When the allelic pairs of genes are identical then they are called homozygous and when the alleles of gene are different it is heterozygous. For example in case of the trait : height; we have TT and tt to show homozygous and Tt to show heterozygous.
Now in a pair of dissimilar factors one dominates the other and hence called dominant factor while the other called as recessive factor. The dominant one is denoted by upper case and recessive one with lower case. For example T is used for tall and t for dwarf of the same character- height.
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