遗传学-双语课-练习题.doc

Medialian genetics
1 Shown are F2 results of two of Mendel's monohybrid crosses. State a null hypothesis that you will test using chi-square analysis. Calculate the x2 value and determine the p value for both crosses, then interpret the p values. Which cross shows a greater amount of deviation?
a)Full pods 882
Constricted pods 299
(b)Violet flowers 705
White flowers224
2 In Dmsophihla, gay body color is dominant over ebony body color, while long wings are dominant over vestigial wings. Work the following crosses through thee F2 generation, and determine the genotypic and phenotypic ratios for each generation. Assume that the Pl individuals are homozygous:
(a)gray, long × ebony, vestigial (b) gray, vestigial × ebony, long (c) gray, long × gray, vestigial
3.How many different types of gametes can be formed by individuals of the following genotypes? What are they in each case?
(a) AaBb,(b) AaBB, (c)AaBbCc, (d)AaBBcc, (e)AaBbcc and (f) AaBbCcDdEe?
4.Using the forked-line method, detennine thee genotypic and phenotypic ratios of these trihybrid crosses:
(a)AaBbCc×AaBBCC, (b)AaBBCc×aaBBCc, and (c) AaBbCc × AaBbCc.
5. Mendel crossed peas with round, green seeds with peas having wrinkled, yellow seeds. All Fl plants had seeds that were round and yellow-Predict the results of test crossing these F1 plants.
6. In one of Mendel's dihybrid crosses, he observed 315 round, yel low; 108 round,green; 101 wrinkled, yellow; and 32wrinkled, green F2 plants. Analyze these data using chi--square analysis to see whether (a) they fit a 9:3:3:1 ratio; (b)the round, wrinkled trait fit for a 3:1 ratio; or (c) the yellow, green traits fit a 3:1ratio.
7. A geneticist, in assessing data that fell into two phenotypic classes, observed values of 250:150. He decided to perform chi-square analysis using two different null hypotheses: (a) The data fit a 3:1 ratio; and (b) the data fit a l:1 ratio.Calculate the X2 values for each hypothesis-What can you conclude about each hypothesis?
8. The basis for rejecting any null hypothesis is arbitrary. The researcher can set more or less stringent standards by deciding to raise or lower the critical p value. Would the use of a standard of p=0.10 be more or less stringent in failing to reject the null hypothesis? Explain.
9. Consider three independently assorting gene pairs, A/a, B/b, and C/c, where each demonstrates typical dominance (A-, B-, C- ), and recessiveness (aa, bb, cc). What is the probability of obtaining offspring that is AABbCc from parents that are AaBbCC and AABbCc?
10. What is the probability of obtaining a triply recessive individual from thee parent as shown in Problem 9?
11. Of all offspring of the Parents in Problem 9, what proportion will express all three dominant traits?
12. For the following pedigree, predict the mode of inheritance and the resulting genotypes of each individual. Assume that the alleles A and a control the expression of the trait.

13. Which of Mendel's postulates are demonstrated by the pedigree in Problem 12? List and define these postulates.
14. The following pedigree follows the inheritance of myopia (near-sightedness) in humans. Predict whether the disorder is identified as a dominant or a recessive trait. Based on your predic6on, indicate the most probable genotype for each individual.

