Summary

This chapter traced classical (transmission) genetics from Mendel's original pea experiments through to its more complex extensions. Mendel's particulate theory, tested and confirmed by the monohybrid cross, showed that heredity is carried by discrete factors (genes) rather than by blending, and produced the characteristic 3:1 Mendelian ratio in F2, formalised as the Law of Dominance and the Law of Segregation. The test cross, Mendel's tool for finding out whether a dominant-phenotype plant is homozygous or heterozygous, and the back cross of which it is a special case, extend that same logic. The dihybrid cross tracked two genes at once, showing that they assort independently to give the 9:3:3:1 F2 ratio (Law of Independent Assortment) and generate new recombinant phenotype combinations not seen in either parent - a pattern that scales further, in exactly the same way, to the trihybrid cross's 27:9:9:9:3:3:3:1 ratio. Beyond these baseline ratios, the chapter worked through the extensions of Mendelian genetics: intragenic interactions (incomplete dominance, codominance, lethal genes, pleiotropy) between the alleles of one gene, and intergenic interactions (epistasis in its several forms) between the alleles of different genes, each producing its own characteristically modified F2 ratio. Polygenic inheritance in wheat kernel colour then showed how the same segregation-and-recombination machinery, scaled up across several genes acting additively, produces continuously graded (quantita …