E X E R C I S E S
I. Interpreting Your Gel
1. Observe the photograph of the stained gel containing your sample and those from other students. Orient the photograph with the sample wells at the top. Interpret the band(s) in each lane of the gel:
2. Determine the genotype distribution for the class by counting the number of students of each genotype (+/+, +/-, and -/-).
3. What can you say about any person in the class who has at least one + allele?
4. An allele frequency is a ratio comparing the number of copies of a particular allele to the total number of alleles present. Imagine a class of 100 students that list their genotype distribution as follows: +/+ 20 +/- 50 -/- 30 Since humans are diploid, the total number of alleles in the class is 2 x 100 = 200. The allele frequency for PV92+ is: 2 x 20 (homozygotes) + 50 (heterozygotes) / 200 = 90 / 200 = 0.45 Likewise, the allele frequency for PV92- is: 2 x 30 (homozygous) + 50 (heterozygotes) / 200 = 110 / 200 = 0.55 Using the genotype distribution from your class, calculate the frequencies of the + and - alleles of PV92.
5. If a population is genetically stable, then the allele frequencies will remain constant from one generation to the next. Such a population is said to be in Hardy-Weinberg equilibrium. Once the allele frequencies have been determined, the distribution of genotypes are described by the equation: p + 2pq + q = 1 where p and q represent the allele frequencies; p and q are the homozygote frequencies; and 2pq is the heterozygote frequency. Use the allele frequencies calculated for your class in Step 3 to determine the expected genotype frequencies. How do they compare with the actual genotype frequencies? How can you account for differences?
A L U I N S E R T I O N P O L Y M O R P H I S M
DNA Learning Center, Cold Spring Harbor Laboratory
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