Hérédité par mélange

Blending inheritance is an obsolete theory in biology from the 19th century. The theory is that the progeny inherits any characteristic as the average of the parents' values of that characteristic. As an example of this, a crossing of a red flower variety with a white variety of the same species would yield pink-flowered offspring. Charles Darwin's theory of inheritance by pangenesis, with contributions to egg or sperm from every part of the body, implied blending inheritance. His reliance on this mechanism led Fleeming Jenkin to attack Darwin's theory of natural selection on the grounds that blending inheritance would average out any novel beneficial characteristic before selection had time to act. Blending inheritance was discarded with the general acceptance of particulate inheritance during the development of modern genetics, after 1900. 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. One of those processes was competition for resources, as Thomas Malthus had indicated, leading to a struggle to survive and to reproduce. Since some individuals would by chance have traits that allowed them to leave more offspring, those traits would tend to increase in the population. Darwin assembled many lines of evidence to show that variation occurred and that artificial selection by animal and plant breeding had caused change. All of this demanded a reliable mechanism of inheritance. Pangenesis was Darwin's attempt to provide such a mechanism of inheritance. The idea was that each part of the parent's body emitted tiny particles called gemmules, which migrated through the body to contribute to that parent's gametes, their eggs or sperms. 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. However, it had some serious weaknesses.