animals A and B shows dominance for skin colour? [1 mark]
(ii) Give one reason for the answer in 2(f)(i). [1 mark]
(g) Assuming B represents the allele for spotted skin colour, while b represents the allele for plain skin colour, determine:
(i) the genotype of each of the animals A and B; [2 marks]
(ii) the phenotypic ratio of the first filial generation. [1 mark]
(h) Name four other traits in the animal that can be inherited. [4 marks]
Observation
Only a few candidates performed well in this question.
Some candidates could identify the sex of the animals and state the correct reason for the identification. Many of the candidates could not identify the stages labelled I to IV and give reasons. Many candidates appeared not to understand the process of meiosis and interpretation of the cross sign. The candidates who performed well in this question were able to: name the organs where spermatozoa and ova are produced; identify the animal that showed dominance for skin colour; name four other traits in animals that could be inherited.
The expected answers are:
(a) (i) Sex of organisms
A – Male
B – Female
(ii) Reason for the answer
A – produces spermatozoa
B – produces ovum/ova/egg.
(b) Interpretation of letter X
Crossing/mating
(c) (i) Stages labelled I to IV
- Parental genotypes
- Gametes
- Fusion of gametes/fertilization
- Zygote
(ii) Reason
I/Parental genotypes: presence of four chromosomes/diploid number of chromosomes/two pairs of chromosomes;
II/Gametes: two chromosomes/haploid condition;
III/Fusion of gametes/fertilization: fusion of the gametes leading to fertilization/sperm cell and egg cell coming together;
IV/Zygotes: four chromosomes restored/diploid condition/two pairs
of chromosomes.
(d) Process in C and D
Meiosis
(e) (i) Organ of the animal A where C can be found
Testis/testes
(ii) Organ of the animal B where D can be found
Ovary/ovaries
(f) (i) Animal that shows dominance for skin colour
A
(ii) Reason
All the offspring/F1 generation show resemblance with the spotted skin parent/ all the offspring have the same skin pattern/phenotype/colour as A.
(g) (i) Genotype
A – BB
B – bb
(ii) Phenotypic ratio
4:0
(h) Other inheritable traits in animals
Size; height; weight; eye colour; ear shape; ear length; length of horn; length of tails;
hair colour.