Question

explain what X inactivation is in Calico cats and how it could affect gene expression in a multicellular organism.

Answer 1

In XX-XY system of sex determination, females have 2 X chromosomes whereas males have only 1 X chromosome. So, the gene products of X-linked genes in females is twice the amount that of in males. To overcome this problem, X chromosome inactivation takes place.

According to Lyon hypothesis, in each cell of female body, one of the X chromosome is inactivated randomly and if a cell contains more than 2 X chromosomes, then only 1 X chromosome remains active and others are inactivated. The inactivated X chromosomes become barr bodies.

Mechanism

The cell decides how many X chromosomes are to be inactivated and only 1 X chromosome remains active. In the inactivated X chromosomes, the inactivation is brought about by by a gene called Xist (X-inactivation specific transcript). The Xist gene is active in those X chromosomes, and produces a RNA that coats the X chromosome and changes the chromatin structure. On the active X chromosome, the activity of Xist gene is repressed so that the chromosome remains active.

Effect on gene expression in multicellular organisms

The X chromosome inactivation occurs randomly that makes the cells functionally hemizygous. However, if a X-linked gene is heterozygous, then 50% of the cells will express one allele and rest 50% of the cells will express another allele. This results in a patchy or mosaic distribution of expression of alleles which are known as calico or tortoiseshell cats.

In cats, gene for coat colour has black and orange alleles. Homozygous conditions produce either black or orange phenotypes but heterozygous condition produces mosaic or calico phenotype i.e. several patches of black and orange colour in coat is observed.

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