Test Questions & Answers Linkage, Genes & Chromosomes Ch.5

Genetics, 6e (Hartwell)

Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

In Drosophila, the genes y, f, and v are all X-linked. Females who are homozygous for recessive alleles of all three genes (y f v / y f v) are crossed to wild-type males. The resulting trihybrid F1 females are testcrossed. The F2 are distributed as follows:

1) Which of the following linkage maps correctly shows the order and distance between the y, f, and v genes?

A) f——35 m.u.——y——15 m.u.——v

B) f——22 m.u.——y——15 m.u.——v

C) y——35 m.u.——f——22 m.u.——v

D) y——22 m.u.——v——15 m.u.——f

E) y——15 m.u.——v——22 m.u.——f

2) What is the coefficient of coincidence in this region?

A) 0

B) 0.2

C) 0.4

D) 0.6

E) 0.8

Females heterozygous for the recessive second chromosome mutations px, sp, and cn are mated to a male homozygous for all three mutations. The offspring are as follows:

3) What is the genotype of the females that gave rise to these progeny?

A) px+ sp cn / px sp+ cn+

B) px+ sp cn+ / px sp+ cn

C) px+ sp+ cn+ / px sp cn

D) px sp cn+ / px+ sp+ cn

E) insufficient data

4) Which of the three genes is in the middle?

A) px

B) sp

C) cn

D) insufficient data

5) Which of the following linkage maps correctly shows the order and distance between the px, sp, and cn genes?

A) sp——0.21 m.u.——px——30.01 m.u.——cn

B) sp——30.01 m.u.——px——0.21 m.u.——cn

C) sp——0.2 m.u.——px——30 m.u.——cn

D) px——0.2 m.u.——sp——30.2 m.u.——cn

E) px——30.2 m.u.——sp——0.2 m.u.——cn

6) What is the coefficient of coincidence in this region?

A) 0

B) 0.16

C) 0.33

D) 0.5

E) 0.66

In peas, tall (T) is dominant to short (t), red flowers (R) is dominant to white flowers (r), and wide leaves (W) is dominant to narrow leaves (w). A tall plant that has red flowers and wide leaves is crossed to a short plant that has white flowers and narrow leaves. The resulting progeny are shown in the table.

7) What is the genotype of the tall plant that has red flowers and wide leaves?

A) T R W / t r w

B) T R W / t r W​

C) T R W / T R W​

D) T R W / T r w​

E) T r W / t R W​

8) This cross is not useful to determine if one of the genes is linked to the others. Which gene?

A) gene T

B) gene R

C) gene W

D) This cross shows that all three genes are linked.

9) If two or more of the genes are linked, what map distance separates them?

A) 4 m.u.

B) 12 m.u.

C) 24 m.u.

D) 50 m.u.

E) None of the genes are linked to each other.

A dihybrid testcross is made to determine if genes C and D are linked. The results are shown in the table.

10) The chi-square value is the sum for all progeny classes of (observed-expected)2/expected. Using the chi-square test for goodness of fit, calculate the chi-square value to test the null hypothesis that genes C and D are unlinked. What is the chi-square value?

A) 0

B) 0.0576

C) 10.8

D) 14.4

E) cannot be determined

11) Using the chi-square test for goodness of fit, how many degrees of freedom are in this data set?

A) 1

B) 2

C) 3

D) 4

12) Given this data, use Table 5.2 to find the most accurate range within which the p value falls.

A) 0.001 < p < 0.01

B) 0.01 < p < 0.05

C) 0.05 < p < 0.10

D) 0.10 < p < 0.50

13) What is a reasonable conclusion based on the chi-square analysis?

A) There is a high probability that the deviation from the expected results is due chance.

B) One can say with a high degree of confidence that genes C and D are linked.

C) The analysis supports the null hypothesis.

D) Genes C and D are most likely unlinked.

14) If only 100 progeny had been counted and the same proportions of progeny genotypes observed, how would the p value and the conclusion drawn about linkage change?

A) The p value would increase, and the likelihood of linkage decreases.

B) The p value would decrease, and the likelihood of linkage increases.

C) Neither the p value nor the likelihood of linkage would change.

D) The p value would decrease, and the likelihood of linkage decreases.

15) The R and S genes are linked and 10 map units apart. In the cross R s / r S × r s / r s what fraction of the progeny will be R S / r s?

A) 5%

B) 10%

C) 25%

D) 40%

E) 45%

16) If the map distance between genes A and B is 20 map units and the map distance between genes B and C is 35 map units, what is the map distance between genes A and C?

A) 15 map units

B) 55 map units

C) More information is needed to distinguish between 15 and 55 map units.

