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Lecture 15


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Biology (Biological Sciences)
Darren De Lorey

Lec15 BIOL207 2014-02-10 Population genetics cont’d A. Does recessive mean rare? (e.g. wrong answers to Midterm Question C2) NO !!! e.g. rare, dominant (AD, XD) diseases in humans… e.g. O blood type is most common and is homozygous recessive True: (1) mutations are rare (2) most mutations are recessive … … but this does not mean that mutations are recessive because they are rare (or vice versa). B. Hardy Weinberg Equilibrium (review) If population is in HWE (5 conditions: no selection, migration, mutation; random mating; large populatio2 size), 2hen observed genotypic frequencies will match expectations calculated as p , 2pq, q . C. Are all populations in HWE ? NO !!! But they are often close; even for humans, for a rare recessive disease can estimate the frequency of carriers based on number affected = q2 D. Can one locus be in HWE and the other not? YES !!! e.g. there can be random mating in a large population with no migration, but selection could affect the frequency of alleles at one locus, but not another Another example to demonstrate HWE is for sickle cell anemia; if you are homozygote for a particular polymorphism and have a defective allele, you will get a sickle shaped cell instead of a round cell; this cell will cause circulation + clotting issues; the sickle cell allele is acutally most common Africa and India; why is this allele really common in these population?  These countries ten to have a great deal of malaria; this is where the sickle cell allele is in higher frequency since the heterozygotes for sickle cell actually have a higher resistance to the malaria virus; useful in areas. E. How about those cats? … 99 “real” cats described in survey; raw data is on e-class White locus 5 pure white (W_) 94 not pure white (ww) Long locus 78 short (L_) 21 long (ll) Dilute locus 64 not dilute (D_) 35 dilute (dd) Piebald spotting s S total alleles 41 0% white (ss) 2 × 41 0 2 × 41 39 0% to 50% white (Ss) 1 × 39 1 × 39 2 × 39 19 >50% white fur (SS) 0 2 × 19 2 × 19 TOTAL 121 77 198 p = 121/198 = 0.61 q = 77/198 = 0.39 p2 = 0.612 = 0.37 2pq = 2(0.61)(0.39) = 0.48 2 2 q = 0.39 = 0.15 expected if HWE observed 0.37 × 99 cats = 36 (ss) 41 no white 0.48 × 99 cats = 48 (Ss) 39 part white 0.15 × 99 cats = 15 (SS) 19 mostly white … Orange/Tortoiseshell O o total alleles O O 5 oran
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