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).