(i) Gene - Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the appearance and inheritance of character.
(ii) Allele - It refers to the alternative forms of a gene which control the same character and located at the same locus in the homologous chromosomes.
(iii) Gene Locus - It is the region on chromosome representing a single gene. The alleles of a gene are present on the same gene locus on the chromosome.
(iv) Heterozygous - The prefix hetro means 'different' and zygous means 'a pair'. The organism in which both the factors of a character are unlike is said to be heterozygous or hybrid. Such organisms do not breed true on self fertilization eg. Tt.
(v) Homozygous - The prefix homo means 'the same' and zygous means 'a pair'.The organism in which both the factors of a character are similar is said to be homozygous or genetically pure for that character. It gives rise to offsprings having the same character on self breeding. e.g TT or tt.
(vi) Dominant and Recessive - A heterozygote possesses two contrasting factors or alleles but only one of the two is able to express itself, while the other remains hidden. The factor which gains expression in F hybrid is known as dominant factor while its allele is unable to express itself in presence of the dominant factor is recessive factor.
(vii) Genotype - It is the genetic constitution of an organism which determines the characters.
(viii) Phenotype - It is the appearance of an individual.
Phenotype of an individual refers to the expressed or observable structural and functional traits produced by interaction of genes and environment.
Note :- The term Genotype and Phenotype were coined by Johanssen.
(ix) Pureline - Generations of homozygous which produce offsprings of only one type i.e. they breed true for their phenotype and genotype.
(x) Genome - Sum total of genes in the haploid set of chromosomes and inherited as unit from parents to offspring.
(xi) Genepool - All the genotypes of all organisms in a population form the gene pool.