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.

Key Concepts

Sex Determination in C. elegans

In C. elegans, sex is determined by the ratio of X chromosomes to autosomes, denoted as X/A. A ratio of 1.0 (XX) results in a hermaphrodite, while a ratio of 0.5 (XO) leads to a male. This mechanism is crucial for understanding how genetic mutations can affect sexual phenotype and the underlying genetic pathways involved.

Gene Interactions and Epistasis

Epistasis refers to the interaction between genes where the effect of one gene is suppressed or modified by another. In the context of the tra and her genes in C. elegans, understanding how these genes interact can help elucidate their roles in sex determination. Genetic screens and double-mutant analyses are often used to study these interactions.

Mutations and Phenotypic Outcomes

Mutations in specific genes can lead to distinct phenotypic outcomes, such as the development of XX males or XO hermaphrodites in C. elegans. The tra-1, tra-2, and her-1 genes are critical in this process, and their mutations can disrupt normal sex determination pathways, leading to abnormal sexual phenotypes. Analyzing these mutations helps in constructing genetic models of sex determination.