Sexual differentiation

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Sexual differentiation is the process by which an embryo develops the traits that distinguish males and females. It begins after fertilization; at first, an embryo is the same regardless of sex. Sex determination decides which path development will follow, usually based on genes (such as the presence of a Y chromosome) or, in some species, environmental cues. Differentiation then builds the gonads (testes or ovaries), shapes hormones, and leads to the body’s internal and external sex organs, as well as secondary traits like body hair, breasts, and bone structure. In humans, most people have XX or XY chromosomes. The Y chromosome carries a gene that starts testis development, and hormones from the gonads drive the rest of the differences. Brain development and behavior are also influenced by hormones and the environment, so sex differences in these areas vary and are not universal.

Different species use different rules. Some rely on environmental factors, such as temperature or social situation, to determine sex. Some can even change sex during life, or have individuals that function as both sexes (hermaphrodites). Birds have a different chromosome system (Z and W), and in some cases sex decisions can happen at the level of individual cells. In many fish and other animals, specific genes help start male or female development, but the exact steps vary across species.

Sexual differentiation can produce more than a single male and female type, with different male morphs or hermaphroditic individuals. The brain and behavior can show sex-related differences, but these differences arise from a mix of biology and environment and are not the same in every species or situation.

In short, sexual differentiation is a flexible, species-specific process that shapes how males, females, and other reproductive forms develop.