The Concept of Identity of Genes by Descent

The idea that the underlying mechanism of inheritance is the transmission according to elementary laws of probability of particulate units was, of course, due to the genius of Mendel. If one of the parents of an individual only has an A gene and an offspring has an A gene, then the A gene of the offspring is a copy of the A gene of the particular parent, and the two A genes are identical by descent. Mendel knew this, even though he did not use precisely these words to express the fact. Early in the rediscovery of Mendels work, mathematicians or statisticians or biometricians whatever one wishes to call the individualswere impressed by the mathematical interest of the Mendelian system. It was realized early, of course, that inbreeding, that is, the mating of related individuals was an important tool for the understanding of genetic phenomena. It, therefore, became necessary to develop some of the theory of inbreeding. Pioneers in this work were Robbins and Jennings. But the great advance was made by Sewall Wright 7, who, it appears to me, singlehandedly developed the method of path coefficients to enable the answering of many important questions of inbreeding.