D) Gene C must be located on a different nonhomologous chromosome.

17) In Drosophila, singed bristles (sn) and cut wings (ct) are both caused by recessive, X-linked alleles. The wild-type alleles (sn+ and ct+) are responsible for straight bristles and intact wings, respectively. A female homozygous for sn and ct+ is crossed to a sn+ct male. The F1 flies are interbred. The F2 males are distributed as follows:

What is the map distance between sn and ct?

A) 12 m.u.

B) 13 m.u.

C) 25 m.u.

D) 50 m.u.

E) 75 m.u.

18) In Drosophila, singed bristles (sn) and cut wings (ct) are both caused by recessive, X-linked alleles. The wild type alleles (sn+ and ct+) are responsible for straight bristles and intact wings, respectively. A female homozygous for sn and ct+ is crossed to a sn+ ct male. The F1 flies are interbred. The F2 males are distributed as follows:

Of these 4 genotypic classes of offspring, which arose from a parental gamete produced by the F1 females? (Select all that apply.)

A) sn+ ct+

B) sn ct

C) sn+ ct

D) sn ct+

19) Suppose the L and M genes are on the same chromosome but separated by 100 map units. What fraction of the progeny from the cross  L M / l m  ×  l m / l m would be L m / l m?

A) 10%

B) 25%

C) 50%

D) 75%

E) 100%

20) The pairwise map distances for four linked genes are as follows: A-B = 22 m.u., B-C = 7 m.u., C-D = 9 m.u., B-D = 2 m.u., A-D = 20 m.u., A-C = 29 m.u. What is the order of these four genes?

A) ABCD

B) ADBC

C) ABDC

D) BADC

E) CADB

21) The zipper-like connection between paired homologs in early prophase is known as a

A) spindle fiber.

B) synaptic junction.

C) synaptonemal complex.

D) chiasma.

E) None of the choices is correct.

22) The measured distance between genes D and E in a two-point testcross is 50 map units. Where are genes D and E in relation to each other? (Select all that apply.)

A) D and E are on different homologous chromosomes.

B) D and E are on the same chromosome, at least 50 map units apart.

C) D and E are on the same chromosome, exactly 50 map units apart.

D) D and E are on the same chromosome, less than 50 map units apart.

23) The R and S genes are linked and 10 map units apart. In the cross R s / r S × r s / r s what percentage of the progeny will be R s / r s?

A) 5%

B) 10%

C) 25%

D) 40%

E) 45%

24) If the recombination frequency between two genes is close to 50%, what could be true about the location of the two genes? (Select all that apply.)

A) They are on nonhomologous chromosomes.

B) They are far apart on the same chromosome.

C) They are very close together on the same chromosome.

D) None of the choices could be true.

25) A dihybrid testcross is made between genes H and I. Four categories of offspring are produced: H I, H i, h I, and h i. How many degrees of freedom would there be in a chi-square test for goodness of fit of the null hypothesis that the H and I genes are unlinked?

A) 1

B) 2

C) 3

D) 4

E) 0

26) Which process(es) can generate recombinant gametes? (Select all that apply.)

A) crossing-over between two linked heterozygous loci

B) independent assortment of two unlinked heterozygous loci

C) segregation of alleles in a homozygote

D) crossing-over between two linked homozygous loci

27) Crossing-over takes place in bivalents (tetrads) consisting of ________ chromatids, and one crossover involves ________ chromatids.

A) 2; 2

B) 2; 4

C) 4; 2

D) 4; 4

E) 8; 4

28) In Drosophila, the genes y (yellow body) and car (carnation eyes) are located at opposite ends of the X chromosome. In doubly heterozygous females (y+ car+ / y car), a single chiasma is observed somewhere along the X chromosome in 90% of the examined oocytes. No X chromosomes with multiple chiasmata are observed. What percentage of the male progeny from such a female would be recombinant for y and car?

A) 5%

B) 10%

C) 45%

D) 55%

E) 90%

29) The Q gene locus is 10 map units from the R gene locus which is 40 map units from the S gene locus:

Q——10 m.u.——R——40 m.u.——S

Which interval would likely show the higher ratio of double to single chiasmata?

A) Q-R

B) R-S

C) The ratios would be the same in the two intervals.

D) Two chiasmata never occur in the same interval.

30) The map of a chromosome interval is:

A——10 m.u.——B——40 m.u.——C

From the cross A b c / a B C × a b c / a b c, how many double crossovers would be expected out of 1000 progeny?

