Blending inheritance

Example of blending inheritance using the color of flowers to show how a species color variation would converge upon one color in relatively few generations if its offspring's color variations were truly bounded by the parent's colors.

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). It took the experiments of 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.

Darwin himself also had strong doubts of the blending inheritance hypothesis, despite incorporating a limited form of it into his own explanation of inheritance published in 1868, called pangenesis. Not least of all, his objections likely arose because it conflicted with his own theory of natural selection. This incompatibility was also 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.

Moreover, prior to Jenkin, Darwin expressed his own distrust of blending inheritance to both T.H. Huxley and Alfred Wallace. In a letter to Wallace, dated February 6, 1866 (coincidentally, this was the same year Mendel formally published the aforementioned article), Darwin mentioned conducting hybridization experiments very similar to Mendel's, with pea plants no less, to prove to himself that blending inheritance did not work as a model for inheritance in certain varieties of species:

"... 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..."

Also, in an earlier letter to Huxley, Dated November 12, 1857, nearly nine years before the letter to Wallace, Darwin expressed his beginnings of doubt regarding the idea of blending inheritance:

"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."

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).