Yellow / Green, Round / Wrinkled peas versus White / Purple people eaters

Fleming Jenkin along with Charles Darwin and other 19th century biologists supposed that inheritance was a blending process. Gregor Mendel showed instead that inheritance is particulate.

Suppose the White allele (W) is genetically dominant to the Purple allele (w).

Line 1: A white sailor introduced on the island of Purple People Eaters is homozygous WW and introduces two W alleles into the populations. The PPEs are all homozygous ww.

Line 2: Suppose the sailor has a two-fold reproductive advantage over locals: he has four offspring, all of whom are heterozygous Ww, and White. The number of Whites has quadrupled, and the number of W alleles has doubled to four.

Line 3: In the next generation, each of the white Ww offspring again has a two-fold advantage, doubling the number of W alleles to eight. Random assortment produces a 1:2:1 genotypic ratio of WW, Ww, and ww offspring, thus reconstituting two WW and four Ww white offspring, and two purple ww offspring (3:1 phenotypic ratio).

Lines 4 & 5: The number of W alleles doubles every generation, and the proportion of WW and Ww individuals increases indefinitely relative to ww individuals. Eventually the population will become (almost) entirely white, as predicted by Darwinian Natural Selection in combination with Mendel's Genetics.

Mendelian particulate inheritance ensures that the selective advantage of a genotype will be perpetuated as an increase in the number of alleles in that genotype in each generation. This in turn means that a new, advantageous phenotype can predominate a population quickly. This disproves Jenkin's argument from blending inheritance, that a new favorable trait cannot expand in a population.

HOMEWORK: Suppose W is semi-dominant OR recessive to w: will the same results be obtained? Diagram & prove your answer.

HOMEWORK: The move from Line 2 to Line 3 includes a slight over-simplification: Identify it.