Genes occurring in homologous chromosomes at the same locus are called alleles or allelomorphic pairs. Being particulate they are also called factors or units of inheritance. They may express the same trait or different traits of a character, e.g., TT, Tt, tt, RR, Rr, rr. Here capital letter represents the dominant factor (T for tallness) while t significant recessive factor (t for dwarfness).

Homozygous- It is an organism with identical alleles, e.g., TT, RR, rr, tt. Homozygous individuals are generally genetically pure breed. A homozygous individual produces only one type of gametes.

Evolution is the process by which different kinds of living organisms are believed to have developed from earlier forms during the history of the earth. It is the result of the introduction and accumulation of variations in every generation.