-lrb- check -rrb- fragile-x syndrome : 1 in 1,000 males ; 1 in 2,500 females . ; -lrb- check -rrb- neurofibromatosis type 1 -lrb- elephant man 's disease -rrb- characterized by numerous skin growths : 1 in 3,000 births. ; -lrb- check -rrb- duchenne muscular dystrophy , a degenerative muscle disease : 1 in 4,000 males . ; -lrb- check -rrb- hemophilia a , a blood-clotting disorder : 1 in 6,700 males . ; -lrb- check -rrb- phenylketonuria : a disorder involving the processing of an enzyme that if untreated , can cause severe mental retardation ; 1 in 10,000 births. ; -lrb- check -rrb- retinoblastoma , an eye cancer : 1 in 14,000 births. ; -lrb- check -rrb- huntington 's disease : a fatal neurological disease ; 1 in 20,000 births .