Blending inheritance

Charles Darwin developed his theory of evolution by natural selection on the basis of an understanding of uniform processes in geology, acting over very long periods of time on inheritable variation within populations.

Darwin assembled many lines of evidence to show that variation occurred and that artificial selection by animal and plant breeding had caused change.

The theory had an intuitive appeal, as characteristics of all parts of the body, such as shape of nose, width of shoulders and length of legs are inherited from both the father and the mother.

That directly contradicts the observed facts of inheritance, not least that children are usually either male or female rather than all intersex, and that traits such as flower colour often re-emerge after a generation, even when they seem to disappear when two varieties are crossed.

Huxley, dated November 12, 1857, Darwin wrote: I have lately been inclined to speculate very crudely & indistinctly, that propagation by true fertilisation, will turn out to be a sort of mixture & not true fusion, of two distinct individuals, or rather of innumerable individuals, as each parent has its parents & ancestors:— I can understand on no other view the way in which crossed forms go back to so large an extent to ancestral forms.

[1][5][6] The evolutionary biologist Richard Dawkins commented that blending inheritance was observably wrong, as it implied that every generation would be more uniform than the one before, and that Darwin should have said as much to Jenkin.

[8][9] In 1892, August Weismann set out the idea of a hereditary material, which he called the germ plasm, confined to the gonads and independent of the rest of the body (the soma).

Flowers would converge to a single coloration in a few generations if inheritance blended the characteristics of the two parents.
Diagram of Charles Darwin 's pangenesis theory. Every part of the body emits tiny particles, gemmules , which migrate to the gonads and contribute to the fertilised egg and so to the next generation. The theory implied that changes to the body during an organism's life would be inherited, as proposed in Lamarckism , and that inheritance would be blending.
Blending inheritance leads to the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, would make natural selection impossible if blending were the mechanism of inheritance. [ 1 ] [ 5 ] [ 6 ]
A Punnett square for one of Mendel's pea plant experiments - self-fertilization of the F1 generation, shows that inheritance is particulate , not blending.