Lab 8: Population Genetics and Evolution

1. Lab 8:Population Genetics and Evolution This may leave a bad taste in your mouth…

2. Pre-Lab Orientation • Recall that the Hardy-Weinberg Equation helps us identify allele frequencies throughout a population. • Given certain assumptions like “large population size,” “random mating,” et cetera… • A great example for a classroom is ability to taste PTC (phenylthiocarbamide). • This stuff is, oddly, used to grow transparent fish by inhibiting melanin production.

3. PTC • Not everyone can taste PTC. • In North America, 55% of people can taste it (it tastes quite bitter), while 45% cannot. • Which one are you? • Let’s take a taste, shall we? (you don’t have to) • I’m going to wash my hands and give you a small piece. • Place it on the tip of your tongue and wait a few seconds. • If you are a PTC taster, you’ll get a bitter taste. If not, you’ll just taste something papery. • Don’t swallow it. Just awkwardly peel it off your tongue and toss it in the trash.

4. Starting the lab… • Take some time to read the Introduction (Page 80) and Exercise 8A while I’m coming around with PTC. • Once we all taste the PTC strips, we’ll take some classroom data and record it in Table 8.1, then answer the Topics for Discussion on Page 82. • Save Exercise 8B – Case Studies for another time.

5. Exercise 8B • Now we start the Case Studies… • …so I’m sorry if this seems a little forward, but you need to go find a mate. • A random mate – remember this is simulating Hardy-Weinberg equilibrium conditions. Find someone with whom you wouldn’t normally pair. • Gender has no role here. • Need a team of three? One of you is going to have to two-time it and act as partner to two others. Only write one result down, just like everyone else.

6. Case I • Start by turning to the back page of your lab packet – the Data Page. • Under Case I, note that we are simulating Initial Class Frequencies of: • AA: 25% • Aa: 50% • aa: 25% • Your Initial Genotype: Aa • On your lab desks is a bag full of letters on card stock. • These letters represent those same alleles that can be inherited – either A or a.

7. Case I • Start by taking four total for each of you – two A and two a. • These are the products of meiosis. • Meiosis, if you forgot, is the production of gametes – haploid cells destined for reproduction. • From one cell, four are produced, but they only have one set of chromosomes so there’s only one allele each. • These are the only alleles you can pass on.

8. Case I • Now put the cards into a pile, face down, and shuffle them. • Take one card off the top. That’s one of the two alleles for your offspring. Have your partner do the same. • These two alleles comprise the first offspring. One of you (and only one) should write the genotype in the data section on the last page (F1 Genotype).

9. Case I • Now put the letters back, repeat the process, develop a second offspring, and have the other partner write that down (F1 Genotype). • Let’s record our data. (you don’t need to do this – just me)

10. Case I • Okay…now the tricky part… • Each of you needs to assume the role of the F1 generation. • Leave the cards at the table but remember your genotype. • Find a new mate and settle down at another lab table. • Now that you have found a new mate, take a new set of cards. • Remember that the letters represent the products of meiosis, so if you’re AA, take four A cards. • If you’re Aa, take two A cards and two a cards. • aa? Take four a cards.

11. Case I • Class data time! What’s your genotype? • Now you’re going to (randomly) find a new mate with the alleles you’ve determined. • Find someone, then repeat the process. • After each generation, pause so we can collect genotype data. • Once we’re done with five generations, record the data in #4 after Case 1 on Page 93. Complete the questions and the Data Page section.

12. Case I Discussion • Did our allele frequencies change? Importantly, would we expect them to change? • Is our population size large enough? • Are we at Hardy-Weinberg equilibrium?

13. FYI • Wondering why our initial frequencies are 0.25 AA, 0.50 Aa, and 0.25 aa if everyone starts Aa? • The answer is because you need to look forward a little. • The Hardy-Weinberg equation tells us that, since p=0.5 and q=0.5 (we’re all heterozygous), p2 should be 0.25, 2pq is 0.50, and q2 is 0.25. • Further, take a look at a Punnett Square for your first cross.

14. AA: 0.25 Aa: 0.50 aa: 0.25 Punnett Square

15. Case II • Now we’re going to repeat Case I but change a condition – as in reality, not every genotype will have an equal chance of survival. • An example is sickle-cell anemia, which can kill humans prior to reproduction if they are homozygous recessive. • Each time you draw aain this process, don’t record it; it doesn’t reproduce. • You and the other parent must keep trying until you get a non-homozygous recessive offspring. • Again, after five generations, we’re going to count frequencies,and then you’ll complete data and questions.

16. Case III • By now you know that sickle-cell anemia, while a disease, helps prevent a far worse disease in malaria. • Individuals that are heterozygous for sickle-cell anemia have some sickle cells, but not enough to make them ill. • At the same time, having those sickle-cells increases resistance to malaria because the parasites can’t infect the erythrocytes (red blood cells).

17. Case III • We’re going to simulate this heterozygote advantage with Case III. • The procedure’s the same except: • If your offspring is AA, flip a coin (or card). • Heads = Does not survive (try again). • Tails = Does survive. • aa still doesn’t survive, by the way, and allele frequencies start the same (two A, two a, et cetera). • We’ll do this for five generations, then record data, then do it for five more generations and record. • And answer the questions as usual.

18. Case IV • Case IV further explores the concept of genetic drift. • Use the rules of Case I, but we’ll do so with a smaller population size. • We will divide the class into three populations – no gene flow between groups!

19. Hardy-Weinberg Problems • To finish the lab, complete the Hardy-Weinberg Problems starting after Case IV.