Blending inheritance



Many biologists and other academics held to the idea of blending inheritance during the 19th century, prior to the discovery of genetics. Blending inheritance was merely a widespread hypothetical model, rather than a formalized scientific theory (it was never formally presented to a scientific body, nor published in any scientific journals, nor ascribed to any specific person),  in which it was thought inherited traits were determined, randomly, from a range bounded by the homologous traits found in the parents. The height of a person, with one short parent and one tall parent, was thought to always be of some interim value between its two parents' heights. The shortcoming to this idea was in how it required the person of interim height, in turn, to then become one of the limiting bounds (either upper or lower) for future offspring, and so on down the entire lineage. Thus, in each family, the potential for variation would tend to narrow, quite dramatically, with each generation, and, so it would go for the entire population with every trait. If blending inheritance were true, in this example, all members of a species would eventually converge upon a single value for height for all members. In short, "blending inheritance is incompatible...with obvious fact. If it were really true that variation disappeared, every generation should be more uniform than the previous one. By now, all individuals should be as indistinguishable as clones."

In addition, blending inheritance failed to explain how traits that seemingly disappeared for several generations often reasserted themselves down the line, unaltered. Blue eyes and blond hair, for example, often could disappear from a family's lineage for several generations, only to have two brown-haired, brown-eyed parents give birth to a blond, blue-eyed child. If blending inheritance were fact, this could not be possible.

History
The obvious shortcomings, such as those mentioned above, with the blending inheritance model were not completely lost to every 19th century thinker. In fact, these inadequacies made for an atmosphere in which many lesser-known, and equally unconvincing, 19th century "arm-chair" hypotheses to be formulated and circulated in attempts to explain inheritance more adequately (see inheritance of acquired characters, maternal impression, telegony, preformationism, Geoffroyism, Pangenesis).

Because he did not know about Mendelian inheritance, Darwin himself incorporated a form of blending inheritance into his own explanations of inheritance. His theoretical claims were published in 1868, and the theory became widely known as pangenesis.

Formerly, Darwin had privately expressed his doubts about the nature of inheritance, struggling with the theoretical difficulty of explaining individual variations that would tend to blend into a population. In a letter to T.H. 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."

In a letter to Alfred Wallace, dated February 6, 1866, Darwin mentioned conducting hybridization experiments with pea plants: "... I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility; an instance I will explain. I crossed the Painted Lady and Purple sweetpeas, which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect but not intermediate. Something of this kind I should think must occur at least with your butterflies & the three forms of Lythrum; tho’ those cases are in appearance so wonderful. I do not know that they are really more so than every female in the world producing distinct male and female offspring..."

The same year, Darwin sent a letter to Carl Wilhelm von Nägeli. In reference to the "position of the leaves not having been acquired tho' natural selection", Darwin wrote: ""I can offer no explanation of such facts & only hope to see that they may be explained, yet I hardly see how they support the doctrine of some law of necessary development for it is not clear to me that a plant with its leaves placed at some particular angle or with its ovules in some particular position, thus stands higher than another plant"."

- 12 June, 1866

Two years later, in his work "The Variation of Animals and Plants under Domestication" (1868), Darwin openly embraced the idea of blending inheritance. He gave a number of examples in favor of the hypothesis, reporting the case of a man who lost part of his little finger and all his children had been said to were born with deformed little fingers, as well as the reports of Jewish children that were said to have been born with aposthia, due to the circumcision of their progenitors. In his own words, Darwin considered this as "conclusive evidence that the effects of operations are sometimes inherited" and concluded that "it must be admitted... that the effects of injuries, especially when followed by disease, or perhaps exclusively when thus followed, are occasionally inherited."

Moreover, Darwin wrote: ""In all cases in which a parent has had an organ injured on one side, and two or more of the offspring are born with the same organ affected on the same side, the chances against mere coincidence are almost infinitely great. Even when only a single child is born having exactly the same part of the body affected as that of his injured parent, the chances against coincidence are great; and Professor Rolleston has given me two such cases which have fallen under his own observation,–namely of two men, one of whom had his knee and the other his cheek severely cut, and both had children born with exactly the same spot marked or scarred.""

The problems of these ideas were noted by a contemporary critic of Darwin's, Fleeming Jenkin, in a now infamous excoriation of Darwin's Origin of Species. Jenkin, a strong proponent of the blending inheritance idea, used blending inheritance to argue against the plausibility of natural selection itself. If one was to assume that blending inheritance was at work, Jenkins argued that any favorable trait that might arise in a lineage for which natural selection could possibly work upon, would naturally be blended away from that lineage long before the much slower processes of natural selection could act upon it and improve it.

It took the experiments of the Augustinian monk Gregor Mendel, presented in Experiments on Plant Hybridization, to finally provide a better model than the one proposed by blending inheritance, and to dismiss the myriad of other speculative ideas erupting at this time. Mendel discredited blending inheritance theory by proposing the theory of particulate inheritance.

Having read Darwin's work, Mendel wrote in a letter to Carl Nägeli: ""Darwin’s statements concerning hybrids of the genera mentioned in “The variation of animals and plants under domestication,” based on reports of others, need to be corrected in many respects.""

Legacy
Blending inheritance is similar to the modern legitimate idea of incomplete dominance and the terms are rarely, but incorrectly, used interchangeably by some. However, incomplete dominance results in blending only of the phenotype, keeping the alleles within the heterozygote distinct (and, thus still inheritable in successive generations), whereas the theory of blending inheritance referred to an actual blending of the genetic material (i.e. in modern terms, alleles would blend together to form a completely new allele).