(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...

(a)(i) What is a gene?

A gene is a unit of heredity located at a specific position (locus) on a chromosome, made up of a segment of DNA, which carries the coded information for a particular protein and is responsible for the transmission of a particular trait or character from parents to offspring.

(a)(ii) Difference between genotype and phenotype

(b) Explanation of terms

(i) Hybrid: An offspring produced from a cross between two genetically different parents that possess contrasting forms of a character; such an offspring is heterozygous, e.g. \(Ss\).

(ii) Pure breeding: A condition in which an organism is homozygous for a trait (e.g. \(SS\) or \(ss\)) so that, when self-pollinated or crossed with an identical type, it produces offspring showing only that same trait generation after generation.

(iii) Nucleotide: The basic structural unit (building block) of a nucleic acid, each one consisting of a pentose sugar (deoxyribose in DNA or ribose in RNA), a phosphate group and a nitrogenous base (adenine, guanine, cytosine, and thymine or uracil).

(c) Genetic cross

Let the allele for smooth seed coat be \(S\) (dominant) and the allele for rough seed coat be \(s\) (recessive).

The homozygous rough pea has the genotype \(ss\).

The smooth-seeded plant had rough-coated parents. Since a rough parent can only supply a recessive \(s\) allele, this smooth plant must have received one \(s\) allele; because it is smooth it must also carry one dominant \(S\) allele. It is therefore heterozygous, genotype \(Ss\). (This is a test cross.)

Genetic diagram of the cross

Results:

Genotypic ratio of offspring \(=\;1\;Ss\;:\;1\;ss\).

Phenotypic ratio of offspring \(=\;1\) smooth seed coat \(:\;1\) rough seed coat, i.e. 50% smooth-seeded and 50% rough-seeded plants.