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http://www.yucatantravel.com/hotels/celestun/images/flamingos.jpg Initially… scientists believed that natural selection “should always favor an optimal form.” -why does variation continue? -shouldn’t natural selection choose the “best” allele for that trait and then cause other alleles to disappear? -Don’t dominant genes push recessive genes out of the gene pool? http://news.bbc.co.uk/media/images/39092000/jpg/_39092214_pg_squirrel_pa.jpg 1908- G.H. Hardy and G. Weinberg Gene frequencies remain constant from generation to generation (equilibrium) if: 1. The population is large 2. Mating is random 3. No new mutations 4. No migration from other distinct populations 5. All genotypes are reproductively equal (natural selection not occurring) When H-W equation does NOT accurately predict gene frequencies for generation after generation, it is safe to assume that the population is evolving (not in equilibrium). “Frequency”= # within a given category total # in population Example: What is the phenotype frequency of individuals in a given population with attached or unattached earlobes? 200 =.2 1000 200 800 800 =.8 1000 http://www.cartage.org.lb/en/themes/Sciences/LifeScience/PhysicalAnthropology/HeredityGenetics/lobe.gif The allele for attached earlobes (e) is recessive and for unattached (E) is dominant. What is the frequency of ee in this population? 200 =.2 1000 200 800 800 =.8 1000 What is the frequency of EE in this population? That’s a little trickier because EE and Ee look the same… So, Hardy and Weinberg provided us with a familiar equation to calculate this. The Hardy-Weinberg Equation: (p + q)2 = p2 + 2pq + q2 when (p + q) = 1 Where: p = frequency of the more common allele (E) q = frequency of the less common allele (e) Therefore: p2 = frequency of homozygous dominant (EE), in this case q2 = frequency of homozygous recessive (ee) 2pq = frequency of heterozygous (Ee) So the question was, what is the frequency of EE in this population? (p + q)2 = p2 + 2pq + q2 (EE) (Ee) (ee) 200 What do we already know? •ee = q2 = .2 •so, q = !(.2) = .447 We also know that p + q = 1 •so, 1-.447 = .553 = p EE is p2, so (.553)2 = .305 800 It’s a smart idea to check your work… p2 = EE = (. 553)2 = .305 2pq = Ee = 2(.553)(.447) = .494 q2 = (.447)2 = .2 These three numbers should add up to approx. 1 .305 + .494 + .2 = .999 So what did we just figure out??? 30.5% of the population is EE 49.4% of the population is Ee 20% of the population is ee