隔离法

When Gregor Mendel performed monohybrid crosses of pea plants differing by single traits, such as pod color, he only saw one parental phenotype disappear in the first filial, or F-one, generation. And then, reappear in the second filial, or F-two generation, meaning that one phenotype was dominant over the other. The offspring with the dominant phenotype, comprised 75% of the F-two generation, while those with the recessive phenotype comprised 25%.

From his experiments, Mendel concluded that there must be a pair of particles, now known to be genes, that are responsible for transmitting each trait from one generation to the next. Mendel summarized one of the key concepts underlying heredity in his Law of Segregation, which states that every diploid organism carries two copies of each gene, called alleles, that are segregated into the gametes at random during meiosis. Thus, when organisms reproduce, all offspring receive one allele from each parent.

Because segregation is random, heterozygous parents, those bearing two different alleles, are equally likely to pass on a dominant or recessive allele, and the 3:1 phenotypic ratio observed in a cross between two heterozygotes, illustrates the Law of Segregation. 25% of the offspring will have a homozygous dominant genotype and display the dominant phenotype, 50% will be heterozygous and display the dominant phenotype, and 25% will be homozygous-recessive and display the recessive phenotype.