Tay-Sachs disease is a genetic disorder with an increased incidence in those of Ashkenazi Jewish descent. Carrier gene testing prior to conception can identify couples who have a high likelihood of having a child with the condition. β-thalassemia has a higher incidence in Mediterranean populations. Trisomy 21 occurs more frequently in mothers of advanced age but has no racial or ethnic associations. There is an increased incidence of severe combined immunodeficiency in some Amish populations.