A) 5

B) 10

C) 20

D) 40

E) 80

31) The cross L p q / l P Q × l p q / l p q is carried out. If the L gene is in the middle, between genes P and Q, what would be the genotypes of the double crossover gametes in this cross?

A) L P Q and l p q

B) L p Q and l P q

C) l p Q and L P q

D) L p q and l P Q

E) cannot be determined

32) Suppose a three-point testcross was conducted involving genes X, Y, and Z. If the most abundant classes of progeny are X Y z and x y Z and the rarest classes are x Y Z and X y z, which gene is in the middle?

A) X

B) Y

C) Z

D) cannot be determined

33) In Drosophila, the genes b, c, and sp are linked and arranged as shown below:

b——30 m.u.——c——20 m.u.——sp

This region exhibits 90% interference. How many double crossovers would be recovered in a three-point cross involving b, c, and sp out of 1000 progeny?

A) 3

B) 6

C) 54

D) 60

E) 600

34) In tetrad analysis, which result would indicate that two genes are linked?

A) NPD = T.

B) PD = T.

C) PD = NPD.

D) PD > NPD.

E) PD > T.

35) In tetrad analysis, NPD asci can result from

(Select all that apply.)

A) independent assortment of unlinked genes.

B) double crossovers between linked genes.

C) single crossovers between linked genes.

D) single crossovers between a gene and a centromere.

36) When analyzing octads (ordered tetrads), second-division (MII) segregations result from

A) single crossovers between linked genes.

B) double crossovers between linked genes.

C) single crossovers between a gene and a centromere.

D) independent assortment of unlinked genes.

E) nondisjunction of homologs.

37) Tetrad analysis shows that crossing-over occurs at the four-strand stage (i.e., after replication) because, when two genes are linked,

A) NPD > T.

B) T > NPD.

C) T > PD.

D) PD > NPD.

E) PD > T.

38) Sturtevant's detailed mapping studies of the X chromosome of Drosophila supported what genetic principle?

A) That genes are arranged in a linear order on the chromosomes.

B) That genes are carried on chromosomes.

C) That sex determination is controlled by the X and Y chromosomes.

D) That segregation of an allelic gene pair is accompanied by disjunction of homologous chromosomes.

E) That different pairs of chromosomes assort independently.

39) Suppose an individual is heterozygous for alternate alleles of gene A (Aa). Under what conditions would a crossover in a somatic cell of this individual lead to a clone of cells that are homozygous for a? (Pick the most precise answer.)

A) The crossover would have to occur between the A locus and the centromere and involve two homologous (nonsister) chromatids.

B) The crossover would have to occur between the A locus and the end of the chromosome and involve two homologous (nonsister) chromatids.

C) The crossover would have to occur between the A locus and the end of the chromosome and involve two nonhomologous chromosomes.

D) The crossover would have to occur between the A locus and the centromere and involve two sister chromatids.

40) If an individual is heterozygous at two loci (A b / a B) that are located on the same chromosome arm with A closer to the centromere than B, under what conditions would a crossover in a somatic cell generate a twin spot?

A) The crossover would have to occur between the A locus and the centromere and involve two homologous, nonsister chromatids.

B) The crossover would have to occur between the A locus and the B locus and involve two homologous, nonsister chromatids.

C) The crossover would have to occur between the B locus and the end of the chromosome and involve two homologous, nonsister chromatids.

D) A double crossover would have to occur, with one crossover between the A locus and the centromere and a second crossover between the A and B loci, and both crossovers would have to involve two homologous, nonsister chromatids.

E) No crossover in a somatic cell could generate a twin spot.

41) Individuals heterozygous for the RB+ and RB− alleles can develop tumors as a result of (Select all that apply.)

A) a mitotic crossover that leads to homozygosity for RB+ in some cells and RB− in other cells.

B) a somatic mutation in the RB+ allele that leads to homozygosity for RB−.

C) a somatic mutation in the RB− allele that leads to homozygosity for RB+.

D) the fact that RB− is dominant to RB+.

42) Which type of tetrad contains two recombinant and two parental spores?

A) PD

B) NPD

C) T

D) ordered tetrads

E) None of these types contain two recombinant and two parental spores.

In Drosophila, singed bristles (sn) and carnation eyes (car) are both caused by recessive X-linked alleles. The wild-type alleles (sn+ and car+) are responsible for straight bristles and red eyes, respectively. A sn car female is mated to a sn+ car+ male and the F1 progeny are interbred. The F2 are distributed as follows:

43) If you want to analyze this data for evidence of linkage between sn and car, what is the null hypothesis?

A) Genes sn and car are linked.

B) Genes sn and car are not linked.

C) Genes sn and car are located close together on the same chromosome.

