True about Lyonisation of X chromosome:
**Core Concept**
Lyonisation refers to the random inactivation of one of the two X chromosomes in female mammals, resulting in a mosaic pattern of X chromosome activity in somatic cells. This process ensures dosage compensation for X-linked genes and prevents a doubling of gene expression associated with having two active X chromosomes.
**Why the Correct Answer is Right**
During embryonic development, the X chromosome undergoes a process called X-chromosome inactivation, also known as Lyonisation. This process involves the formation of a Barr body, which is a compact, heterochromatic structure that represents the inactivated X chromosome. The inactivation is random, meaning that either the maternal or paternal X chromosome can be inactivated in any given cell. This random inactivation results in a mosaic pattern of X chromosome activity, where some cells have an active maternal X chromosome and some cells have an active paternal X chromosome.
**Why Each Wrong Option is Incorrect**
**Option A:** Lyonisation is not specific to the X chromosome; it can occur on any chromosome that is subject to dosage compensation.
**Option B:** Lyonisation is not a type of genetic mutation; it is a normal process that occurs during embryonic development.
**Option C:** Lyonisation is not a result of X-chromosome duplication; it is a mechanism to prevent the doubling of gene expression associated with having two active X chromosomes.
**Clinical Pearl / High-Yield Fact**
Lyonisation is a critical process that ensures dosage compensation for X-linked genes and prevents genetic disorders associated with X-chromosome anomalies. It is essential for understanding the genetic basis of various diseases, including those caused by X-chromosome abnormalities.
**Correct Answer: D. Lyonisation is a normal process that occurs during embryonic development to ensure dosage compensation for X-linked genes.**