BIOL 205 Chapter Notes - Chapter 6: Phenylalanine Hydroxylase, Osteogenesis Imperfecta, Null Allele
5 Nov 2016
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In full dominance, the homozygote cannot be distinguished from the heterozygote since both appear as wildtype. Example: pku (phenylketonuria) and many other single gene human diseases are fully recessive, whereas their wildtype alleles are dominant. Pku is caused by a defective allele of the gene encoding the enzyme phenylalanine hydroxylase (pah) In absence of normal pah, the phenylalanine entering the body in food is not broken down and hence accumulates. Under such conditions, phenylalanine is converted into phenylpyruvic acid, which is transported to the brain through the bloodstream and there impedes normal development, leading to mental retardation. The reason that the defective allele is recessive is that one dose of the wildtype allele p produces enough pah to break down the phenylalanine. People with p/p or p/p have enough p to be wildtype, and p/p is the only allele combination that results in non- functional pah. The wildtype allele of a gene is haploinsufficient.
