The examples discussed so far in this chapter involve genes with only two alleles, such as A for yellow seeds and a for green seeds. Similarly, as noted in Chapter 15, most single-nucleotide polymorphisms (SNPs) have only two alleles, differing only in which particular base pair is present at a particular position in genomic DNA. On the other hand, because a gene consists of a sequence of nucleotides, any nucleotide or set of nucleotides in the gene can undergo mutation. Each of the mutant forms that exists in a population constitutes a different allele, and hence a population of organisms may contain many different alleles of the same gene, which are called multiple alleles. Some genes have so many alleles that they can be used for individual identification by means of DNA typing, such as the VNTR (variable number tandem repeat) sequences discussed in Chapter 15.

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In considering genetic diseases, multiple alleles are often grouped into categories such as “mutant” and “normal.” But there are often many different “mutant” and many different “normal” alleles in a population. For the “mutant” alleles, the DNA sequences are different from one another, but each produces a protein product whose function is impaired under the usual environmental conditions. For the “normal” alleles, the DNA sequences are also different, but they all are able to produce functional protein.

For example, more than 400 different recessive alleles that cause phenylketonuria (PKU) have been discovered across the world. PKU is a moderate to severe form of mental retardation caused by mutations in the gene encoding the enzyme phenylalanine hydroxylase. Children affected with PKU are unable to break down the excess phenylalanine present in a normal diet, and the buildup impairs the development of neurons in the brain. About 1 in 10,000 newborns inherits two mutant alleles, which could be two copies of the same mutant allele or two different mutant alleles, and is affected.

The “normal” form of the gene encoding phenylalanine hydroxylase also exists in the form of multiple alleles, each of which differs from the others, but nevertheless encodes a functional form of the enzyme.

Quick Check 6 How is it possible that there are multiple different alleles in a population and yet any individual can have only two alleles?