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