A cat's coloring is mostly determined by genes on their X chromosomes, which contain alleles for colors, such as black, orange, gray, and cream. The allele for white fur is located on a different gene. Calico cats, by definition, must display three different colors in their fur - white plus two of the other colors. This is easily possible in female cats, because females normally possess two X chromosomes. However, this occurs rarely in male cats, because males typically possess only one X chromosome plus one Y chromosome. What must be the genetic make-up of a male calico cat, and what type of chromosome disorder does this most resemble?

XXY, Klinefelter's syndrome

Explanation:

In humans, as well as in many mammals, biological sex is determined by a pair of sex chromosomes (X and Y) which combine to produce a female phenotype (XX) or a male phenotype (XY). In humans, the Klinefelter syndrome (KS) is a chromosomal disorder resulting from two or more X chromosomes in males. In cats, there are documented rare cases in which male cats can also inherit two X chromosomes in addition to the Y chromosome, so they are XXY instead of XY (similar to the Klinefelter syndrome in man). Also in cats, the calico-colored is a phenotype result from the inactivation of one X chromosome, which usually occurs in cat females. However, approximately one in every 3,000 male cats have the XXY condition, and these male cats may also exhibit a “calico” phenotype.