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The orange mutant gene is found only on the X, or female, chromosome.
As with humans, female cats have paired sex chromosomes, XX, and male cats have XY sex chromosomes.
The female cat, therefore, can have the orange mutant gene on one X chromosome and the gene for a black coat on the other. If expressed, this gene codes for white, or no color, and is dominant over the alleles that code for a certain color (i.e.
orange or black), making the white spots on calico cats.
In contrast, male placental mammals, including chromosomally stable male cats, have one X and one Y chromosome.
Since the Y chromosome does not have any locus for the orange gene, there is no chance that an XY male could have both orange and non-orange genes together, which is what it takes to create tortoiseshell or calico coloring.
In contrast a non-white-spotted tortoiseshell usually has small patches of color or even something like a salt-and-pepper sprinkling.In the absence of other influences, such as color inhibition that causes white fur, the alleles present in those orange loci determine whether the fur is orange or not.Female cats — like all female placental mammals — normally have two X chromosomes.In any event, because the genetic conditions for calico coloring are X linked, a fertile male calico's coloring would not have any determination in the coloring of any male offspring (who would receive the Y, not the X chromosome from their father).As Sue Hubble stated in her book Shrinking the Cat: Genetic Engineering before We Knew about Genes, The mutation that gives male cats a ginger-colored coat and females ginger, tortoiseshell, or calico coats produced a particularly telling map.