Introduction to Genetic Analysis 10

Introduction to Genetic Analysis 10 - 44200_01_p1-26 3/2/04...

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9 1.2 Genetic variation mented and unpigmented skin is caused by different al- leles of a gene that encodes an enzyme involved in melanin synthesis. The alleles of a gene are conventionally designated by letters. The allele that codes for the normal form of the enzyme involved in making melanin is called A , and the allele that codes for an inactive form of that enzyme (resulting in albinism) is designated a, to show that they are related. The allelic constitution of an organism is its genotype, which is the hereditary underpinning of the phenotype. Because humans have two sets of chromo- somes in each cell, genotypes can be either A / A , A / a ,or a / a (the slash shows that the two alleles are a pair). The phenotype of A / A is pigmented, that of a / a is albino, and that of A / a is pigmented. The ability to make pig- ment is expressed over inability ( A is said to be domi- nant, as we shall see in Chapter 2). Although allelic differences cause phenotypic differ-
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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.

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