X chromosomes began to work twice as hard and produce twice as much protein. Sort of like a woman who raises children, runs the household, and also has a full time job. But women do not need the doubled output from two X chromosomes, so early in the development of the female fetus one of the X chromosomes in each cell shuts down. This is where calico cats come in. Mitten is a mosaic of colors.
Some areas are orange, and some are black.
The gene that produces the black color is found on the X chromosome. While Mitten was still a microscopic little ball of cells in the uterus, one or the other of the X chromosomes in each of those cells switched off. Some of the cells, and all their descendants as the fetus grows, were destined to produce black hair. The rest produced orange hair. A female cat, with two X chromosomes, has twice as much chance to have a gene for black.