Lyonizáció

Lyonizáció is a biological process that refers to the inactivation of one of the two X chromosomes in female mammals. This phenomenon is essential for dosage compensation, ensuring that females, who have two X chromosomes, do not produce twice the amount of X-linked gene products compared to males, who have only one X chromosome.

The process of lyonizáció begins early in embryonic development, typically around the blastocyst stage. In each somatic cell of a female embryo, one of the two X chromosomes is randomly chosen to be largely inactivated. This inactivated X chromosome condenses into a structure called a Barr body, which is a darkly stained heterochromatic mass visible within the nucleus.

The inactivation is not always permanent. Some genes on the inactivated X chromosome can escape inactivation and remain transcriptionally active. The pattern of X chromosome inactivation is also variable, meaning that in different cells, either the paternal or the maternal X chromosome can be inactivated. This mosaicism results in females expressing genes from both of their X chromosomes, albeit in different cell populations.

Lyonizáció is crucial for normal female development. Its absence can lead to developmental abnormalities. The random nature of X inactivation has implications for the expression of X-linked traits and diseases in females, as they can be either carriers or exhibit a milder form of the condition depending on which X chromosome is inactivated in specific tissues. The concept is named after the British geneticist Mary Lyon, who proposed the theory of X chromosome inactivation in 1961.