Question

Are the sex chromosomes (for humans X and Y ) expressed in all somatic cells? Or just those cells of organs that are related somehow to sexual phenotype (primary and secondary sexual characteristics)?

Answer 1

First thing first: There are genes on chromosomes and the genes are expressed. All genes on a chromosome are not expressed at one go.

Explanation:

• The X chromosome has hundreds of genes on it and many of these are controlling characters related to organs other than the reproductive system.
• For example, gene for colourblindness present on X, is responsible for maintenance of photoreceptors in healthy cone cells of retina.
• Then there are genes on X chromosome which allow hepatic cells to produce clotting factors ; hence mutation in such X linked genes cause hemophilia.
• So, It can not be said that genes on X chromosome are expressed only in cells associated with reproductive system; some of these genes are actually expressed in various somatic tissues of both sexes through entire life.
• That is not entirely true for genes present on Y chromosome, because Y chromosome is present only in males: i.e. Y linked genes are absent in all female cells .
• There is one AMELY gene on Y, that codes for extracellular enamel protein amelogenin which expresses during development of tooth enamel, a nonreproductive tissue but this is rather an exception.
• Most of the Y-linked genes are expressed in organs of male reproductive system such as testes, prostate gland, etc.
• Expression of SRY gene of Y chromosome initiates sex differentiation very early in embryonic life.
• So, expression of genes on Y chromosome are restricted in male reproductive organs only, and some of these genes express for only a brief period of embryonic life.