Define the Lyon hypothesis.

Verified step by step guidance

1. The Lyon hypothesis, also known as X-inactivation, is a process that occurs in female mammals where one of the two X chromosomes is randomly inactivated during early embryonic development.
2. This inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell, balancing the dosage of X-linked genes between males and females.
3. The inactivated X chromosome condenses into a structure known as a Barr body, which is transcriptionally inactive.
4. The choice of which X chromosome (maternal or paternal) to inactivate is random in each cell, leading to a mosaic pattern of X-linked gene expression in females.
5. This hypothesis was proposed by Mary F. Lyon in 1961 and explains phenomena such as the coat color patterns in calico cats, which are female and exhibit different colors due to X-inactivation.

Key Concepts

Lyon Hypothesis

The Lyon hypothesis, also known as X-inactivation, is a genetic phenomenon that occurs in female mammals where one of the two X chromosomes is randomly inactivated during early embryonic development. This process ensures dosage compensation between males (who have one X chromosome) and females (who have two). The inactivated X chromosome condenses into a structure called a Barr body, which is largely transcriptionally inactive.

Dosage Compensation

Dosage compensation is a biological mechanism that balances the expression of genes on sex chromosomes between males and females. In mammals, this is achieved through X-inactivation, where one X chromosome in females is silenced to prevent an overexpression of X-linked genes. This ensures that both sexes have a similar level of gene expression from their X chromosomes, which is crucial for normal development and function.

Barr Body

A Barr body is the inactive X chromosome in a female cell, which becomes highly condensed and transcriptionally inactive after X-inactivation. This structure can be observed in the nuclei of female somatic cells and serves as a visual marker of X-inactivation. The presence of Barr bodies is a key feature in understanding the Lyon hypothesis and the mechanisms of dosage compensation in mammals.