Formal genetics of humans: Modes of inheritance

Abstract

Mendel's fundamental discoveries are usually summarized in three laws: 1. Crosses between organisms homozygous for two different alleles at one gene locus lead to genetically identical offspring (F1 generation), heterozygous for this allele. It is unimportant which of the two homozygotes is male and which is female (law of uniformity and reciprocity). Such reciprocity applies only for genes not located on sex chromosomes. 2. When these F1 heterozygotes are crossed with each other (intercross), various genotypes segregate: one-half are heterozygous again, and one-quarter are homozygous for each of the parental types. This segregation 1:2:1 is repeated after crossing of heterozygotes in the following generations, whereas the two types of homozygotes breed pure. As noted previously (Chap. 1), Mendel interpreted this result correctly, assuming formation of two types of germ cells with a 1:1 ratio in heterozygotes (law of segregation and law of purity of gametes). 3. When organisms differing in more than one gene pair are crossed, every single gene pair segregates independently, and the resulting segregation ratios follow the statistical law of independent segregation (law of free combination of genes).