PPP_Ch 4 b - CHAPTER 4 Extensions of Mendelian Genetics Dr....

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CHAPTER 4 Extensions of Mendelian Genetics Dr. Sanghamitra Mohanty
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MAIN AREAS OF DISCUSSION: How alleles influence phenotypes Gene interaction X-linkage Sex-limited and sex-influenced inheritance Conditional inheritance Genetic anticipation
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Alternative forms of a gene are called alleles . Mutation is the source of alleles. The wild-type allele is the one that occurs most frequently in nature and is usually, but not always, dominant . A mutant-allele contains modified genetic information 4.1 4.1 Alleles Alter Phenotypes in Different Ways
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Often, a mutation causes the reduction or loss of the specific wild-type function. Such a case is called a loss of function mutation. If the loss is complete, the mutation has resulted in what is called a null allele . Gain-of-function mutation enhances the function of the wild-type product.
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4.2 4.2 Geneticists Use a Variety of Symbols for Alleles Dominant alleles are usually indicated either by: an italic uppercase letter ( D ) letters ( Wr ) Recessive alleles are usually indicated either by: an italic lowercase letter ( d ) an italic letter or group of letters with the + superscript ( Wr + )
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4.3 4.3 Neither Allele Is Dominant in Incomplete, or Partial Dominance In incomplete dominance : neither trait is dominant offspring from a cross between parents with contrasting traits may have an intermediate phenotype
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The phenotypic ratio is identical to the genotypic ratio in cases of incomplete dominance ( Figure 4.1 ).
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4.4 4.4 In Codominance, the Influence of Both Alleles in a Heterozygote Is Clearly Evident Codominance occurs when joint or equal expression of two alleles of a gene in a heterozygote results in phenotypic detection of both gene products.
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Sample problem on codominance: Q: Found on chromosome 4 in humans, two blood  group alleles, L M  and L N , are equally expressed in  heterozygous individuals. Such heterozygous  individuals are said to have MN blood group. 
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This note was uploaded on 07/04/2011 for the course BIO 325 taught by Professor Saxena during the Fall '08 term at University of Texas.

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PPP_Ch 4 b - CHAPTER 4 Extensions of Mendelian Genetics Dr....

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