Define the following terms: [5 MARKS]
(i) Allele
(ii) Dominant gene
(iii) Genotype
(iv) Phenotype
(v) Heterozygous dominant

Each Part: 1 Mark

(i) Allele: These are alternative forms of a gene, occupying the same place on a chromosome and affecting the same characters but in a different way. For example, in the character of tongue rolling (projecting the tongue out of the mouth in the form of a tube), there are two possibilities - either one may roll the tongue or one may not be able to do so. Thus there are two alleles - one for rolling and the other for non-rolling.

(ii) Dominant gene: A dominant gene is one which is able to express itself in a heterozygote.

(iii) Genotype: It is the genetic constitution or gene complement of an individual or an organism.

(iv) Phenotype: It is the external manifestation of a genotype. In other words, it is the observable characteristics which are genetically controlled.

(v) Heterozygous dominant: It is a pair of genes in which one gene is dominant and the other one is recessive. So the effect of recessive one is marked by the dominant gene.
Example: Rr.