“Mendel proposed the concept of genetic dominance based on phenotype of heterozygotes. In a garden pea cross between purple- and white-flowered homozygous parents (PP and pp), all F1 plants were purple heterozygous (Pp). Selfing of F1 plants produced both purple and white-flowered plants. In snapdragon heterozygotes (pink flowered; RW) show an intermediate phenotype compared to red and white flowers of its parents (red; RR and white flowered-plants; WW) and this is referred to as incomplete dominance. Codominance occurs when phenotypes of both alleles are expressed in heterozygote. For example, fur in cattle can be red (RR; only red hairs), white (WW; only white hairs) or roan (RW; both red and white hairs together).

Alleles are different versions of the same gene found at the same locus of homologous chromosomes. A diploid organism has a gene pair for each trait, one from each parent. In molecular terms, one of the two alleles of a gene may or may not be transcribed, and if transcribed may produce different versions of a protein due to a variation. These altered protein forms may influence the functional protein resulting in haplosufficiency or dominance (where a functional copy of the gene is sufficient to produce wild type phenotype), incomplete dominance (both variants of the gene contribute to phenotype) and rarely haploinsufficiency (the non-functional allele is dominant and influences phenotype).

During breeding, plants or animals may be crossed for selecting 1 (monohybrid), 2 (dihybrid) or 3 (trihybrid) traits. For example, in a heterozygous purple (Pp) plant, the probability of egg or sperm having white (p) allele is 1/2. On selfing, the chance for being white flowered is 1/2 x 1/2 = 1/4. For more than one trait, the product of their chances of occurring separately is determined.”
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