97_07_LB_LastLec

97_07_LB_LastLec - Bio 97 Population Genetics II 1. Forces...

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Population Genetics II ©1999-2005 Lee Bardwell Bio 97 1. Forces that keep “bad” alleles in gene pools 2. Inbreeding
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Population Genetics Allele freqs Genotype freqs Gene pool Hardy Weinberg Law: ©2001 Lee Bardwell A a AA Aa aa (p+q) 2 = p 2 +2pq +q 2 = 1 HW equilibrium: allele freqs remain constant from generation to generation, under certain conditions
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Forces that keep “bad” alleles in gene pools Delayed age of onset (e.g. Huntington’s) Slow selection against recessive alleles in diploids Mutation-selection equilibrium Heterozygote superiority ©2000 Lee Bardwell Sometimes, bad alleles become good
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Forces that keep “bad” alleles in gene pools Delayed age of onset (e.g. Huntington’s) ©2000 Lee Bardwell
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When the allele is rare Why? - because then most recessive alleles are in heterozygotes Thus, rare disease-causing recessive alleles persist in the population in heterozygote carriers, even if they are lethal when homozygous ©2002 Lee Bardwell Selection for/against recessive alleles is inefficient
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Carrier/Affected Ratio for an autosomal recessive disease ©2001 Lee Bardwell q2 2pq Disease Incidence Carrier Frequency % of disease alleles in carriers 0.1 0.43 90.00% 0.01 0.18 99.00% 0.001 0.06 99.90% 0.0001 0.02 99.99%
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Selection vs. Mutation New mutations generate harmful new alleles Selection eliminates harmful alleles These forces push the pop out of HW equilibrium, but they push in opposite directions Over time, an equilibrium is reached where allele frequencies are stable - new alleles are remove by selection at the same rate they are created by mutation --> bad alleles are a stable part of the gene pool ©2000 Lee Bardwell
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We’re all genetically defective ©2007 Lee Bardwell One new mutation per kid… Plus the mutations that were inherited… We are all carriers of at least 5 defective alleles of different genes
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Is modern medical science ruining our gene pool? Does saving the life of people with recessive genetic diseases decrease the fitness of our population? (in class discussion) ©2000 Lee Bardwell
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Heterozygote Superiority Fitness (a measurement of viability and fertility) of heterozygote is greater than either homozygote Example: sickle cell anemia - heterozygote carriers are more resistant to malaria HS keeps both alleles in gene pool Example of a “bad” allele becoming good ©2000 Lee Bardwell
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INBREEDING
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Assume your grandfather is a carrier (heterozygote) of a very rare genetic disease (autosomal recessive, incidence 1/250,000). (A) What’s the chance that both
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97_07_LB_LastLec - Bio 97 Population Genetics II 1. Forces...

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