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Recessive and dominant heredity in humans Most genetic disorders are caused by recessive alleles (Why?). Many of these alleles (and the disorders) are rare, but can be more common in certain ethnic groups. Simple recessive heredity What Disorder Happens Why it Happens Who Gets It Most 1/2000 white Americans Cystic Fibrosis Thick mucus in lungs and digestive tract defective protein in the cell membrane Tay-Sachs disease dysfunction of central nervous system missing a Amish, normal enzyme E. European Jews Phenylketonuria phenylalanine builds up, damages CNS missing a normal enzyme originally Swedish + Norwegian people Simple dominant heredity Unattached earlobes earlobe not attached to head no skin grew anyone polydactyly six fingers on a hand extra bone grows from other fingers mostly Amish people Huntington disease brain breaks down after age 30-50 not yet known certain families; and a village in S. America