(For further details of the process, check out my post on Nessa Carey's book Junk DNA.) In short, a long non-coding RNA known as Xist (X-inactive specific transcript) turns off one copy of the X chromosome in each cell on the opposite strand of DNA, a gene called Tsix is found. The specifics of how this happens is actually pretty amazing. In organisms like the fruit fly, the XY males just double the expression of the genes on the X chromosome. In mammals, this involves silencing one of the X chromosomes in each cell with the XX genotype. Organisms have different ways to level the compensate for these differences. If an XX individual expressed all the genes on both X chromosomes, they would have a huge imbalance in protein expression in comparison with someone with XY. In contrast, the X chromosome contains lots of protein-encoding genes. The Y chromosome is rather small and does not encode many genes. Somatic chromosomes come in even pairs, but the sex chromosomes are not created equal. The patterns observed in calico and tortoiseshell cats can be explained by a phenomenon called X chromosome inactivation. If fur color followed the rules of Mendelian genetics, you would expect cats to be only black and white or orange and white, not two or three colors at once. The genetics of the tortoiseshell/calico cat The gene for white spotting (S) has been linked to the KIT locus more recent publications have shown that a retroviral insertion in the KIT oncogene caused white spotting, with a full insertion leading to the recessive all white mutation. However, recessive versions of the MC1R allele create orange/amber fur color in Norwegian forest cats. A recent study of the Syrian hamster, which shows sex-linked inheritance of yellow fur color, described the sex-linked yellow gene ( Sly) as being independent of MC1R function and not likely homologous to the Orange gene in cats. Interestingly, the MC1R gene, which has been linked to red hair in humans and other mammals, is not involved in the coloring of calico cats because it isn't on the X chromosome. The Orange gene has not yet been identified, but its position on the X chromosome has been mapped. There are rare cases of male cats with Klinefelter syndrome (XXY) that can have these fur color variants.Īgouti (gene name ASIP) encodes a protein that inactivates the melanocortin receptor and thus controls the distribution and amount of pigment cells (melanocytes) in the hair. Because the Orange gene is on the X chromosome, most torties and calico cats are female. Based on the combined expression of these genes, some cats are tortoiseshell, a mix of black and orange and some cats are calico, a mix of black, orange, and white. For this discussion, I will focus only on the gene loci that create the calico pattern: agouti (A), orange (O), and spotted (S). In other mammals, white fur color alone has been linked to at least six genes ( MITF, EDN3, EDNRB, PAX3, SOX10, and SNAI2). There are many different genes that create the spectrum of coat colors and patterns found in domestic cats. The genetics of cat fur color (like the genetics of human hair color) are rather complex. While researching the post, I found myself in another internet rabbit hole and I wanted to share some of the things I learned about cat genetics because, it turns out, calico cats are an excellent lesson in genetics. Nettie Stevens is known for her discovery of the X and Y chromosomes as the basis for sex determination. In an earlier post, I detailed the life and work of Nettie Maria Stevens, the namesake of our new calico cat.
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