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Mendel's laws[′men·dəlz ‚lȯz]
principles describing the discrete, corpuscular nature of inheritance, discovered by G. Mendel.
Mendel himself formulated only the law of combination of differentiating characters, which explained his discovery of the phenomena of segregation and the independent assortment of hereditary factors (later called genes) in the offspring of a cross. Three laws were usually accepted during the early phase of Mendelism—dominance, segregation, and independent assort-ment—thought to apply equally to the organism’s characters and to the hereditary factors in its germ cells. Therefore, the first law was thought to be that of dominance, according to which, in the first generation produced by crossing individuals differing in analogous (allelic) characters, only one (the dominant) is manifested, while the second, or alternative, remains hidden (recessive). A violation of the law of dominance was soon found with the intermediate manifestation of both characters in a first-generation hybrid. As a result, the name of Mendel’s first law was changed to that of the uniformity of first-generation hybrids.
Mendel’s second law, usually called the law of segregation, becomes operative when first-generation hybrids are crossed or self-pollinated. The pairs of allelic genes are segregated, so that both dominant characters and the recessive characters that were latent in the preceding generation appear in the offspring in definite numerical ratios.
Mendel’s third law, that of the independent assortment of characters, applies to crosses in which more than one pair of allelic genes combine: all pairs of participating alleles combine freely in the offspring, so that all possible combinations can occur—again, in definite numerical ratios. The third law is a direct consequence of segregation; it is more correct, therefore, to call it the law of the independent segregation of different pairs of alleles.
Mendel demonstrated and calculated all possible types of segregation and combination of different pairs of genes, and he derived a general formula for the ratios he observed in the crosses. However, this formula is valid only for the combination of the genes themselves. As for the characters actually manifested in development, subsequent research disclosed a number of complications caused by the patterns of interaction of the various genes during the development of the characters they determine (pleiotropy, polygenism, epistasis). These interactions should therefore not be regarded as contradicting the law of independent segregation or combination, which is in fact partly violated only in regard to the phenomena of gene linkage (discovered subsequently).
A careful distinction must be made between the principles related to the transmission and distribution of hereditary factors in the offspring and those principles related to the realization of these factors in the development of the organism. The former, which are genotypic, include Mendel’s laws of segregation and independent assortment, the latter, which are phenotypic, are concerned with dominance, intermediate manifestation, and many other forms of allelic and nonallelic interactions. Mendel’s laws have been completely confirmed and explained by the chromosome theory.
A. E. GAISINOVICH