Because the black and orange colors are determined by joining two separate pairs of X-chromosomes, calicos are nearly always female, except in the event of an extra Y mutation on one of the pairs.
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One would hardly expect calico cats to hold the key for treating and preventing obesity in humans, known for sleeping all day and being spoiled by their owners, but their unusual black and orange color patterned coat may hold the secret. Because the black and orange colors are determined by joining two separate pairs of X-chromosomes, calicos are nearly always female, except in the event of an extra Y mutation on one of the pairs. The way the genes interact is what is most intriguing to biologists. In order to produce the pattern, both sex-linked X-chromosomes come together, pushing out other traits. In order for them to join together, one X-chromosome is silenced in the pairing of cells. The consequence is that the genes produce one phenotype over the other, preventing one color from dominating, for example. This determination results in a compromise of both.
Now, using a new technology called soft X-ray tomography which allows for three-dimensional views of cellular components, researcher Elizabeth Smith from the University of California- San Francisco is able to spot the activity of chromosomes and discovered that obesity is also determined by a similar interaction among X-chromosomes.