There are over 1,000 X-linked genes, including the genes for red-green color blindness, hemophilia, male pattern baldness, and body fat distribution. And if you remember your high school genetics, males have one “X” and one “Y” chromosome, and females have two “X” chromosomes.
Since females have two “X” chromosomes, only one of the X chromosomes will be expressed in any given cell. The determination of which one is expressed is random. Tortoiseshell and calico cats (all females) are the perfect example- they have a gene for orange fur on one of their X chromosomes, and a gene for black fur on the other. Their random coat patterns are due to the random expression of X-chromosomes; areas where the fur is black express the X-chromosome with the black fur gene, and areas where the fur is orange express the X-chromosome with the orange fur gene.
OK, so the cats look pretty awesome. But it doesn’t stop there. Researchers are working with calico cats to try to understand how X-chromosomes are inactivated, in an attempt to figure out a way to turn certain genes on or off in a way that isn’t random. How cool would it be if genes linked to obesity or other diseases could be selectively silenced without altering a person’s DNA? Or if X-chromosome linked disorders could be silenced in a way that they wouldn’t be passed down to our offspring?
If your daughter is on the low end of the birth weight curve, it may not be a good idea to encourage her to ‘catch up’ to get within the normal curve. A surprising new study, perfomed at Baylor College of Medicine, showed that female mice who were growth-restricted in utero (and thus born at a lower birth rate) were more prone to obesity as adults. After realizing this, researchers looked at historical reports of people born in famine conditions (growth-restricted early in life) and found that these women were more likely to be obese. This trend was not seen in males.
It seems that the reason for this is not over-eating later in life, but lack of activity. From an evolutionary sense, growth-restricted females may be more likely to conserve energy for future bearing of children. As there are millions of babies born each year with low birth weights- and obesity is certainly a growing problem worldwide- this study might prompt pediatricians to rethink their recommendations to moms with small little girls. While further research will yield more information, in the meanwhile, it wouldn’t hurt to make sure that your little ones are getting plenty of exercise and learning healthy habits at a young age!