Based on the information in Problem and the analysis of the phenotypes of single-and double-mutant strains, a model for sex determination in C. elegans has been generated. This model proposes that the her-1 gene controls sex determination by establishing the level of activity of the tra-1 gene, which, in turn, controls the expression of genes involved in generating the various sexually dimorphic tissues. Given this information,
(a) does the her-1 gene product have a negative or a positive effect on the activity of the tra-1 gene?
(b) What would be the phenotype of a tra-1, her-1 double mutant?
The identification and characterization of genes that control sex determination has been a focus of investigators working with C. elegans. As with Drosophila, sex in this organism is determined by the ratio of X chromosomes to sets of auto-somes. A diploid wild-type male has one X chromosome and a diploid wild-type hermaphrodite has two X chromosomes. Many different mutations have been identified that affect sex determination. Loss-of-function mutations in a gene called her-1 cause an XO nematode to develop into a hermaphrodite and have no effect on XX development. (That is, XX nematodes are normal hermaphrodites.) In contrast, loss-of-function mutations in a gene called tra-1 cause an XX nematode to develop into a male. Deduce the roles of these genes in wild-type sex determination from this information.
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