Abstract
Two hundred second chromosomes were extracted from a Japanese population in October of 1972, and the viabilities and productivities of homozygotes and heterozygotes from them were examined. Viability was measured by the Cgamma method and productivity by the number of progeny produced per female. The frequency of lethal-carrying chromosomes was 0.1315. When the average heterozygote viability was standardized as 1.000, the average homozygote viability was 0.595 including the lethal lines, and 0.866 excluding them. The frequency of recessive sterile chromosomes among 131 non-lethal lines was 0.092 in females and 0.183 in males. There were two instances in which homozygosis for the second chromosome caused sterility in both sexes, which was close to the number expected (2.2) on a random basis of 0.0092 S 0.183 X 131. When the average heterozygote productivity of 200 lines was standardized as 1.000, the average homozygote productivity was 0.532 including female steriles, and 0.584 excluding them. The ratio of detrimental load to lethal load was 0.383, while the ratio of partial sterility load to complete sterility load was 7.767. The average viability of lethal heterozygotes was slightly, but not significantly, lower than that of lethal-free heterozygotes, while the average productivity of lethal heterozygotes was significantly lower than that of lethal-free heterozygotes. There was a significant association of sterility in either sex with low viability of homozygotes. However, no statistically significant differences in viability and productivity were detected between sterile heterozygotes and non-sterile heterozygotes. The heterozygous effects of viability and productivity polygenes were examined by regressions of the heterozygotes on the sum of corresponding homozygotes. The regression coefficients were slightly positive for both viability and productivity if lethal and sterile chromosomes were excluded. The correlation between viability and productivity in homozygotes was significantly positive when sterile chromosomes were included, but the significance disappeared when the sterile chromosomes were excluded. In the heterozygotes there were no detectable correlations between them.
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