QUESTIONS AND PROBLEMS 3

1. Define the following terms:

Chromosome-mediated transfer

Genetic distance

Heterokaryon

Hybrid cell

Interference (positive vs negative)

Linkage group

Linkage map

Liked genes

Ordered tetrad

Somatic cell hybridization

Syntenic genes

Unordered tetrad

2. Suppose that all the gene loci in particular organism map into seven separate linkage groups. How many chromosomes are present in the somatic cells of this species?

3. Criticize the following statements:

(a) Crossing over results in the formation of recombinant type.

(b) Genes that are present on the same chromosome fail to assort independently

(c) A genetic map shows the dimensional relationship of genes on a chromosome

(d) The farther apart two genes are on a chromosome, the greater is their frequency of recombination.

4. Consider the following linkage data:

Gene loci: a-b b-c c-d d-e c-eMap distance: 8 6 2 4 6Construct a chromosome map of these loci

5. Albinism in mice can arise as the homozygous expression of one or the other (or both) of two recessive genes, c and d. Individual must have the dominant alleles of both genes ( C-D-) to express normal color. Suppose that mating between a dihybrid male (CcDd) and females of a doubly recessive albino strain (ccdd) produce 148 albino and 52 colored offspring. Are the two loci linked?

6. In tomatoes, the gene for round fruit shape (O) is dominant to its allele for elongate shape (o), and the gene for smooth fruit skin (P) is dominant to its allele for peach (p). Two series of crosses involving these genes produced the following results:

Cross 1: P: round, smooth X elongate, peach

F1: all round, smooth

Cross 2: P: round, peachX elongate, smooth

F1: all round, smooth

When the F1 of Cross 1 were test crossed, they produce offspring in the following proportions: 46% round, smooth; 4% round, peach; 4% elongate, smooth; 46% elongate, peach. In contrast, testcrosses involving the F1 of Cross 2 gave the following result: 4% round, smooth; 46% round, peach; 46% elongate, smooth; and 4% elongate, peach. Explain these results, designating the genotypes of all individuals concerned.

7. The following data were obtained from a series of two-factor testcrosses. Explain these results in term of the relative locations of the different genes on linkage map.

Gene loci : a-b b-c a-c b-d a-e d-e a-d% Recomb: 42 44 50 48 50 50 508. In rabbits, genes are two loci, b and c, interact to produce coat color; B-C- individuals are black, bbC- individuals are brown, and cc (B-cc or bbcc) individuals are albino. Certain crossing data suggest that these are loci are linked. In one series of testcrosses, mating between a dihybrid male and female of the doubly recessive albino strain resulted in 65 black, 34 brown, and 101 albino offspring. (a) Are the genes linked in the cis or trans arrangement in the dihybrid parent? (b) Calculate the map distance between these loci. (c) The dihybrid male in this series of testcross was the product of a mating between members of two homozygous strains. What were the genotypes of these homozygous strain?

9. Nail-patella syndrome in human is characterized by congenital abnormalities of the finger nails (and sometime toenails) and of the patellae (kneecaps). The gene for this disorder is dominant and is located in chromosome 9 about 10 map units away from the ABO locus. Suppose that a man with nail-patella syndrome and type A blood marries a normal woman with B blood. The mothers of both the husband and wife are normal and have blood type O. (a) The husband and wife have two children. One with type A blood and the other with type B blood. What is the probability that both children are normal (that is, do not have nail-patella syndrome)? (b) The couple are now expecting another child. What is the chance that the child will have nail-patella syndrome and O blood? (c) An amniocentesis are performed, which reveals that the fetus has AB blood. What is the chance that this fetus has nail-patella syndrome.

