Most animal species and many plants are dioecious, meaning that there are two sexes, female and male. Sexual identity develops by genetically regulated pathways, but not by genes alone is any trait determined. Two principles are at work here. First, an environmental or genetic trigger channels development in a male or female direction. Second, the trigger initiates a cascade of gene-environment interactions that ends in the expression of mature sexual traits.
Species with XX females and XY males have a problem: both sexes have two copies of each autosomal gene, but females have twice as many copies of X-linked genes as do males. Dosage compensation solves the problem.
This chapter surveys mechanisms of sex determination and then compares both sex determination and dosage compensation in mammals and Drosophila.
Triggers for Sex Determination
Sex determination can be initiated by environmental factors, including temperature, chemical signals between individuals, and amount of light.
The developmental trigger in some fish and reptiles is incubation temperature. In some turtles, eggs incubated at above 30°C develop into females; eggs incubated at cooler temperatures develop into males. In other turtles, females develop at extreme incubation temperatures and males at intermediate incubation temperatures. Perhaps temperature-sensitive transcription factors bind to the enhancers or promoters of sex-determining genes.
Sex is determined by short-range chemical signals in some mollusks (e.g., the slipper limpet) and a marine worm Bonellia.