Discussion
In this study we have used strongly selected, highly inbred biased sex
ratio lines to assess heritability of sex in a marine copepod species,T. californicus , without sex chromosomes using an incomplete
diallel cross. We show: 1) Substantive extra-binomial variation for sex,
that persists through two generations of random crosses in a controlled
laboratory environment; 2) Mean phenotypic values for sex ratio in F1
and backcrossed offspring match the midpoint of the parental values as
predicted in a normally distributed polygenic trait with many genes of
small effect; 3) Heritability for sex (on the observed scale) of 0.09
and heritability for the underlying threshold trait of 0.271.
Heritability estimates for binary traits are necessarily limited to
lower values due the nonadditive affects created by the link function
and it is unclear what the maximum observable heritability is for sex in