Evolution II.2 Answers.

Transcription

1 Evolution II.2 Answers. 1. (4 pts) Contrast the predictions of blending inheritance for F1 and F2 generations with those observed under Mendelian inheritance. Blending inheritance predicts both F1 and F2 hybrids to be intermediate between the two parents. For F1 hybrids, Mendelian inheritance predicts either (a) all individuals like one of the parents (dominance), or (b) individuals intermediate (incomplete dominance). For the F2 hybrids, Mendelian inheritance predicts either (a) both parental phenotypes (dominance) or both intermediate and parental phenotypes (incomplete dominance). 2. (2 pts) When would you expect independent assortment to break down? When the genes in question are linked, i.e., on the same chromosome and close to each other. Historically, deviation from independent assortment was the basis for the construction of genetic maps, i.e., the closer together a pair of genes, the less likely a crossing over between them. 1

2 3. (6 pts) Continue the preceding table into the 5 th generation. Selfing Leads to Heterozygote Elimination. (Assume Four Seeds per Cross) Cross / Genotype AA Aa aa 4 th Generation 28 (AA x AA) (Aa x Aa) (aa x aa) th Generation Total 120 (.46875) 16 (.0675) 5 th Generation 120 (AA x AA) (Aa x Aa) (aa x aa) th Generation Total 496 ( ) 32 (.03125) 120 (.46875) 496 ( ) 4. (8 pts) Suppose there are three alleles, A1, A2, A3, with gene frequencies p, q, r. How many genotypes are there? What are their H-W frequencies? There are six genotypes: A 1 A 1, A 1 A 2, A 1 A 3, etc. The proportions are p 2 : 2pq : 2pr : q 2 : 2qr : r 2. 2

3 5. (8 pts) Suppose the genotypic frequencies, p AA, etc., are 0.10, 0.50 and a. What are the gene frequencies? b. Is the population in Hardy-Weinberg equilibrium? a. AA homozygotes have two copies of gene A, Aa heterozygotes have one copy and aa homozygotes have none. Then p A = = =. 35 Likewise, aa homozygotes have two copies of gene a, heterozygotes have one copy and AA homozygotes, none. Then p a = = =. 65 Alternatively, it may be observed that p a = 1 p A =. 65 b. H-W frequencies as follows: p AA = =. 1225; p Aa = =. 455; p aa = = Thus there is an excess of heterozygotes (.50 vs..445) and an insuffi- 3

4 ciency of homozygotes (.10 vs and.40 vs..445). Depending on population size, these deviations might or might not be statistically significant. 6. (2 pts) What might one conclude if a population is not in H-W equilibrium? Other forces selection, migration, mutation, etc. are affecting gene frequencies. 7. (4 pts) In the case of heterozygote superiority, is p* stable or unstable? Stable. If the heterozygote is superior to both homozygotes, p increases if p < p (but not equal to 0 or 1) and decreases if p > p (but not equal to 0 or 1). In other words, p p. 8. (4 pts) In the case of heterozygote inferiority, is p* stable or unstable? Unstable. The argument is as given above but with the signs reversed. In other words, p moves away from p, unless, of course, p equals 0 or 1. 4

5 9. (6 pts) Using Eq (6), compute the equilibrium frequency of HbS q * (1 p*) (1.88) (1.14) (8 pts) Do you expect the frequency of HbS to be higher or lower among American than African blacks? Give two reasons why or why not? Lower in U.S. due to (1) eradication of malaria there is no longer selection for heterozygotes and (2) intermarriage with individuals of nontropical descent. 5

6 11. (8 pts) Tay-Sachs disease (TSD) occurs in frequencies of in Jews of Eastern European extraction. TSD is a neurodegenerative disorder caused by a single autosomal mutation. In its most common form, it is almost invariably fatal by age 4. Assume the disease only presents in homozygotes (two copies of the mutation) and a mutation rate of 2 X a. Compute the equilibrium frequency of TSD according to Eq (10). Hint: What is the value of the coefficient of selection, s? q μ s = = b. What do you conclude? The observed frequency is an order of magnitude greater than that predicted by mutation-selection balance. Something else involved. Possibilities include heterozygote advantage and a small population bottleneck in recent past. 6

7 12. (4 pts) Parent-offspring comparisons in humans would yield high heritability for life-time earning and religion i.e., the children of the rich tend to be rich, etc. Does this necessitate the existence of poverty genes? Explain. Of course not. Environmental factors are often involved and, in many cases, predominate. 13. (6 pts) In assessing whether or not intelligence has a heritable basis, twin studies are often used. Design such a study. Hint. Not all twins are the same. Compare correlations between monozygotic (identical) and dizygotic (non-identical) twins reared apart and together. Also compare correlations between siblings, and parent-child correlations. The table below gives data compiled from multiple studies. The bottom-line conclusion is that both heredity and environment are important. 7

8 IQ correlations from multiple studies compiled by Devlin, B. et al The heritability of IQ. Nature. 388: Column 0 is the average of the various studies weighted by sample size. Columns I-IV are the correlations predicted by models that allow for various genetic, maternal and environmental effects. Monozygotic twins manifest the highest correlations, but even here, environment plays a role correlations are higher if the children are reared together. If only genetics mattered, monozygotic twins would manifest a correlation of 1.0; dizygotic twins, sibling and parent-child, a correlation of 0.5 and adoptive parent-child, a correlation of 0. 8

9 15. (6 pts) Asume the average IQ in a population is 100 and h 2 = 0.4. What is the expected IQ of the daughter of parents whose IQs are 120 and 110? What about the IQ of a child born to parents with IQs of 90? Use Eq (11) with 115 and 90 for Tp. Yields 106, (6 pts) Comparing the offspring s expected IQ to that of the parents and to the population mean, what do you conclude? Offspring will be intermediate between parents and the population average. This is sometimes referred to as reversion to the mean. 17. (2 pts) Which of the three modes (stabilizing, directional, disruptive) of selection always increases phenotypic variance? Which mode shifts trait frequency distributions? Disruptive; directional. 9

10 18. (2 pts) Which of the three modes of selection is often a consequence of trade-offs and could be responsible for evolutionary stasis? Stabilizing. 19. (8 pts) How might you account for the fact that the observed mean birth weight of humans is slightly less than the mean? One possibility is parent-offspring conflict. What s good for junior s fitness is not necessarily good for mom s. Specifically, Mom has multiple shots at reproduction; each of her children has only one shot at life. 20. (8 pts) The following species nest in Alaska where summer nights can be cold: golden eagle, great horned owl, kingbird, rufous hummingbird, yellow-shafted flicker. One of these goes torpid (reduced metabolic rate and body temperature) at night. Which species do you imagine does this? Explain. Rufous hummingbird. It s the smallest, has the largest surface area to volume ratio and rate of heat loss. It is therefore the most likely to :run 10

11 out of gas if it attempts to maintain its body temperature at night. 21. (8 pts) Small cliff swallows are more agile fliers than large individuals and therefore more likely to be able to capture insects on the fly. How does this affect selection for body size in this species? Works against selection for large body size to reduce death from hypothermia. One can imagine selection in different years for small or large body size depending on the weather. 11