Why Boys Inherit More Genetic Diseases Than Girls
The book I'm reading is discussing a gene therapy project to treat Duchenne muscular dystrophy, which is the most common form of the disease, afflicting about one in 3,500 boys. Their muscles waste, they are confined to wheelchairs by the age of 13 or 14, and die by their early twenties. The gene that causes Duchenne, the dystrophin gene, is located on the X chromosome. Girls have two X chromosomes, and can make healthy muscle molecules even if they only have one good copy of the dystrophin gene, so they do not get Duchenne muscular dystrophy. Boys have one X and one Y chromosome. The Y chromosome is small and atrophied, and has lost its copy of the dystrophin gene, as well as many other genes. If a boy inherits a bad dystrophin gene on his X chromosome, he does not have a healthy copy on another X chromosome to compensate. That is why boys inherit more genetic diseases than girls.

You may wonder why I am interested in this stuff. My nephew Noah, my brother Pat and my mother Carol have a neurological disorder called CMT (Charcot-Marie-Tooth Disease). Unlike Duchenne, CMT is not life-threatening, but it does cause progressive muscle wasting. Noah recently had heel-cord surgery to repair contracted tendons.

Some potentially good news about research on Duchenne's: A study last summer showed that RNA injections produce positive results in Duchenne mice.