Mendelian Genetics Problem Set Name: Biology 105 Principles of Biology Fall 2003 These problem sets are due at the beginning of your lab class the week of 11/10/03 Before beginning the assigned problem set below, it is strongly suggested that you go through the Mendelian Genetics Tutorial at The Biology Place, an internet resource for learning biology. At this internet site, there are practice problems in monohybrid, dihybrid, and sex-linked crosses. In particular you should definitely go over the problems listed below, because these problems are structured similar to, and are at the depth of, the problems that will be on your third exam. Answers to the practice problems, and detailed tutorials on those answers, are available at the Biology Place: http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html Monohybrid crosses: Problems 1, 3-11. Dihybrid cross problems: Problems 1-4, 11, 12 Sex-linked Problem Set: Problems 1, 3, 5-9 There are several similar problems& solutions, at the back of chapter 10 of your text. Mendelian Genetics Assignment The following problems are worth 2 points each. While you are allowed to consult with other students if you are having difficulty, the work that you turn in is expected to represent your own effort. 1. A pure-breeding, tall pea plant is crossed with a pure-breeding, short pea plant. All of the offspring of this cross are tall. a. Which allele is dominant, and how do you know? b. What are the genotypes of these offspring? c. Suppose two of the offspring from problem 1 are crossed and produce 1000 offspring. How many tall and how many short plants will you expect? d. How many of each genotype would you expect among the 1000 offspring? 1
2. In guinea pigs, black vs. albino is due to alleles of one gene, with the allele for black being dominant to that for albinism. Castle and Phillips (Science 30: 312,1909) replaced the ovaries of an albino female with those from a homozygous black female. The female was then mated with an albino male and produced two black offspring. Are these results expected on the basis of Mendelian inheritance? Explain. 3. It is the nurse's first day in the maternity ward, and he manages to mix up three babies born around the same time. The babies' blood types are O, A, and AB, while the blood types of the three pairs of irate parents are: AB and O, A and B, and B and B. Which baby belongs with which pair of parents, and why? What are the possible genotypes of all the parents with respect to the blood type? 4. Use the laws of probability to predict the likelihood of getting an AaBbDdee from a cross between two AaBbDdEe individuals. 2
5. Long after Mendel s Laws were accepted it was observed that crosses between homozygous red snapdragons and homozygous white snapdragons produced pink snapdragons. This is exactly what the blending inheritance model told Mendel to expect when he bred his peas! Is this still a case of Mendelian inheritance? If not, why not? If so,how do you explain it? What would happen if you crossed two of the pink flowers? 6. Polydactyly is a rare dominant disorder that affects the human hand. If two heterozygous polydachtylous humans marry and produce a total of four offspring, what is the probability of obtaining: a) four polydachtylous children. b) first= polydachtylous, second = normal, third = normal, fourth = polydachtylous 3
7. The MN blood group is another system due to a single gene with two codominant alleles, M and N. Codominant means that the phenotype for the heterozygote is MN. Give the expected frequency and genotypes from the following matings: MM x NN MN x NN MN x MN 8. Suppose that you actually gathered data for the MN blood group and obtained the following results (expressed in terms of numbers of offspring) from three parental matings on the MN blood group Father's Mother's Number of Offspring Genotype Genotype Genotype offspring MM MM 110 108 MM, 2 MN, 0 NN MM MN 121 58 MM, 62 MN, 1 NN MM NN 136 0 MM, 134 MN, 2 NN a. Do these data fit your predicted results? Point out each and every place you think there is a discrepancy. b. What do you think might have caused these discrepancies? Try to come up with multiple hypotheses (Note: Mutations occur in a given locus in ~1 in 10 8 gametes) 4
9. In 1901, Bateson reported the first post-mendelian study of a cross involving two characters. White leghorn chickens, having white feathers and large "single" combs, were crossed to Indian Game Fowl, having dark feathers and small "pea" combs. The F1 were white with pea combs, and the F2 distribution was: 111 white pea, 37 white single, 34 dark pea, and 8 dark single. What were the genotypes and phenotypes of the parents, and what was the expected F2 distribution of phenotypes? 10. In garden peas a pure line that is tall with purple flowers is crossed to another pure line that is short with white flowers. The F1s are all short and white. You cross the F1s with each other and get 900 short white, 315 short purple, 307 tall white, and 99 tall purple. You allow 90 of the short purple F2 plants to self-fertilize and then select a single seed from each one. What phenotypes would you expect to see in the plants reared from these seeds and in what numbers? Why? 5
11. Taupe eyes is a dominant autosomal mutation found on chromosome 3 of Drosophila. Eyeless is a recessive autosomal mutation found on chromosome 2. If a true-breeding taupe-eyed male is crossed with a true-breeding eyeless female (assume that she has no taupe alleles, but instead has the normal red-eyed alleles), and then the F1 flies are crossed, what would be the expected phenotypic distribution of the F1 and F2 flies? 12. In Drosophila, what are the predicted F1 and F2 phenotypic ratios of a cross between true-breeding parents with the following phenotypes: a white-eyed female and a red-eyed male? 13 In Drosophila, what are the predicted F1 and F2 phenotypic ratios of a cross between true-breeding parents with the following phenotypes: a red-eyed female and a white-eyed male? 6
14. In sesame both the number of seed pods per leaf axil and the shape of the leaf are determined by single genes. The allele for one pod is dominant to that for three pod, while the allele for normal leaf is dominant to that producing wrinkled leaves (Langham, D. G., J. Hered. 36: 245, 1945). The results of five crosses, each between a single pair of plants, gave the results shown below. Determine the genotypes of the parents of each cross. Parents Progeny 1-pod 1-pod 3-pod 3-pod normal wrinkled normal wrinkled a.1-pod, norm x 3-pod, norm 318 98 323 104 b.1-pod, norm x 1-pod, wrink 110 113 33 38 c.1-pod, norm x 3-pod, norm 362 118 0 0 d.1-pod, norm x 3-pod, wrinkled 211 0 205 0 e.1-pod, wrink x 3-pod, norm 78 90 84 88 7
15. A human female "carrier" who is heterozygous for the recessive, sex-linked trait red color blindness, marries a normal male. a. What proportion of their female progeny will show the trait? b. What proportion of their male progeny will show the trait? c. If one of their daughters marries a normal male, what is the probability that the first son of this marriage will show the trait? d. If one of their daughters marries a normal male, what is the probability that the first child of this marriage will show the trait? Extra Credit - 6 points You are breeding fruit flies for use in genetics experiments. Every once in a great while you notice a fly that has a golden body color. Early one morning, you have the great fortune of finding two such flies: one a male, and the other (in another culture) a virgin female. Delighted, you cross the two golden-colored flies. You get 34 golden offspring and 16 normal colored. a. What do you think is the dominant phenotype? Why? b. What do you think are the genotypes of the parents and the progeny? c. Carry out a chi-squared analysis of your results to test how well the data fit your hypothesis d. You select fifteen F1 golden females, and fifteen F1 golden males. In individual vials you cross these flies with true-breeding normal flies. In each cross, you get approximately equal numbers of golden and normal progeny. Explain these results. e. Given your new information, what was the expected phenotypic distribution in your first cross? f. Given your new information, Carry out a chi-squared analysis of your results to test how well the data fit your new hypothesis. g. What does this tell you about the predictive power of the chi-squared calculations? 8