D) Crossing-over sometimes occurs between sn and car.

44) What is the χ2 value for a chi-square test for goodness of fit of the null hypothesis?

A) 0.5

B) 1.0

C) 2.0

D) 0.4

E) 20

45) What is the p value from this test? (Pick the most accurate choice.)

A) p > 0.5

B) 0.1 < p < 0.5

C) p < 0.1

D) p < 0.05

E) p < 0.01

46) What does the data analysis allow you to conclude about linkage between sn and car?

(Select all that apply.)

A) There is a high probability that the deviations from the expected number of F2 in each genotype class are due to chance.

B) The data do not allow rejection of the null hypothesis.

C) The p value is high meaning that the data is significant.

D) There is good evidence that cn and car are linked.

In humans, the genes for red-green color blindness (R = normal, r = color blind) and hemophilia A (H = normal, h = hemophilia) are both X-linked and only 3 map units apart.

47) Suppose a woman has four sons, and two are color blind but have normal blood clotting and two have hemophilia but normal color vision. What is the probable genotype of the woman?

A) H R / h r

B) H r / h r

C) h r / h R

D) H r / h R

E) H R / H r

48) A woman whose mother is color blind and whose father has hemophilia A is pregnant with a boy. If the alleles for color blindness and hemophilia A are rare in the population, what is the probability that the baby will have normal vision and normal blood clotting?

A) 0

B) 0.03

C) 0.485

D) 0.47

E) 0.015

In Drosophila, the autosomal recessive pr and cn mutations cause brown and bright-red eyes, respectively (wild-type flies have brick-red eyes). Flies who are homozygous recessive at both pr and cn have orange eyes. A female who has wild-type eyes is crossed to an orange-eyed male. Their progeny have the following distribution of eye colors:

49) Which phenotypes are parental?

A) wild-type and orange

B) brown and bright-red

C) wild-type and brown

D) bright-red and orange

E) There is no way to determine this.

50) What is the genotype of the wild-type mother of these progeny?

A) pr cn / pr+ cn+

B) pr+ cn / pr+ cn

C) pr+ cn / pr cn+

D) pr cn+ / pr cn+

E) pr cn / pr cn

51) The mother of these progeny resulted from a cross between two flies from true-breeding lines. What are the genotypes of these two lines?

A) pr cn+ / pr cn+ and pr+ cn / pr+ cn

B) pr+ cn+ / pr+ cn+ and pr cn / pr cn

C) pr+ cn+ / pr cn and pr cn / pr cn

D) pr+ cn / pr cn and pr cn+ / pr cn

E) More than one of these could be true.

52) What is the map distance between the pr and cn genes?

A) 20 m.u.

B) 2 m.u.

C) 4 m.u.

D) 46 m.u.

E) 8 m.u.

Consider a pair of homologous chromosomes heterozygous for three genes (e.g. A B C / a b c) during prophase I of meiosis. Let the sister chromatids of one homolog be numbered 1 and 2; and the sister chromatids of the other homolog be numbered 3 and 4.

53) A crossover that would result in genetic recombination (e.g., A b c or a B C) could involve which pair of chromatids? (Select all that apply.)

A) 1 & 2

B) 3 & 4

C) 1 & 3

D) 2 & 4

E) 1 & 4

F) 2 & 3

54) Assume a double crossover occurs in this pair of chromosomes that results in chromatids of the genotypes A b C and a B c. If the first crossover (the one between A and B) involves chromatids 1 (A B C) & 4 (a b c), which chromatids could be involved in the second crossover?

A) 1 & 2

B) 3 & 4

C) 1 & 3

D) 2 & 4

E) 1 & 4

F) 2 & 3

A haploid yeast strain has mating type a and genotype his4 TRP1. A different haploid yeast strain has mating type α and genotype HIS4 trp1. The two strains mate and produce diploid offspring with genotype his4 TRP1 / HIS4 trp1. The diploid cells undergo meiosis to produce a variety of tetrad types. In the following questions, the genotypes of the spores in a tetrad are shown. For each tetrad, tell what type it is: parental (PD), non-parental (NPD), or tetratype (T).

