Mendel, Gregor Johann: Mendel's First Law

Mendel's First Law

The law of segregation (Mendel's first law) states that in the process of the formation of the gametes (see meiosis) the pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. Mendel found that when a pure strain of peas bearing one form of a gene (that is, a strain in which both members of the gene pair being studied are the same), inbred for many generations, was crossed with a pure strain carrying an alternative form of the gene, one of these forms consistently prevailed over the other in determining the visible characteristics of the offspring; he therefore termed the two forms dominant and recessive, and called the phenomenon itself the law of dominance. Given A as the dominant factor and a as the recessive, the offspring of the purebred strains having genes of the form AA and aa are hybrids, individuals each being Aa. When the hybrids are crossed, the offspring exhibit the characteristic in question in a ratio of three dominant to one recessive; i.e., the four possible combinations of the genes in Aa and Aa are AA, aA, Aa, and aa. By the same rule, when a hybrid is crossed with a purebred recessive (Aa with aa) the ratio is one to one. Breeders often use these ratios to trace the hybrid or purebred nature of the parent stock.

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