Gen.+Class+notes_6

Gen.+Class+notes_6 - Penetrance and Expressivity:...

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Penetrance and Expressivity : Penetrance is a concept that refers to the percentage of individuals expressing the expected phenotype Expressivity refers to the degree to which a trait is expressed. For example in polydactyly, some people have a full six finger, others have just a fold in the skin. The variability observed in penetrance and expressivity is due to effects of other genes and the environment on the phenotype.
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3) Lethal alleles have a profound effect on the cross phenotypic ratio. The animal usually dies early during development, thus the progeny will be missed completely. Lethal alleles can be dominant or recessive. Yellow mice cannot breed true meaning the Y allele is never present as a homozygous YY.
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4) Multiple alleles in the population can affect the outcome of the cross . The same principles of Mendelian inheritance apply but a bigger range of genotypes and phenotypes is possible. MR= restricted M = mallard md= dusky MR is dominant to M M is dominant to md How many genotypes are possible in the population? [n(n+1)/2] were n=# of alleles. [3(3+1)/2]= 6
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Factors at multiple locus : Gene interactions occur between different locus (non- allelic) but they affect the same phenotype. Remember that in dihybrid crosses the F2 phenotypic ratio is 9:3:3:1. This reflects independence in the inheritance of the two locus: independent assortment. In gene interactions, this ratio is sometimes affected by the fact that phenotypic expression of one gene is influenced by a gene at a different locus. Sometimes the interaction of two genes produces novel phenotypes. Example : the color of peppers.
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The color of peppers is determined by a common enzymatic pathway were the Y locus codes for the first enzyme and the C locus for the last. Y locus x x x C locus
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Same ratio as a dyhybrid cross without interaction.
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Gene interactions can affect the phenotypic ratios when there is epistasis. Epistasis refers to the concept of gene masking: the
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Gen.+Class+notes_6 - Penetrance and Expressivity:...

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