15. Two true-breeding pea plants are crossed. One parent is round, terminal, violet, constricted, while the other expresses the contrasting phenotypes of wdnkled, axial, white, full. The four pairs of contrasting traits are controlled by four genes, each located on a separate chromosome. In the F1 generation only round, axial, violet, and full are expressed. In the F2 generation, all possible combinations of these traits are expressed in ratios consistent with Mendelian inheritance.
(a) What conclusion can you draw about the inheritance of these traits based on the Fl results?
(b) Which phenotype appears most frequently in the F2 results? Write a mathematical expression that predicts the frequency of occurrence of this phenotype.
(c) Which F2 phenotype is expected to occur 1east frequently? Write a mathematical expression that predicts this frequency.
(d) How often is either Pl phenotype likely to occur in the F2 generation?
(e) If the Fl plant is testcrossed, how many different phenotypes will be produced, and how does this number compare to the number of different phenotypes in the F2 generation discussed in part (b)?
16. Tay-Sachs disease (TSD) is an inborn error of metabolism that results in death, usual1y before the age of five. You are a genetic counselor, and you interview a phenotypically normal couple who consult you because the man had a female first cousin (on Ms father's side) who died tom TSD, and the woman had a maternal uncle with TSD. There are no other known cases in either family, and none of the matings were/are between related individuals. Assume that this trait is rare in this population.
(a)Using standard pedigree symbols, draw a pedigree of these individuals, families, showing the relevant individuals.
(b)The couple asks you to calculate the probability that they both are heterozygous for the TSD allele.
(c)They also want to know the probability that neither of them is heterozygous.
(d)They also ask you for the probability that one of them is heterozygous but the other is not.
[Hint: thee answers to (b), (c), and(d) should add up to1.0.]
17 Chlamydomonas, a eukaryotic green alga, is sensitive to the antibiotic erythromycin, which inhibits protein synthesis in prokaryotes. There are two mating types in this alga, mt+ and mt-, If an mt+ cell sensitive to the antibiotic is crossed with an mt- cell that is resistant, all progeny cells are sensitive. The reciprocal cross (mt+ resistant and mt- sensitive) yields all resistant progeny cells. Assuming that the mutation for resistance is in the chloroplast DNA, what can you conclude from the results from these crosses?
sex determination
1. As related to sex determination, what is meant by (a) homomorphic and heteromorphic chromosomes; (b) isogamous and heterogamous organisms?
2. Contrast the life cycle of a plant such as zea mays with an animal such as C.elegans-.
3. Discuss the role of sexual differentiation in the life cycles of chlamydomonas, ZE mays, and C .elegans.
4. Distinguish between the concepts of sexual differentiation and sex detem1ination.
5. Contrast the Protenor and Lygaeus modes of sex determination.
6. Describe the major difference between sex determination in Drosophila and in humans
7. What specific observations (evidence) support the conclusions you have drawn about sex determination in Drosophila and humans?
8. Describe how nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete.
9 An insect species is discovered in which the heterogametic sex is unknown. An X-linked recessive mutation for reduced wing (rw) is discovered. Contrast the F1 and F2 generations from a cross between a female with reduced wings and a male with normal sized wings when (a)the female is the heterogametic sex; (b) the male is the heterogametic sex.
10. Based on your answers in boblem9, is it possible to distinguish between Protenor and Lygaeus mode of sex determination based on the outcome of these crosses?
11.When COWS have twin calves of unlike sex (fraternal twins), the female twin is usually sterile and has masculinind reproductive organs. This calf is referred to as a freemartin .In cows, twins may share a common placenta and thus fetal circulation. Predict why a freemartin develops.
12. It has been suggested that any male-determining genes contained on the Y chromosome in humans cannot be located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region?
13. What is a Barr body, and where is it found in a cell?
14. Indicate the expect ed number of Barr bodies in interphase cell of the following individuals; KIinefelter syndrome; Tumer syndrome; and karyotypes 47,XYY, 47,XXX, and 48,XXXX.
15. Define the Lyon hypothesis.
16 .Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?
17. Predict the potential effect of the Lyon hypothesis on the region of a human female heterozygous for the X-linked red-green color blindness trait.
18. Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoise shell pattern are almost invariable females. Why?
19. What does the apparent need for dosage compensation mechanisms suggest about the expression of genetic information in normal diploid individuals?
Linkage and Chromosome Mapping in eukaryotyes
1. What is the significance of crossing over (which leads to genetic recombination to the process of evolution?
2. Describe the cytological observation that suggests that crossing over occurs during the first meiotic prophase.
3. Why does more crossing over occur between two distantly linked genes than between two genes that aree very close together on the same chromosome?
4. Why is a 50 percent recovery of single-crossover products the upper limit, even when crossing over always occurs between two linked genes?
5. Why are double-crossover events expected less frequently than single-crossover events?
6. What is the proposed basis for positive interference?
7. What three essential criteria must be met in order to execute a successful mapping cross?
8. The genes dumpy wings ( dp ),clot eyes (cl), and apterous wings (ap) are linked on chromosome II of Drosophilia .in a series of two-point mapping crosses, the genetic distances shown below were determined-What is the sequence of the three genes?
dp-ap 42 dp-cl 3 ap-cl 39
9. Consider two hypothetical recessive autosomal genes a and b, where a heterozygote is testcrossed to a double-homozygous mutant. Predict the phenotypic ratios under the following conditions: (a) a and b are located on separate autosomes; (b) aand b are linked on the same autosome but are so far that a crossover always occurs between them; (c) a and b are linked on the same autosome but are so close together that a crossover never occurs (d) a and b are linked on the same autosome about l0mu apart.
10.Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r when homozygous, produces colorless aleurone. The plant color (not kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homologous presence of recessive allele causes the plant to appear yellow. I a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained:
CY 88; Cy12; cY8; cy92
Explain how these results were obtained by determining exact genotype and phenotype of the unknown plant, including the precise association of the two genes on the hormologs (i.e. the arrangement).

