![]() ![]() The relationships between relatives for quantitative traits are described in terms of regressions and correlations, as defined by the early biometricians Galton and Pearson. These continuously varying phenotypes, now recognized as quantitative traits, are more common in populations than phenotypes with the inheritance properties that Mendel reported. Indeed, Mendel deliberately excluded such phenotypes (or traits) from consideration. Mendel’s principles of inheritance were contrary to the common observation at the time that crosses between organisms with different phenotypes were often intermediate between the 2 parents, and that phenotypic variation in populations is continuous and not discrete. In a separate series of crosses between 2 species of common bean with different flower colors and unexpected ratios of flower color in hybrids, Mendel correctly inferred multiple loci with recessive epistasis (where the expression of one gene is modified by another). Luckily for Mendel, the 7 loci were each on a different autosome. Mendel also performed crosses in which the hybrids differed for 2 or 3 of the 7 loci, analyses of which led him to conclude “the relation of each pair of different characters in hybrid union is independent of the other differences in the 2 original parental stocks” (the Law of Independent Assortment). ![]() He inferred that the F 1 hybrids have 2 alleles at each locus (A, a), which are present in equal proportions in the female and male gametes, and combine at random to produce F 2 offspring in the ratio 1 AA:2 Aa:1 aa (the Law of Segregation). ![]() When Mendel crossed each of the 7 F 1 hybrids among themselves, he observed a 3:1 ratio of dominant to recessive phenotypes in the F 2 generation. He excluded phenotypes that were intermediate between the 2 parents in the F 1 hybrid. Mendel chose these loci because the hybrid (F 1) between the homozygous parental genotypes (to use modern terminology) was indistinguishable from one of the parents i.e., one of the alleles was dominant and the other recessive (Mendel’s original terms). Working with the garden pea, Pisum sativum, Mendel chose 7 “characters” (polymorphic loci each affecting a different phenotype in today’s parlance) that “permit of a sharp and certain separation,” excluding those for which “the difference is of a ‘more or less’ nature, which is often difficult to define”. The field of genetics was born with the publication in 1866 of Gregor Mendel’s Experiments in Plant Hybridization. ![]()
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