Question

4. (a)(i) What is a gene? [2 marks] (ii) Differentiate between the terms genotype and phenotype. [2 marks]
(b) Explain the following terms: (i) hybrid; (ii) pure breeding; (iii) nucleotide. [6 marks]
(c) In garden pea seeds, smooth seed coat is dominant over rough seed coat. With the aid of a genetic diagram, determine the result expected if a homozygous rough pea is crossed with a smooth seed coat plant whose parents were rough coated. [10 marks]

Accepted Answer

(a)(i) A gene is defined as a unit of inheritance.
(ii) Genotype refers to the genetic constitution of an organism while phenotype refers to the observable features of an organism. The phenotype of an organism depends on the genotype and the environment.

(b)(i) Hybrid refers to the progeny of a cross between organisms with contrasting characters.
(ii) Pure breeding occurs when a particular trait is retained in subsequent generations.
(iii) A nucleotide is a unit of DNA. Each nucleotide contains a five-carbon sugar, phosphate group and an organic nitrogen-containing base (adenine, guanine, cytosine or thymine).

(c) Note: since smooth is dominant over rough, if a smooth seed coat is dominant over rough seed coat, the said parent could only be rough coated if they have no gene for smooth seed coat at all; consequently, there is no way such parents could produce an offspring that would be smooth coated. (i.e. the cross of homozygous recessive rough (rr) with a plant that is itself rough-coated, having rough-coated parents — meaning it too is homozygous recessive rr — produces only rough-coated offspring, all rr.)

WAEC Biology 2012 Theory — Question 4

Sign up free to unlock