Abstract
Samples of natural populations of Drosophila pseudoobscura were taken in certain localities in Caiifornia and in Arizona. The laboratory-bred progeny of the wild flies were utilized to make crosses, the offspring of which had inbreeding coefficients of F = 0, 0.125 and 0.250. Batches of 50 eggs (in one of the experiments with replications) were taken from each cross, and the numbers of adults developing from these eggs were recorded. As expected, the percent survival was lower in inbred than in outbred progenies. The A and B statistics were computed; the B/A ratios varied from 4.5 to 5.8. The estimates of the genetic loads turned out to be no less than 0.5 and no more than 0.9 lethal equivalents per average gamete. Series of crosses utilizing appropriate genetic markers in the second, third, and fourth chromosomes were then made in order to estimate the proportions of the lethal and semilethal chromosomes and the mean viabilities of the homozygotes and heterozygotes for these chromosomes. The estimate of the genetic load derived with the aid of this method comes to about 1.4 lethal equivalents per average gamete. It is suggested that an appreciable fraction of the genetic load is balanced, and thatmore » the components of the genetic load tend to act synergistically when brought to manifestation in homozygous condition. (auth)« less
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