In humans and other animals, sex is usually determined by a single gene.
that in some species, such as platyfish, it takes a whole 'parliament' of genes acting together to determine whether offspring develop as a male or female., we took a close look at these claims. We found they describe abnormal situations, such as hybrids between two species with different sex-determining systems, or when one sex system is in the process of replacing another.
We conclude that sex is normally determined by a single gene. Evolutionary theory suggests this is the most stable state of affairs, as it ensures a 1:1 ratio of male and female animals.In mammals, females have two X chromosomes, whereas males have an X and a Y. The Y chromosome bears a gene called SRY, which acts as a"": an XY embryo, carrying SRY, develops into a biological male, and an XX embryo, lacking SRY, develops into a biological female.
This makes the inheritance of sex simple. Females make eggs, which carry a single X chromosome, while males make sperm, half carrying an X and half carrying a Y.Sex in other vertebrates Among animals with backbones , there is a huge variety of systems that determine sex. However, they usually come down to the action of a single gene., in which a male-dominant gene on the Y chromosome directs testis development. Some vertebrates have the opposite – a female-dominant gene on the X chromosome.. In birds, males have two copies of a Z chromosome with the sex-determining gene DMRT1. Females have a single Z and a W chromosome that lacks DMRT1.
These single-gene systems deliver equal numbers of males and females, which theory says is the optimal balance for a stable system. If the ratio favors one sex, individuals that produce more of the other sex will leave more descendants and their genes will spread until a 1:1 ratio is achieved.
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