Introduction
This topic covers how biological sex is determined and differentiated, the genes and hormones involved, and the disorders of sex development (DSD), which include the conditions historically called male and female pseudohermaphroditism and hermaphroditism, with examples.
Definitions
- Sex determination: the biological process that establishes whether an individual will develop as male, female, or both. It is set at fertilization and influenced by genetic or environmental factors.
- Sex differentiation: the process by which the undifferentiated gonads and reproductive structures develop into male or female organs and secondary sexual characteristics, according to the initial determination.
Types of sex determination
Mnemonic "GEN CHeck SEX": Genetic, Environmental, Not nuclear (cytoplasmic), CHermaphroditism.
A. Genetic sex determination (GSD)
- Based on inheritance of specific sex chromosomes at fertilization. In humans XX = female, XY = male.
- The Y chromosome and the SRY gene are crucial for initiating male development.
- Common in mammals, birds, insects and many plants.
B. Environmental sex determination (ESD)
- Sex is determined by environmental conditions during embryonic development.
- Temperature-dependent sex determination (TSD) in reptiles: alligator eggs incubated at lower temperatures develop as females, higher temperatures produce males.
- Chemical cues or hormones can influence sex in certain fish and amphibians.
C. Cytoplasmic sex determination
- Sex is influenced by cytoplasmic factors such as endosymbiotic bacteria. Wolbachia, an intracellular bacterial genus in many insects, alters host reproduction to favour its own spread: parthenogenesis induction (females reproduce without males), feminization of genetic males, and male killing (male embryos eliminated, boosting infected female populations).
- Applications: pest and disease control, e.g. releasing Wolbachia-infected mosquitoes to reduce spread of dengue, Zika and chikungunya; it reduces mosquito lifespan and virus transmission capability.
D. Hermaphroditism (in the animal kingdom)
- An individual has both male and female reproductive organs.
- Simultaneous hermaphrodites: both sets of gonads functional at the same time (earthworms, some snails).
- Sequential hermaphrodites: protandry (male first, then female, e.g. clownfish); protogyny (female first, then male, e.g. wrasse fish).