12.8: X-linked diseases and mono-allelic gene expression

While calico spots occur only in female cats, there are a number of genetic susceptibilities that are more commonly seen in males. These arise because males have only a single X chromosome. The result is that, in contrast to the rest of the genome, genes on the X are effectively haploid in males. The result is that phenotypes associated with recessive alleles of genes located on the X chromosome are more often visible in males. In females, who are formally heterozygotic for such genes, some cells express one allele while others express the other. The female is mosaic, composed of clones expressing one or the other allele on the X chromosome.

In a typical diploid cell, it is sometimes the case that one gene is active while the other copy of the gene, on the homologous chromosome is inactive, due to stochastic "transcriptional silencing" events.^[455] In some cases of such stable monoallelic expression there is what is known as somatic selection, which we will return to. Given that there are two alleles, when they are different which one is expressed may influence cell growth, division, and even survival, so that over time, cells expressing one allele may come to dominate (in numbers) those that express the other (→) in particular regions of the body. The extent to which random monoallelic expression influences human development and disease is just now being recognized and examined carefully.

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