(a) (i) What is a gene? (ii) Differentiate between the terms genotype and phenotype (b) Explain the following terms: (i) hybrid (ii) pure breeding (iii) nuc...

Answer Details

(a)(i) What is a gene?

A gene is a unit of heredity; it is a small segment of DNA on a chromosome that carries the coded information for a particular characteristic (trait) and is passed on from parents to offspring.

(a)(ii) Difference between genotype and phenotype

The genotype is the genetic make-up of an organism, that is, the actual combination of genes (alleles) it possesses for a trait (for example TT, Tt or tt). The phenotype is the observable or physical expression of that trait (for example tall or short), resulting from the interaction of the genotype with the environment.

(b) Explanation of terms

• (i) Hybrid - An offspring that is heterozygous for a trait, resulting from a cross between two genetically different (pure-breeding) parents (for example Tt).
• (ii) Pure breeding - An organism that is homozygous for a trait and always produces offspring identical to itself for that trait when self-crossed (for example TT or tt).
• (iii) Nucleotide - The basic structural unit (building block) of a nucleic acid, made up of a nitrogenous base, a pentose sugar and a phosphate group.

(c) Genetic diagram

Let smooth seed coat be S (dominant) and rough seed coat be s (recessive).

A homozygous rough pea is ss. The smooth plant whose parents were both rough (ss) must have received an s allele from each rough parent, so it is heterozygous smooth, Ss.

Cross: ss (rough) \( \times \) Ss (smooth)

Gametes: ss produces all s; Ss produces S and s.

Result: Offspring genotypes are Ss : ss in the ratio 1 : 1. Phenotypes are smooth : rough = 1 : 1 (that is, 50% smooth-seeded and 50% rough-seeded).