Heterosis.

In Genetics 35:420, 1950, Dr. F.G. Brieger published as footnote 2 a comment which is largely unrelated to his main paper. In this footnote, Brieger questions Richey's (J. Herd. 36:243‑244., 1946) giving Bruce priority in proposing the hypothesis of dominant favorable factors as a reason for heterosis, on the basis of Bruce's 1910 paper (reproduced in Richey, 1946).

Brieger states that Bruce "missed the essential point of the dominance hypothesis, namely the covering up, in hybrids, of all or most recessive genes by their respective dominant alleles, contributed in part by one or by the other parent." He states that it is shown by the formulae "that the mean frequency of heterozygotes in the hybrid population is inferior to the combined mean of the two parent populations." Actually this is the opposite of what the formulae show. Bruce demonstrates that the mean number of recessives (not heterozygotes) in the hybrid is less than the collective mean of the parent families. He then points out that the aggregate of the dominants and heterozygotes must be larger. It follows that the frequency of the heterozygotes must be greater, not less as Brieger concludes.

Brieger then states, "It would, however, be required that the hybrid population contains less homozygous recessives than either parent population individually, if we want to explain heterosis." Whether the number of expressed recessives in the hybrid population must be less than that in either parental population, or whether it must only be less than the number in the mean of the two parental populations, collectively, depends upon a definition of heterosis. If this term denotes a phenomenon in which the hybrid is more vigorous than the mean of the parents (a very common definition), it is only necessary that there are fewer expressed recessives in the hybrid than in the mean of the parents.

In reading my 1946 paper, I find that I have given greater priority to Bruce than I really intended. It was not my intention to belittle in any way the contribution of Keeble and Pellew (Jour. Genetics 1:47‑56, 1910) even though I seem to have done so.

F.D. Richey