Evolution__3rd_Edition

38 PART 1 / Introduction
Figure 2.10
(a) Blending inheritance. The
parental genes for dark green
(A) and white (a) color blend in
their offspring, who produce a
new type of gene (A′) coding for
light green color. (b) Mendelian
inheritance. The parental genes
are passed on unaltered by the
offspring.
“Blending” heredity is a
(theoretical) alternative to
Mendelian heredity
Individuals
Gametes
Offspring
Gametes
Grandchildren
(a) Blending heredity
(b) Mendelian heredity
AA X aa AA X aa
A a A a
Aa X Aa Aa X Aa
A' A' A a A a
A'A' AA Aa aa
synthetic theory of evolution, or neo-Darwinism. The problem was Darwin’s lack of a
sound theory of heredity, and indeed it had even been shown in Darwin’s time that natural
selection would not work if heredity was controlled in the way that, before Mendel,
most biologists thought it was. Before Mendel, most theories of heredity were blending
theories. We can see the distinction in much the same terms as have just been used for
Mendelism (Figure 2.10). Suppose there is a gene A that causes its bearers to grow up
dark green in color, and another gene a that causes its bearers to grow up white. We can
imagine that, as in the real world of Mendelism, so in our imaginary world of blending
heredity, individuals are diploid and have two copies of each “gene.” An individual
could then either inherit an AA genotype from its parents and have a dark green phenotype,
or inherit an aa genotype and have a white phenotype, or an Aa genotype and
have a light green phenotype. (Thus in the Mendelian version of the system, we should
say there is no dominance between the A and a genes.)
The interesting individuals for this argument are the ones that have inherited an Aa
genotype and have grown up to be light green. They could have been produced in a
cross of a dark green and a white parent: then the offspring will be light green whether
inheritance is Mendelian (with no dominance) or blending. But now consider the next
generation. Under Mendelian heredity, the light green Aa heterozygote passes on intact
to its offspring the A and a genes it had inherited from its father and mother. Under
blending heredity, the same is not true. An individual does not pass on the same genes
as it inherited. If an individual inherited an A and an a gene, the two would physically
blend in some way to form a new sort of gene (let us call it A′) that causes light green
coloration. And instead of producing 50% A gametes and 50% a, it would then produce
all A′ gametes. This makes a difference in the second generation. Whereas in Mendelian
heredity, the dark green and white colors segregate out again in a cross between two
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