What is the most common genetic disorder in the u.s.?

Too complex to answer: There are Mendelian disorders: X-linked recessive, X-linked dominant, autosomal dominant, and autosomal recessive. There are genetic disorders caused by abnormal amounts of chromosomal material: trisomies, deletions, and duplications. The largest group are disorders in which there is clearly a genetic component, but non-genetic factors are also important. No easy answer.