44 Chapter 2 • Patterns of Inheritance the amino acid tyrosine by the enzyme phenylalanine hydroxylase: However, if a mutation in the gene encoding this en-zyme alters the amino acid sequence in the vicinity of the enzyme’s active site, the enzyme cannot bind phenyl-alanine (its substrate) or convert it to tyrosine. Therefore phenylalanine builds up in the body and is converted instead into phenylpyruvic acid. This compound inter-feres with the development of the nervous system, lead-ing to mental retardation. Babies are now routinely tested for this processing deF-ciency at birth. If the deFciency is detected, phenyl-alanine can be withheld by use of a special diet and the development of the disease can be arrested. Cystic Fbrosis is another disease inherited according to Mendelian rules as a recessive phenotype. Cystic F-brosis is a disease whose most important symptom is the secretion of large amounts of mucus into the lungs, re-sulting in death from a combination of effects but usu-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.