The Genetic Structure of Natural Populations

Recent reports (MUKAI et al. 1974; KATZ and CARDELLINO 1978; COCKERHAM and MUEAI 1978) have indicated that the Cy chromosome is not always dominant over its homologous chromosome with respect to viability. Thus, the genetic parameters previously estimated using viabilities determined by the Cy method are biased. In the present paper, the biases of the estimates for the polygenic mutation rate, the degree of dominance and the homozygous load are examined. The results indicate that the biases for the mutation rate and the degree of dominance are small and that the estimate of the homozygous load relative to the average viability of the population is not biased. INCE some experimental evidence suggested that the rearranged second chromcsome , Zn(2LR)SMI, which is marked with the Cy mutation, almost completely suppresses the effect of deleterious mutations located in homologous chromosome (MUKAI 1964; MUKAI et al. 1972). The object was to measure the method (WALLACE 1956) or its equivalent to estimate the relative viabilities of second or third chromosomes of Drosophila melanoigaster (MUKAI and YAMAGUCHI 1974, and earlier). However, recent careful experiments conducted by COCKERHAM and MUKAI (1978) clearly showed that the C y chromosome is not always completely dominant. Thus, it is necessary to examine how the estimates of genetic parameters such as the polygenic mutation rate, the degree of dominance and the homozygous genetic load, all of which are based on the assumed complete dominance of the Cy chromosome ( MUKAI 1964; MUKAI and YAMAZAKI 1968; MUKAI and YAMAGUCHI 1974; WATANABE, YAMAGUCHI and MUKAI 1976, etc.), are biased. Fortunately, the results of the examination show that the biases are small. This article presents the results, along with the effect of synergistic interaction (MUKAI 1969a) on the estimate of the polygenic mutation rate. * Paper No. 4 from the Laboratory of Population Genetics, Faculty of Science, Kyushu University 33, Fukuoka 812, Japan. This work was mainly supported by Grants 144001 and 348003 from the Ministry of Education, Science and Culture of Japan. Genetics 94: 169-184 January, 1980.