Introduction to Genetic Analysis 36

Introduction to - 44200_02_p27-72 11:37 AM Page 35 2.1 Autosomal inheritance 35 leles of one gene the letter x represents a difference in the

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35 2.1 Autosomal inheritance leles of one gene; the letter “x” represents a difference in the nucleotide sequence: erozygous, the one functional copy generally provides enough enzyme function to allow the synthesis of enough pigment to make the petals purple. Hence the allele for purple is dominant, as Mendel found (see Table 2-1). If we represent a wild-type allele as A , and the cor- responding recessive mutant allele as a , we can summa- rize the molecular picture as follows: A/A !: active enzyme !: purple pigment A/a !: active enzyme !: purple pigment a/a !: no active enzyme !: white Note that inactivation of any one of the genes in the an- thocyanin pathway could lead to the white phenotype. Hence the symbol A could represent any one of those genes. Note also that there are many ways a gene can be inactivated by mutation. For one thing, the mutational damage could occur at many different sites. We could represent the situation as follows, where dark blue indi- cates normal wild-type DNA sequence and red with the letter X represents altered sequence resulting in non- functional enzyme: In studying any kind of variation such as allelic vari- ation, it is often helpful to have a standard to act as a ±xed reference point. What could be used as the “stan- dard” allele of a gene? In genetics today the “wild type” is
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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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