Mendel's Laws

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Mendel's laws

[′men·dəlz ‚lȯz]
Two basic principles of genetics formulated by Mendel: the law of segregation of alleles of a unit factor (gene), and the law of independent assortment of alleles of different unit factors.

Mendel’s Laws


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.


References in periodicals archive ?
5) that describe the genotype and phenotype probabilities for monogenic Mendelian inheritance.
Testing Mendelian inheritance from field-collected parasites: revealing duplicated loci enables correct inference of reproductive mode and mating system.
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4] Catalogo de enfermedades Mendelianas, OMIM: online mendelian inheritance in man
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Number of markers-- Primers EcoRI (E) --Primers MseI (M)-- CAG CAT CTA CTC ACC 12 (a) 18 15 0 AAG 15 18 14 16 ACA 0 0 11 10 ACT 14 9 11 7 AAC 5 17 19 14 ACG 0 0 0 0 AGG 3 2 0 0 --Number of polymorphic markers-- E / M CAT CTA CTC Total ACC 8 (b) 4 - 12 AAG 5 8 5 18 AAC - 9 1 10 Total 13 21 6 40 Primer combinations are denoted by the EcoRI--AXX and Msel--CXX selective nucleotides, respectively; (a) Number of markers scored per primer combination; Underlined--AFLP primer combinations selected to investigate the Mendelian inheritance pattern in the full-sibs; (b) Total number of polymorphic markers scored.
These included examples of classic Mendelian inheritance patterns (e.
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The two QTL on SSC1 and SSC7 had Mendelian inheritance mainly with dominance and additive effects, respectively, while the QTL on SSC11 was maternally expressed with an increasing KNP allele (Table 2).
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In the evaluation, genes extracted by the system were compared to those found in two actively curated genetic databases, Genetics Home Reference and Online Mendelian Inheritance in Man (OMIM).