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

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

    Reconsider Jenkin's argument from blending inheritance with a Mendelian particulate model. Suppose the White allele (W) is genetically dominant to the recessive Purple allele (w).

Line 1: A White sailor thrown up 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 again that the sailor becomes King, and gains a two-fold reproductive advantage over locals, who have two offspring each. He has four offspring, all of whom are heterozygous Ww, and therefore 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 giving rise to two WW and four Ww white offspring, and two purple ww offspring (3:1 phenotypic ratio).

Line 4: The number of W alleles doubles every generation, and the proportion of WW and Ww individuals increases indefinitely relative to ww individuals.

Line 5: Eventually the population approaches fixation for the W allele, as predicted by Darwinian Natural Selection in combination with Mendelian Genetics, as proved by the General Selection Model

    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 come to predominate a population. Recall that the disadvantageous recessive w allele cannot be totally eliminated from the population. 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: There is a bit of trickery to move from Line 2 to 3. Identify it: does it change the argument? How could you make the diagram more realistic?