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Bio 201 F07 True lect 5v2post - Checking population data to...

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genotypes AA Aa aa TOTAL p=freq of A q=freq of a observed # 24 128 75 227 =(24+128/2)/227 =(75+128/2)/227 expected FREQ =p^2 =2*p*q =q^2 0.39 0.61 =0.39^2 =2*0.39*0.61 =0.61^2 0.15 0.47 0.37 expected # =0.15*227 =0.47*227 =0.37*227 expected # 34.1 107.8 85.1 observed # minus expected # -10.1 20.2 -10.1 ((observed # minus expected #)^2)/expected # 3.00 3.80 1.20 "chi-square" components TOTAL of chi-square components 8.00 "CHI-SQUARE STATISTIC" TEST OF STATISTICAL SIGNIFICANCE =CHIDIST(0.15, 2 DEGREES OF FREEDOM) RETURNS P VALUE P VALUE 0.0183351 8.00 Checking population data to test for “Hardy-Weinberg Equilibrium” Degrees of freedom = #columns minus 1 = 3-1 = 2 P-value = probability of getting data at least this different from expected, by chance alone (based on expected frequencies)
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P-values and null hypotheses The P-value represents the probability that data is consistent with the null hypothesis In the chi-square test of whether genotype frequencies are in Hardy-Weinberg (HW) proportions Null hypothesis = population in HW equilibrium for this gene P value > 0.05 means we cannot reject the null hypothesis i.e. no evidence of departure from HW P value < 0.05 means we reject the null hypothesis i.e. one or more of the assumptions of HW is violated
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Our data was significantly different from HW expectation P value less than 0.05 (1 in 20) • Reasons? – Fewer AA individuals than expected 24 vs. 34.1 – More Aa individuals than expected 128 vs. 107.8 – Fewer aa individuals than expected 75 vs. 85.1 Looks like heterozygote advantage (also called heterosis )
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A real world example: sickle cell anemia Mutation in gene encoding ! chain of hemoglobin (Hb ! ) -over 100 alleles have been found in humans (most are rare) 3 common alleles in West Africa, where sickle cell anemia is common: Hb ! A , Hb ! C , Hb ! S S/S = severe anemia (low red cell count) abnormal structure of globin protein, red blood cells sickle shaped under low O 2 tension Carrying one S allele confers heterozygote advantage , due to increased resistance to Plasmodium -mediated malaria Normal cells Sickle cells The malaria parasite, Plasmodium falciparum , a protozoan.
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D.L.Hartl and A.G. Clark 1988 Principles of Population Genetics
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