The population sex ratios in the apple snail Pomacea canaliculata are nearly 1:1, but the brood sex ratios vary almost continuously from all male to all female. Here I report results of experiments to study genetics of the sex ratio variation. First, I studied regressions of the offspring sex ratio on the sex ratios of the parentsâ siblings as well as correlations in the brood sex ratios between sisters or brothers. There were significant positive relationships between the offspring sex ratio and the sex ratio of the motherâs siblings (slope = 0.28), and between the offspring sex ratios of two sisters (r= 0.41). On the other hand, the father-offspring regression (slope = 0.10) and correlation between two brothers (r= -0.13) were not significant. These patterns differed from predictions using typical cytoplasmic sex factors, sex-ratio genes or polygenic sex determination. Next, I studied the effect of each parent on the brood sex ratio, by exchanging partners among mating pairs. There were positive correlations between sex ratios of half-sib broods of the mother (r= 0.42) and the father (r= 0.47). Moreover, the correlation between full-sib broods was nearly 1 (r= 0.92). Thus, both parents contributed equally to the sex ratio variation. The most likely mechanism of the sex ratio variation involves a small number of nuclear sex-determining genes that act additively.