Mendel's Laws

Also found in: Dictionary, Thesaurus, Medical, Legal, Wikipedia.
Related to Mendel's Laws: Mendel's Laws of Inheritance

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Despite their clarity and importance, Mendel's laws of inheritance went unnoticed until 1900, when their rediscovery catalyzed formation of the new science of genetics.
The rediscovery of Mendel's laws was a major event in the development of the biological sciences.
By 1900, the year that Mendel's laws resurfaced, the population of the world numbered 1.625 billion people.