Surprisingly, there are some human females who show a rather similar calico pattern that you can actually see on their skin. But it is not revealed as a patchwork of colors. No, you will never see a woman with the distinctive skin patchwork coloration of a calico cat walking down the street.
However, for a very small number of women, if you were to look closely on a hot day, you would see a calico pattern appear on their skin. Not patchworks of colors, but two types of skin — skin that either does or does not sweat. On a hot day you could literally see a calico type patchwork of wet and dry areas on the skin of these women. And, like the calico fur, this is only seen in one sex – women only. This is a rare female disorder called anhidrotic ectodermal dysplasia.
What might explain this calico pattern of fur colors seen only in female cats and the calico patches of skin (with or without sweat) seen on women with this condition? What is it about being female that might generate such calico patterns? In both cats and humans, the cause can be traced to a manifestation of the fundamental chromosomal difference between the sexes – females have two X chromosomes (XX) while males only have one (XY). Let’s see how having two X chromosomes can lead to a calico patchwork.
Men get the one X chromosome that is in each of their cells from their mother (they always get a Y from their father, never an X). In contrast, women have two X’s in each of their cells. Women get one X chromosome from their mother, and another X from their father. But there is a problem. Two active X chromosomes in one cell would lead to conflicting genetic instructions, so this is prohibited by women’s biology. Since only one X chromosome can be active in each cell the second X must be «switched off.» But which one? The X she got from her mother, or the X she got from her father?
In this respect, nature believes in equal representation of the sexes. A few weeks after conception, one of the two X chromosomes in each cell of a female’s body is randomly deactivated. As each of these cells in the developing fetus multiplies, its descendant cells all have the same X chromosome activated. This leads to a patch of cells that all have the same active X chromosome (say, the X from the mother). A different fetal cell may have randomly deactivated the mother’s X chromosome, and so all of its descendant cells each have the X chromosome from the father.
You can probably now see where this is leading. The fur color of calico cats is determined by alleles on the X chromosome. To simplify this discussion a bit, we’ll ignore the white fur color for now, and just discuss the alleles that code for either the orange or black fur color on calico cats.
Say the X chromosome from the mother has an allele for orange fur, while the X chromosome from the father has an allele for black fur. In early fetal development, the random deactivation of one of the X chromosomes in each cell leads to two different cell lines, and we end up with a female calico cat with a patchwork of these fur colors. You can literally see the patches of cells that have an X from one parent, and a different set of cells that have an X from the other parent (although without genetic testing, we don’t know which color came from which parent).
Not so for the male cats. Because the males got their X chromosome in each of their cells from their mother, all of their cells have the same allele for fur color, and they are basically entirely one color, never a patchwork of different colors.
Now, apply this calico pattern to all of the cells in the female body. Females, both in their bodies, and their brains, are a patchwork of two different types of cells – those that have an X chromosome they got from their mother and those with an X chromosome from their father. Females are thus «genetic mosaics.» This is remarkable. There is nothing equivalent to it in males.
Now imagine that we could image the brain with some type of brain scanner so that all of the neurons with an X from the father show up as blue on the screen, and that all the neurons with an X from the mother show up as pink. What color(s) would men’s brains be?
Men’s brains would appear on the imaging screen as entirely one color — all pink (all of their X chromosomes are from their mother — remember, they never get an X from their father, only a Y).
What would women’s brains look like on the imaging screen? Yes, their brains would appear as a patchwork of colors – with patches of pink and blue showing up throughout the brain. So in this situation, what would a woman’s brain resemble? Yes, her brain would appear with a patchwork of colors similar to the fur of a calico cat!