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| Modern Genetic Analysis Chapter 4. The Inheritance of Genes Human Pedigree Analysis Figure 4-20. Genetics and the molecular biology of albinism. In the pedigree, parents heterozygous for the recessive albinism allele produce three A/ progeny, who have melanin in their cells, and one a/a male, who is albino. The three panels at the bottom of the figure show why this is so. The gene for the A allele codes for a polypeptide necessary to convert tyrosine to melanin. A mutation in the gene for the a allele causes the replacement of a threonine with a lysine. The resulting polypeptide is unable to catalyze melanin production. The single A allele in heterozygotes encodes sufficient functional polypeptide to create melanin. The a/a individual produces no functional polypeptide and, hence, no melanin.

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