Dosage Compensation in Humans

Dosage compensation is a genetic mechanism that equalizes the expression of genes on the X chromosome between individuals with different numbers of X chromosomes, such as males (XY) and females (XX). In humans and other mammals, this is achieved primarily through X-chromosome inactivation (also known as Lyonization) in females.

X-chromosome Inactivation Explained

In females, one of the two X chromosomes in each somatic cell is randomly inactivated early in embryonic development. This inactivated X chromosome condenses into a compact structure called a Barr body.

• This process ensures that females, who have two copies of X-linked genes, express them at a similar level to males, who have only one copy.
• The choice of which X chromosome (maternal or paternal) to inactivate is generally random in each cell.
• Once a cell undergoes inactivation, all its descendant cells will inactivate the same X chromosome.

Analyzing the Options

Let's examine each statement based on our understanding of dosage compensation in human females:

1. X-chromosome inactivation in female occurs in a zygote immediately after fertilization.

   This statement is incorrect. X-chromosome inactivation does not happen immediately after fertilization in the zygote. It occurs later, during early embryonic development, after several cell divisions have taken place.

2. X-chromosome inactivation is non-random, in some individuals maternal X is inactivated while in others paternal X-chromosome is inactivated.

   This statement is generally incorrect for most somatic tissues. While there can be exceptions (e.g., imprinted inactivation of the paternal X in the placenta), X-inactivation in the embryo proper is typically a random process. In different cells within the same individual, either the maternal or the paternal X chromosome is randomly chosen for inactivation.

3. Y-chromosome of males is seen as Barr body.

   This statement is incorrect. A Barr body is the inactivated X chromosome found in the somatic cells of females and individuals with more than one X chromosome. The Y chromosome does not form a Barr body.

4. The body of female is a mosaic of cells, some having paternal X- and others having maternal X-inactivated.

   This statement is correct. Due to the random nature of X-chromosome inactivation, different cells in a female's body will have inactivated different X chromosomes. Some cells will have the maternal X active and the paternal X inactivated, while other cells will have the paternal X active and the maternal X inactivated. This makes the female body a genetic mosaic for X-linked genes.

Mosaic Nature of Female Body

The random inactivation of either the maternal or paternal X chromosome in different cells leads to a mosaic pattern of gene expression in females. For X-linked traits, a female might express the allele from her maternal X in some cells and the allele from her paternal X in other cells, depending on which chromosome was inactivated in those cell lineages.