GROUP INBREEDING AND COANCESTRY
I N his classical papers, "Systems of Mating" (1921 ) , SEWALL WRIGHT developed the concept of the inbreeding coefficient in terms of the correlation of uniting gametes, and, among other things, related it to the decrease in heterozygosity and applied it to systems of mating relatives. The next year (1922) he produced procedures for calculating the inbreeding coefficient and a measure of the relationships among different individuals, which was now labeled coefficient of relationship, from pedigrees. In 1931 he showed the consequences of finite population size in terms of the inbreeding coefficient and the related variation and random drift of gene frequencies. In many other papers he and other workers have further refined the theory and extended the applications to more complex situations, but these three papers serve as the bases. MALBCOT (1948) provided an alternative view of the coefficient of inbreeding, making use of the probability of genes being "identical by descent," i.e., of being copies of the same ancestral gene. His concomitant measure of the relationships among individuals, called coancestry herein, is more simply related to the inbreeding coefficient than is the coefficient of relationship. While MAL~COT'S definitions and methods must lead to the same results as does WRIGHT'S, they are generally easier to grasp and apply, requiring only simple probability arguments, for those not well versed in path coefficients.