10. Elliptocytosis is hereditary trait in humans that is characterized by the presence of oval-shaped red blood cells, rather than the normal biconcave-shaped cell. The gene for elliptocytosis is dominant to its normal allele and is located on chromosome 1, about 20 map units away from the Rh locus. The following pedigree shows the joint inheritance of the gene for RH factor and elliptocytosis in three generation of a family. The filled-in symbols represent individuals with elliptocytosis, while the presence or absence of Rh factor is shown below each symbol. (a) Determine the genotypes of the individuals in the pedigree. (b) What is the linkage phase (cis or trans) of individual II-2? (c) Which individuals in generation III are recombinant types? (d) Calculate the recombination frequency for the progeny in generation III. How does your estimated map distance compare with the value given earlier in the problem?

Rh+Rh-

Rh+Rh+ Rh-

Rh- Rh- Rh+ Rh- Rh+ Rh- Rh+ Rh+

11. The recessive genes from hemophilia A and red-green color blindness in human are located about 10 map units apart on the X chromosome. The ABO locus is on chromosome 19. A normal man with type AB blood marries a normal , type AB woman whose mother was colorblind and whose father was hemophilic. They have several children. (a) What is the chance that their first child is color blind? (b) What is the chance that their second child is a boy with type A blood who is both hemophilic and color blind? (c) What is the chance that the third child will, like the parents, be normal in color vision and clotting time and AB in blood type?

12. In corn, the genes for colored kernels (C) vs colorless (cc) and plump kernels (Sh) Vs Shrunken (sh) are about 4 map units apart on chromosome 9. The dihybrid cross C sh//c Sh X C sh//c Sh is made. (a) compute the genotypic ratio in the gametes produced by each of the parents in the cross. (b) What percentage of the offspring will be c sh//c sh in genotype? (c) What percentage of the offspring will have the colored, shrunken phenotype? (d) What percentage of the offspring will be true-breeding?

13. In Drosophila, crossing over occurs in females but not in males. Suppose that the dihybrid cross pr+ b//pr b+ X pr+ b//pr b+ is made, in which the genes for the dominant red eyes (pr+) vs purple (pr) and fro the dominant gay body (b+) vs black (b) are in the trans arrangement. The genes for these traits are located about 6 map units apart on chromosome II. (a) Predict the phenotypic ratio among the progeny of this cross. (b) Does the ratio you predicted indicate anything about how far apart the pr and b loci are on the chromosome II? Explain.

14. The plants that are dihybrid for different pairs of gene loci are selfed. The frequency of double-recessive offspring produced by each mating is given below. In each case, determine whether or not the genes are linked. If the genes are linked, evaluate the linkage phase (cis or trans) in the dihybrid parent, and calculate the map distance between the genes involved.

Genotype of self-

Pollinated plant : AaBb BbCc CcDd

Frequency of of double-

Recessive offspring: 1.00% 6.25% 9.00%

(aabb) (bbcc) (ccdd)

15. A testcross was made using the trihybrid PpRrSs as the dominant parent. The test cross results revealed that the trihybrid produces the following gametes:

PRS

11pRS 15

PRs

238pRs240

PrS

242prS230

Prs

11prs 13

(a) Which loci are linked, and which assort independently? (b) Determine the arrangement (cis or trans) of the linked genes in the trihybrid parent. (c) Calculate the map distances between all linked loci.

16. In Drosophila, the three gene pairs for red-eyes (cn+) vs cinnabar (cn), normal bristle number (rd+) vs reduced (rd), and the long wings (vg+) vs vestigial (vg) are known to have their loci on chromosome II. Suppose that a three-point test cross yields the following offspring:

Cinnabar, reduced, vestigial

406

Cinnabar, reduced, long

46

Cinnabar, normal, vestigial

28

Cinnabar, normal, long

3

Red, normal, long

438

Red, normal, vestigial

45

Red, reduced, long

33

Rd, reduced, vestigial

1

(a) Which are progeny classes of the parental type? Which are the single recombinant types? Which are the double recombinant types? (b) Calculate the map distances between the genes, and construct a linkage map of these loci. (c) Identify the most probable origin of each of the recombinant classes, with regard to the number of location of crossovers. (d) Determine the coefficient of coincidence for this set of loci.