11. In the cross shown here, involving two linked genes, ebony (e) and claret (ca), in Drosophila, where crossing over does not occur in males, offspring were produced in a (2+:1ca:1e) ratio:
These genes are 30 mu apart on chromosome III. What did crossing over in the female contribute to these phenotypes?
12. With two pairs of genes involved (P,p and Z,z), a testeross (to ppzz) with m organism of unknown genotype indicated that the gametes were produced m these proportions :PZ=42.4%; Pz=6.9%; pZ=7.1%; and pz=43.6%. Draw all possible conclusions tom these data.

13 in a serious of two-point map crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed
Pr-adp 29 Pr-vg 13 Pr-c 21 Pr-b 6 Adp-b 35 Adp-c 8 Adp-vg 16 Vg-b 19 Vg-c 8 c-b 27
(a) If the adp gene is present near the end of chromosome II, (locus83), construct a map of these genes.
(b) In another set of experiments, a sixth gene (d) was tested. against b and pr, and the results were d-b =17% and d-Pr =23%. Preedict the results of two-point maps between d and c, d and vg, and d and adp.
14 Two different female Drosophila were isolated, each heterozygous for the autosomally linked genes black body (b), dachs tarsus (d) ,and curved wings (c).These genes are in the order d-b-c with b closer to d than to c. Shown below is the genotypic arrangement for each female, along with the various gametes formed by both. Identify which categories are noncrossovers (NCO),single crossovers (SCO), and double crossovers (DCO) in each case. Then, indicate the relative frequency in which each will be produced.
15 In Drosoophilia, a cross was made between females expressing the three X-linked recessive traits, scute bristles (sc),sable body (s), and vermilion eyes (v) and wild-type males. All females were wild type in the F1, while all males expressed all three mutant traits.The cross was carried to the F2 generation, and 1000 offsprings were counted, with the results shown in the table. No determination of sex was made in the F2 data. (a0) Using proper nomenclature, determine the genotypes of the Pl and F1 parents, (b) Determine the sequence of the three genes and the map distance between them. (c) Are there more or fewer double crossovers than expected? (d)Calculate the coefficient of coincidence; does this represent positive or negative interference?

16. A cross in Drosophila involved the recessive, X-linked genes yellow body (y), white eyes(w), and cut wings (ct). A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal, but whose wings were cut. The F1 females were wild type for al1 three traits, while the Fl males expressed the yellow-body, white-eye traits. The cross was carried to F2 progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed. (a)Diagram the genotypes of the F1 parents; (b)Construct a map, assuming that w is at locus 1.5 on the x chromosome. (c) Were any double-crossover offspring expected? (d) Could the F2 female offspring be used to construct the map? Why or why not?

17.In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye(p) are recessive mutations on chromosome III. Flies from a Diet1aete stock were crossed to homozygous ebony, pink flies, and the Fl progeny with a Diet1aete phenotype were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table, (a)diagram the cross, showing the genotypes of the parents and offspring of both crosses. (b) What are sequence and interlocus distance between these