Results

Only 68 of 332 crosses had data for a second brood and we thus used only one brood from each cross in all analyses to avoid unequal sample size issues. Where two broods were available, we used the brood with the largest size (typically, but not necessarily, the first produced). To avoid imprecise estimates of brood sex ratio, we kept only crosses with brood size >11 for analyses; this led to eight crosses being dropped for a total sample size of 326 crosses (32 parental, 71 F1 and 223 backcrosses).
Phenotypic variance increased substantially within one generation of crosses between selected lines, even within the same selection type and block, and phenotypic variance in F1s was similar regardless of parental selection line types and for both blocks (Table 2). Phenotypic mean values for F1s and backcrosses fell at the midpoints of parental lines and of parental and F1 lines, respectively, as expected for a polygenic inherited trait (Figure 1).
The observed variance in brood sex ratio using all crosses combined was 0.038 and significantly outside the bounds of binomial expected variance (randomization median variance = 0.0055; 95% CI: 0.0046-0.0065); in 5000 simulations a variance as large as that observed did not occur (Figure 2). This result clearly indicates that sex is not inherited as a simple dichotomous trait, as is the case in organisms with a sex chromosome.
In addition, brood sex ratio distribution by generation data show that while variation decreases in brood sex ratio during selection as F1 lines are crossed and backcrossed, observed variance in brood sex ratio far exceeds the range of the expected variance based on a null model of binomial trait inheritance and controlling for observed brood sizes (Figure 2). Similarly, as the selection lines interbreed, while the brood sex ratio distribution goes from bimodal to unimodal, the curve remains flattened with fewer observations of brood sex ratio having 0.4 to 0.6 proportion male than expected and more with proportion male <0.4 and >0.6 than expected under binomial inheritance (Figure 2).