As Schrödinger so famously demonstrated, whenever one is illustrating fundamental scientific principles, the optimum choice for such an illustration is a cat. In that spirit, I’d like to present one of my favorite examples of how fundamental biology phenomena are visible in our everyday life.
Some background: Gene copy number is important. Variations in gene copy number are, perhaps, a subtle sort of problem — having 50% more or less copies of a gene available for expression is conceivably a minor thing in the biochemical world, where feedback loops regulate gene expression to increase or decrease as necessary. (That’s why so many disorders are recessive; as long as one functional copy of a gene exists, things seem to work fine.) Nevertheless, the duplication or deletion of entire chromosomes has a severe effect. Within the autosomes (non-sex chromosomes), no cases of chromosome loss are viable. The only extra chromosome that is mild enough to be viable in humans is trisomy 21, which causes Down syndrome. Chromosome 21 is the smallest autosome.
Because of this, when it comes to the sex chromosomes, mammals are faced with a copy number problem. Males (XY) have only one copy of the X chromosome, while females (XX) have two. The ways biology addresses this issue is called “dosage compensation“. In mammals, dosage compensation is achieved by randomly inactivating all but one X chromosome in all cells. Thus, regardless of an animal being male or female, only one X chromosome is active in any given cell.
This X-inactivation occurs early in embryonic development. Once a cell has decided to inactivate a given X chromosome, that decision is inherited by all its daughter cells. As a result, female mammals exist as a “mosaic” of X-inactivations — in their bodies, whole patches of tissue have one or the other X inactivated.
An interesting consequence of X-inactivation is that, unlike genes on other chromosomes, only one allele of a gene on the X chromosome is expressed in any given cell. This phenomenon is easily visible in tortoiseshell cats — tortoiseshell coloration arises from X-inactivation, so these cats are almost always female. The coat color gene, which has alleles for orange or black coats, exists on the X chromosome. Because of X-inactivation, only one of the two genes is active in various patches of skin, giving rise to a pattern of orange and black patches. Since the process of X-inactivation is random, this pattern of patches is random.
I love this example of X-inactivation so much, I added a picture of a kitty to the wikipedia page on X-inactivation. It’s always cool to have a everyday visualization of what would otherwise be an abstract genetic and developmental phenomenon.