Mendel’s laws [Gregor J. Mendel] , the basic principles of inheritance based on breeding experiments on garden peas by the 19th-century Austrian monk Gregor Mendel. These principles are usually stated as the law of segregation and the law of independent assortment. According to the law of segregation, each trait of a species is represented in the somatic cells by a pair of units, now known as genes, which are segregated during meiosis so that each gamete receives only one gene for each trait. In any monohybrid crossing, the possible ratio for the phenotypic expression of a particular dominant trait is 3:1, whereas the genotypic ratio of pure dominants to hybrids to pure recessives is 1:2:1. According to the law of independent assortment, the members of a gene pair on different chromosomes segregate independently from other pairs during meiosis so that the gametes show all possible combinations of genes. Genes on the same chromosome are affected by linkage and segregate in blocks according to the amount of crossing over that occurs, a discovery made after Mendel’s work. Also called mendelian genetics, mendelian laws. See also chromosome, crossing over, dominant allele, linkage, meiosis, recessive allele.
