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Penetrance - Mendel I Ascertainment J.L Marsh Bio 137B...

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Mendel I & Ascertainment 10/11/05; 10:20 PM J.L. Marsh Bio 137B Page 16 3] COMPLICATIONS 1] FAILURE TO EXPRESS the phenotype = 4 reasons PENENTRANCE/EXPRESSIVITY/epistasis/environment 1] Incomplete PENETRANCE Penetrance = % of expected showing the trait -Ptosis = drooping eyelid. Expresses before birth 40% with gene show no evidence Consider the RARE case of a homozygote for a Dominant trait. --> ALL offspring will be P/+ Homozygote for dominant P/P Ptosis 60% reveal trait = 60% penetrant WHAT % OF OFFSPRING FROM A HETEROZYGOTE WILL BE AFFECTED? 30% because 0.5 X 0.6 = 0.3 Note: If a dominant has reduced penetrance some individuals who don't show the phenotype will carry the gene. This will look like a mating between 2 recessive heterozygotes. 30% will start to look like ~25% & the answer won't be clear - determining dom vs rec by inspection will help 2] EXPRESSIVITIY = THE DEGREE OR INTENSITY OF THE PHENOTYPE Expressivity = the range of severity of trait in different people - e.g. Cleft lip
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