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 bound 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.
Blending inheritance Wikipedia
The shortcomings of the blending inheritance model were not completely lost to every 19th century thinker, despite the predominance of this hypothesis at that time. In fact, these inadequacies made for an atmosphere in which many equally unconvincing 19th century "arm-chair" hypotheses were formulated and circulated in attempts to explain inheritance more adequately (see inheritance of acquired characters, maternal impression, telegony, preformationism, Geoffroyism, Lamarckism). Pangenesis was Charles Darwin's Lamarkian attempt to explain inheritance. But despite Darwin's misguided Lamarkian leanings, he rightly had strong doubts about the blending inheritance hypothesis, as evidenced in his private correspondence:
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 that were not unlike those done by Gregor Mendel:
"... 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..."
Moreover, Darwin's reservations regarding blending inheritance were further reinforced by the inherent conflict this hypothesis had with Darwin's theory of evolution by natural selection, published in his seminal work On the Origin of Species (1859). This incompatibility was best summarized by Fleeming Jenkin in his critique entitled "Review of 'The origin of species'" published in The North British Review, June 1867. As a staunch defender of blending inheritance, Fleming attempted to criticize Darwin's proposed process of natural selection, a very slow and gradual process, by noting that any favorable trait that might arise in a lineage would have "blended away" (via blending inheritance) long before natural selection had time to work.
Blending inheritance (and the many other competing hypotheses) was, ultimately, dismissed by Gregor Mendel's proposal of theory of particulate inheritance, presented in Experiments on Plant Hybridization (1865), which remains the basis of the inheritance model in the modern evolutionary synthesis.
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).