55) his4 TRP1; his4 trp1; HIS4 trp1; HIS4 TRP1

A) PD

B) NPD

C) T

D) cannot be determined

56) his4 TRP1; his4 TRP1; HIS4 trp1; HIS4 trp1

A) PD

B) NPD

C) T

D) cannot be determined

57) his4 trp1; his4 trp1; HIS4 TRP1; HIS4 TRP​

A) PD

B) NPD

C) T

D) cannot be determined

58) HIS4 trp1; HIS4 TRP1; his4 TRP1; his4 trp1​

A) PD

B) NPD

C) T

D) cannot be determined

59) HIS4 trp1; HIS4 trp1; his4 TRP1; his4 TRP1

A) PD

B) NPD

C) T

D) cannot be determined

In Neurospora, the white-spore allele (ws) results in white spores rather than the wild-type black spores. Diploid ws / ws+ cells undergo meiosis followed by one round of mitosis to form ordered tetrads. Indicate if the pattern of spores seen in the tetrads are consistent with first-division or second-division segregation.

60) ws; ws; ws; ws; ws+; ws+; ws+; ws+

A) first-division segregation pattern

B) second-division segregation pattern

61) ws; ws; ws+; ws+; ws; ws; ws+; ws+

A) first-division segregation pattern

B) second-division segregation pattern

62) ws; ws; ws+; ws+; ws+; ws+; ws; ws

A) first-division segregation pattern

B) second-division segregation pattern

63) ws+; ws+; ws+; ws+; ws; ws; ws; ws

A) first-division segregation pattern

B) second-division segregation pattern

64) ws+; ws+; ws; ws; ws+; ws+; ws; ws

A) first-division segregation pattern

B) second-division segregation pattern

65) Two genes are considered linked when there are more F2 progeny with recombinant genotypes than parental genotypes in the offspring of a dihybrid testcross.

66) Chiasmata are structures that show where recombination occurred between sister chromatids.

67) Chiasmata can be seen through a light microscope and are sites of recombination.

68) A linkage group includes all of the genes on a chromosome, including genes that are so far apart from each other on a chromosome that they assort independently during meiosis.

69) The hypothesis that predicts no linkage between genes is known as the null hypothesis.

70) If the p value corresponding to a given χ2 value and number of degrees of freedom is lower than 0.05, then the null hypothesis is rejected and it can be said with some confidence that the two genes being evaluated are linked.

71) Genes that are not syntenic are not linked.

72) The genetic distance from one end of a linkage group and the other may exceed 50 m.u. because the distances between many gene pairs are added together to make the map.

73) Sectors with a different phenotype in an otherwise uniform yeast colony may be evidence of mitotic recombination.

74) Large sectors suggest a mitotic recombination event late in the growth of a yeast colony.

75) A female mouse from a true-breeding wild-type strain was crossed to a male mouse with apricot eyes (ap) and grey body (gy). The F1 mice were wild-type for both traits. When the F1 were interbred, the F2 were distributed as follows:

Females

Males

Which of the following statements is correct?

A) ap and gy are unlinked

B) ap and gy are linked on an autosome and 10 map units apart

C) ap and gy are linked on an autosome and 20 map units apart

D) ap and gy are X-linked and 10 map units apart

E) ap and gy are X-linked and 20 map units apart

76) Suppose the map for a particular human chromosome interval is:

a——1 m.u.——b——1 m.u.——c——1 m.u.——d——1 m.u.——e——1 m.u.——f

In a man heterozygous for all six genes, what fraction of his sperm would be recombinant in the a-f interval?

A) 0%

B) 1%

C) 2.5%

D) 5%

E) cannot be determined

77) A coefficient of coincidence of 0.5 in a region of three genes means that half as many double crossovers were observed as would have been expected if crossovers in the two intervals were independent.

78) Mitotic recombination occurs between homologous chromosomes. In which of the following would you not expect to encounter mitotic recombination?

A) e. coli

B) tobacco plants

C) homo sapiens

D) drosophila melanogaster

79) In some fungi, such as the bread mold Neurospora crassa, the arrangement of spores in the ascus directly reflects the order in which they were produced during meiosis. What is the collection of spores produced by meiosis in Neurospora crassa called?

A) unordered tetrad

B) unordered octad

C) ordered tetrad or ordered octad

D) ordered pentad

80) Twin spotting provides evidence of what genetic event?

A) meiotic recombination

B) mitotic recombination

C) linkage

D) mutation

E) biological evolution

81) Another name for a chromosome is a ________, because it contains alleles that are often inherited together.

A) linkage group

B) crossing over group

C) genetic recombinant

D) bivalent

82) The diploid garden pea plant has 14 chromosomes. The haploid fungus Neurospora crassa has 7 chromosomes. Neither organism has separate male and female individuals. Therefore, the number of linkage groups in these two organisms is

A) the garden pea has 14 linkage groups, and Neurospora has 7.

B) the garden pea has 7 linkage groups, and Neurospora has 7.

C) the garden pea has 8 linkage groups, and Neurospora has 8.

D) the garden pea has 15 linkage groups, and Neurospora has 8.