Lyonization

Lyonization is a biological process that occurs in female mammals. It is named after British geneticist Mary F. Lyon, who first proposed the theory in 1961. Lyonization, also known as X-chromosome inactivation or X-inactivation, is a mechanism that regulates the expression of genes on the X chromosome in a process that results in a functional dosage compensation between males and females.

Process

During the early stages of embryonic development in female mammals, one of the two X chromosomes in each cell is randomly inactivated. This process is irreversible and all cells derived from the original cell will have the same X chromosome inactivated. The inactivated X chromosome condenses into a Barr body, which is transcriptionally inactive.

The choice of which X chromosome to inactivate is random in placental mammals, but in marsupials, the X chromosome inherited from the father is always inactivated. This process ensures that females, like males, have one functional copy of the X chromosome in each body cell.

Mechanism

The mechanism of X-inactivation involves modification of the DNA and histones, making the chromosome less accessible to the transcription machinery and thus silencing it. The X-inactivation center (XIC), a region on the X chromosome, plays a key role in this process. The XIC produces a long non-coding RNA called XIST (X-inactive specific transcript) that coats the X chromosome and initiates its inactivation.

Implications

Lyonization has important implications in genetics and medicine. For example, it is the reason why females can be carriers of X-linked recessive disorders without showing symptoms. However, if a female is heterozygous for a certain gene on the X chromosome, she can be a mosaic for that trait, with some cells expressing one allele and others expressing the other allele. This can have various effects depending on the specific gene and the proportion of cells expressing each allele.

See also

• Dosage compensation
• Epigenetics
• Genetic linkage
• Sex chromosome