Chapter 20, Problem 24P

In C. elegans there are two sexes: hermaphrodite and male. Sex is determined by the ratio of X chromosomes to haploid sets of autosomes (X $\text{/}$ A). An X $\text{/}$ A ratio of $1.0$ produces a hermaphrodite (XX), and an X $\text{/}$ A ratio of $0.5$ results in a male (XO). In the $1970\text{s}$, Jonathan Hodgkin and Sydney Brenner carried out genetic screens to identify mutations in three genes that result in either XX males ( $\text{tra-1}$, $\text{tra-2}$) or XO hermaphrodites ( $\text{her-1}$). Double $-$ mutant strains were constructed to assess for epistatic interactions between the genes (see table). Propose a genetic model of how the her and tra genes control sex determination.