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/A). An X/A ratio of 1.0 produces a hermaphrodite (XX), and an X/A ratio of 0.5 results in a male (XO). In the 1970s, Jonathan Hodgkin and Sydney Brenner carried out genetic screens to identify mutations in three genes that result in either XX males (tra-1, tra-2) or XO hermaphrodites (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.

Genotypeᵃ XX Phenotype XO Phenotype
Wild-type Hermaphrodite Male
tra-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ Male Male
her-1ʳᵉᶜ Hermaphrodite Hermaphrodite
tra-1ᵈᵒᵐ/+ Hermaphrodite Hermaphrodite
tra-1ʳᵉᶜ tra-2ʳᵉᶜ Male Male
tra-1ʳᵉᶜ her-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ her-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ tra-1ᵈᵒᵐ/+ Hermaphrodite Hermaphrodite
ᵃrec = recessive mutation; dom = dominant mutation.

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