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March 25, 2009 | By:  Rachel Davis
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X-chromosome inactivation in the calico cat

Science is so much easier to understand when you can see it. When I was told that an egg, the kind you eat for breakfast, is one large type of cell, it made my first exposure to cell biology easier to assimilate. The calico cat is a cute natural phenomenon that illustrates the important scientific concept of X-chromosome inactivation. Calico cats are always female. The calico cat displays a mixture of red- and black-based colors, depending on which of its two X chromosomes has been inactivated. The allele coding for orange hair color is dominant, while the allele for black color is recessive - although this doesn't matter much since only one allele is expressed. A separate gene for spotting results in the presence of white patches.

X-chromosome inactivation is the answer to a genetic dilemma stemming from the effects of gene copy number on the phenotype of an organism. If the genes represent blueprints for protein synthesis, then one can imagine that more blueprints allow more architects to erect more buildings. Thus the number of RNA transcripts that a gene spawns is proportional to the number of copies of that gene in the cell. There are two copies of each gene on autosomes (non-sex chromosomes), but the sex chromosomes present an exception to the rule. While female mammals have two X chromosomes, males have 1 X and 1 Y chromosome. The X and Y chromosomes are different in nature: while the mammalian X chromosome contains ~1000 genes, there are very few genes on the Y chromosome. The genes on the Y chromosome are only required for the development of males.

Females would therefore produce two times as much transcript in comparison to males if no mechanism had evolved to correct the imbalance. The mechanism which has evolved is known as dosage compensation: random inactivation of one of the X chromosomes in each cell at an early stage in development. This pattern of random inactivation is then passed on to all progeny cells. The same phenomenon is observed in other species, in different manifestations. In Drosophila, females have two X chromosomes and males have one X and one Y. The dosage compensation mechanism invoked here is hypertranscription, where males transcribe more copies of the genes on their single X chromosome. In C. elegans, dosage compensation takes the form of hypotranscription. Since females have two X chromosomes and males have only one X chromosome, the number of RNA transcripts derived from each X chromosome in the female is reduced by half.

This inactivated chromosome in mammals in known as the Barr body. Under the microscope, the Barr body is observable in the nucleus due to its dark-staining, highly condensed, heterochromatic structure. Most genes on the inactivated X chromosome are silenced by DNA hypermethylation, representing an example of epigenetic inheritance. In the germ-line, the inactivated X chromosome becomes reactivated during oogenesis.

For more, see Introduction to Genetic Analysis by Griffiths et al. 9th ed. (2008)

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