Summary

This chapter traced the complete arc of classical genetics, beginning with Gregor Mendel's own carefully designed pea-breeding experiments and building outward, step by step, all the way to the molecular and chromosomal disorders recognised in clinical medicine today. Mendel's three foundational laws — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment — were derived purely from patiently counting phenotypic ratios observed in his monohybrid crosses (yielding the characteristic 3:1 F2 ratio) and his dihybrid crosses (yielding the characteristic 9:3:3:1 F2 ratio) of the garden pea, all of this achieved many decades before genes, chromosomes or DNA had themselves been discovered or understood in any way. The initially simple picture of complete dominance that Mendel's seven chosen pea characters happened to display was then shown, through later work on other organisms and other genes, to be only ONE of several possible relationships that can exist between two alleles: incomplete dominance, illustrated by flower colour in Mirabilis jalapa, produces a distinct, blended intermediate phenotype in the heterozygote, while co-dominance and multiple allelism, illustrated by the human ABO blood group system, instead produce BOTH parental phenotypes expressed together, simultaneously and undiminished, in the very same heterozygous individual. Polygenic inheritance, illustrated by human skin colour, showed how several different genes acting together in an additive fashion can generate smooth, continuous phenotypic variation rather than a small number of sharply distinct classes, while pleiotropy, illustrated by phenylketonuria, showed essentially the reverse situation — a single gene simultaneously affecting several distinct, apparently unrelated phenotypic characters at once. …