It actually depends on the prevalence of a particular disease. For example, Cystic fibrosis (CF) is the most common genetic disease in the United States and the United Kingdom.
Also, according to Regis College in Greater Boston, Down syndrome, thalassemia, Tay-Sachs disease, and Sickle Cella anemia are common amongst US citizens.
On the other hand, in Spain, Fragile X syndrome, Huntington disease, and muscular dystrophy are the most common diseases leading couples to use PGD.
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