<Li> Genetic conditions . Sometimes disability is caused by abnormal genes inherited from parents , errors when genes combine , or other reasons . The most prevalent genetic conditions include Down syndrome , Klinefelter syndrome , Fragile X syndrome ( common among boys ) , neurofibromatosis , congenital hypothyroidism , Williams syndrome , phenylketonuria ( PKU ) , and Prader -- Willi syndrome . Other genetic conditions include Phelan - McDermid syndrome ( 22q13del ) , Mowat -- Wilson syndrome , genetic ciliopathy , and Siderius type X-linked intellectual disability ( OMIM 300263 ) as caused by mutations in the PHF8 gene ( OMIM 300560 ) . In the rarest of cases , abnormalities with the X or Y chromosome may also cause disability . 48 , XXXX and 49 , XXXXX syndrome affect a small number of girls worldwide , while boys may be affected by 49 , XXXXY , or 49 , XYYYY. 47 , XYY is not associated with significantly lowered IQ though affected individuals may have slightly lower IQs than non-affected siblings on average . </Li>
<Li> Genetic conditions . Sometimes disability is caused by abnormal genes inherited from parents , errors when genes combine , or other reasons . The most prevalent genetic conditions include Down syndrome , Klinefelter syndrome , Fragile X syndrome ( common among boys ) , neurofibromatosis , congenital hypothyroidism , Williams syndrome , phenylketonuria ( PKU ) , and Prader -- Willi syndrome . Other genetic conditions include Phelan - McDermid syndrome ( 22q13del ) , Mowat -- Wilson syndrome , genetic ciliopathy , and Siderius type X-linked intellectual disability ( OMIM 300263 ) as caused by mutations in the PHF8 gene ( OMIM 300560 ) . In the rarest of cases , abnormalities with the X or Y chromosome may also cause disability . 48 , XXXX and 49 , XXXXX syndrome affect a small number of girls worldwide , while boys may be affected by 49 , XXXXY , or 49 , XYYYY. 47 , XYY is not associated with significantly lowered IQ though affected individuals may have slightly lower IQs than non-affected siblings on average . </Li>
a genetic disorder that causes a moderate to severe mental disability is called
a genetic disorder that causes a moderate to severe mental disability is called
Genetic conditions
Genetic conditions