Genetics Research: A master switch determines sex in animals, new research says.

Melbourne:Sex in humans and other animals is usually determined by a single gene. However, there are claims that in some species, such as the platyfish, an entire “parliament” of genes work together to determine whether the offspring will develop into males or females. In a new analysis, scientists took a closer look at these claims. We found that they describe unusual situations, such as hybrids between two species with different sex-determination systems, or when one sex system is in the process of replacing another. They came to the conclusion that sex is normally determined by a single gene. Evolutionary theory states that this is the most stable condition, as it ensures a 1:1 ratio of males and females.

Human ‘master switch’ for gender
In mammals, females have two X chromosomes, while males have one X and one Y. The Y chromosome contains a gene called SRY, which acts as a “master switch”: an XY embryo, containing SRY, develops into a biological male, and a XX embryo, lacking SRY, develops into a male. Develops into a biological female. This simplifies inheritance of gender. Females make eggs, which contain a single X chromosome, while males make sperm, half with X and half with Y. The union of eggs and sperms produces half XX females and half XY males in a 1:1 sex ratio.
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one gene theory
In other vertebrate animals with a sexual backbone (backbone), there is a large variety of sex-determining systems. However, they usually depend on the action of a single gene. Many fish, frogs and some turtles have systems similar to ours, in which a male dominant gene on the Y chromosome directs testicular development. Some vertebrates have the opposite – a female-dominant gene on the X chromosome. Other vertebrates use a single gene difference. In birds, males have two copies of a Z chromosome with the sex-determining gene DMRT1. Females with only one Z and one W chromosome lack dmrter1. Sex depends on DMRT1: two copies in Zw males, versus one in Zw females.
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master switch in each type
Intriguingly, many different genes act as master switches in different species. But they all act by triggering the same male or female differentiation pathway. These single-gene systems provide an equal number of males and females, which theory says is the optimal balance for a stable system. If the ratio favors one sex, individuals who produce more of the other sex will leave more descendants and their genes will spread until a 1:1 ratio is achieved. Some exceptional species Some aquarium fish have more complex systems. Genetic crosses in platyfish show two or more genes that determine male or female development; It appears that sea bass have at least three sex genes.

how in frog and lizard
Some frogs and lizards use two or more genes to determine sex. There are also some species that have two or more pairs of sex chromosomes. The platypus has five X and five Y chromosomes. In our recent paper we examine classic examples and recent claims of polygenic sex determination. We conclude some systems represent unusual and transient conditions. Multiple sex chromosomes does not mean multiple sex genes. In the platypus, all five Y chromosomes run together in the sperm, and a gene on the smallest Y directs male development.
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two sex genes
In many systems, two sex genes are detected, but they control different steps of the same pathway that is controlled by a single master gene. In some classic fish systems, such as the platyfish, the different variants all arise from the same chromosome, suggesting that sex is controlled by different variants of the same gene. A Japanese frog has different sex chromosomes on different islands, but they are all variants of the same chromosome.

New System in the European Frog
Other examples suggest transition. The C bass shows different frequencies of variations over its range. A European frog is slowly showing signs of a new system replacing the old one. The zebrafish is particularly interesting. Strains bred freely in laboratories for 30 or 40 years have abnormal sex ratios and multiple sex genes. But it turns out that wild zebrafish have a regular ZW sex chromosome system. The Lab stock independently lost its W chromosome during lab breeding. All lab fish are ZZ, and the sex of hatchlings is determined by weak sex-differentiating genes that were in the background.