Question

Why do XY males have biallelic expression of some sex-linked genes?

Answer 1

Biallelic expression means expression of both the alleles of the same gene. Normally during gene expression only one of the two copies (alleles) of the gene is expressed (monoalleli expression), but in bialleleic both the alleles are expressed. mammalian females have 2 X chromosomes out of which one X chromosome is inactivated during development of female embryos to balance the X-linked gene dosage between the sexes. This process is termed as X-chromosome inactivation. Males have one X and one Y chromosome. In males, inactivation of X-chromosome also occurs. This phenomenon is termed as meiotic sex chromosome inactivation as it occurs in the late stages of first meiotic prophase during spematogenesis. Several of the genes that escape X-chromosome inactivation in female somatic cells are those that have Y-linked homologos in the male such that there is normally expression of two copies of these genes in XY male cells. 86% of miRNA genes in males escape meiotic sex chromosome inactivation as these genes are critically required for the meiotic or postmeiotic phases of spermatogenesis.