Unformatted text preview: can calculate genotype frequencies from a Punnet square: If the probability for an individual to possess allele A is p, then the probably of being homozygous for A is p x p = p2 (fig. 2). Similarly, the probability for an individual to be homozygous for the other allele is q2 and the chance to be heterozygous is equal to 2pq. The sum of the frequencies of all 3 genotypes is equal to 1: p2+q2+2pq=1. A (p) a (q) 2 A (p) AA (p ) Aa (pq) a (q) Aa (pq) aa (q2) Figure 2. Punnet square for a population. p and q represent the frequency of alleles A and a, respectively, in the population The A allele is present in the offspring whose genotype is AA and Aa, which frequencies are p2 and 2pq respectively. The frequency of A is equal to p2+ (2pq) since only half the alleles of genotype Aa contributes to p. Thus we can write than the frequency of A in the next generation (p’) will be: p’ = p2 + (2pq) = p2 + (pq). Since (p+q=1), then it is true that q=1‐p. If we substitute this value in the previous equation we’ll obtain: p' = p2+p(1‐p) = p2+p‐p2 = p or p’ = p 74 Lab4 ‐ Microevolution We can conclude that the frequency of A will not change in the next generation. The same is true for the frequency of a, which means that allele frequencies remain the same between generations. This is the Hardy‐Weinberg (HW) principle, from which two fundamental conclusions can be made: 1‐ The allele frequency, as well as genotype frequency, will not change between generations. 2‐ Genotypes frequencies in the next generation can be calculated from allele frequencies in the present generation. However, this principle is only true under certain circumstances. A population in which the two conclusions of the HW principle hold is said to be in HW equilibrium. The equilibrium of a population relies on several fundamental assumptions: a‐ There is no selection b‐ There is no mutation c‐ There is no migration inside or outside of the population d‐ Chance does not cause some alleles to be transmitted more than others (no genetic drift) e‐ Mating is random If any of these assumptions is violated, the HW equilibrium is broken, the allele frequencies will change during time and genotype frequencies can’t be predicted from the allele frequencies. Thus, the HW equilibrium represents a null hypothesis that can be used to test the presence of an evolutionary force within a population: if the actual (measured) genotype frequencies in a population are different than what the HW principle would predict, it is a sign that evolution may be acting within this population. II‐Methods A‐ Introduction The following exercises will illustrate different aspects of population genetics and microevolution. All steps have to be completed sequentially and simultaneously by the whole class. Keep a record of your own results as well as the combined results of the class. You will be asked to answer a series of question during the lab session and hand in the questionnaire at the end of the lab to your TA. B‐ Diving into the lake… In the lab today, each pair of students...
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 Fall '08
 FENWICK
 TA, Blue locus

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