X chromosome inactivation in calico cats




X chromosome inactivation causes a mosaic tissue effect in calico cats. what makes this female calico cat "calico." Like all mammals, this cat began her life as a single cell. That cell had two X chromosomes, one from each parent. One of the chromosomes carried a dominant allele for the brownish-black pigment melanin. The other carried an allele that specifies rust-colored fur. The cell divided to produce an early embryo. In some of the embryonic cells, the X chromosome carrying the dominant allele condensed to form a Barr body. In other cells, the X chromosome with the allele that specifies rust-colored fur was inactivated. In all descendents of those cells, the corresponding parental chromosome also became inactivated. When we look at the adult cat, we see the descendents of embryonic cells in which the X chromosomes were first inactivated. Cells in the rust-colored areas are descended from cells in which the chromosome carrying the dominant allele was inactivated. Cells in areas where the fur is black are descended from embryonic cells in which the other X chromosome was inactivated. The white areas result from a gene interaction with a spotting gene that blocks melanin production completely.



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