17. In the seedling stage, a completely recessive corn plant was glossy (leaves have a shiny appearance), virescent (poor in chlorophyll), and liguleless (lacking certain appendages at the base of the leaves). This plant is crossed to a trihybrid that as a seedling had dull leaves, normal chlorophyll content, and ligules. Of the many seedlings produced in the next generation, a random sample of 1000 had the following characteristics:

Dull, normal, with ligules

28

Dull, normal, liguleless

179

Dull, virescent, with ligules

69

Dull, virescent, liguleless

250

Glossy, normal, with ligules 198

Glossy, normal, liguleless 70

Glossy, virescent, with ligules

183

Glossy, virescent, liguleless 23

(a)Calculate the map distances between the genes, and construct a linkage map of these loci (glossy = gl, virescent = v, and liguleless = lg). (b) Give the genotype of the trihybrid parent, designating the proper gene arrangement on the pair of chromosomes.

18. Assume the following linkage map:

a10b 6c

(a)Calculate the frequencies of the parental, single recombinant, and double recombinant classes expected among the progeny of the three-point test cross ABC//abc X abc//abc, assuming that crossing over occurs without interference. (b) Repeat your calculations, but now assume interference, with a coefficient coincidence of 0.5.

19. Three two-point testcrosses gave the following recombination frequencies:

Rp-q = 12%

Rq-r = 8%Rp-r = 5%.

(a) Construct a linkage map af the p. q. r loci. Why are the distances not additive?

(b) Calculate the frequencies of the parental, single-recombinant, and double-recombinant classes expected among the progeny of the three-point testcross PpQqRr x ppqqrr, base on the values of Rp-q , Rq-r ,and Rp-r given above. (c) Determine the coefficient of coincidence for this set of loci.

20. Consider the four-point testcross ABCD//abcd X abcd//abcd. (a) How many recombinant classes can we possibly detect in the progeny of this cross? (b) Indentify the most probable origin of each recombinant class, with regard to the number and location of crossovers. (c) Suppose that data from previous two-point crosses indicate that the members of each of the pairs of gene loci (a-b, b-c, and c-d) are two map units apart. How many offspring must we examine from this four-point cross in order to observe about eight individuals in the lowest frequency class?

21. You discover a recessive mutant gene in Drosophila that that is not reported in the scientific literature, and you want to determine on which chromosome this gene is located. Assume that you have access to two indicator strains of flies, one homozygous for a gene on chromosome II, and another for a gene on chromosome III. Describe an experimental breeding procedure you could use that would permit you to assign this mutant gene to one of the four chromosomes in this species.

22. In a series of experiments with Neurospora, the following numbers of FDS and SDS asci were observed for each gene pair. Determine the distance between the gene and its centromere in each case.

NUMBER OF

TETRADS

GENE PAIRFDSSDS

(a) f+ vs. f (fluffy) growth82118

(b) c+ vs. c (colonial) growth7816

(c) r+ vs. r (ropy) growth6742

23. Neurospora of opposite mating types with the genotypes CD an cd are crossed. Analysis of the 389 ordered tetrads produced from this cross yields the following data:

(1)(2)(3)(4)(5)(6)(7)

CDCdCDCDCDCdCD

CDCdcDCdCdcDCd

CdcDCdcDCDCdCd

cdcDcdcdcdcDcD

259142965778

Determine whether or not the genes are linked. If they are linked, calculate the distances between the genes. What are the gene-centromere distances?

24. Gene pairs c+, c (c = compact growth) and leu+. leu (leu + requirement for the amino acid leucine) are observed in a mating of Neurospora srains c+ leu+ X c leu. The following classes of ordered tetrads are obtained:

(1)(2)(3)(4)(5)(6)(7)

+ ++ leu+ ++ ++ ++ leu+ +

+ ++ leuc ++ leuc leuc +c leu

c leuc ++ leuc ++ ++ leu+ leu

c leuc ++ leuc leuc leuc +c +

367 4 11 50 60 2 6

Determine linkage relationships and the map distances between the genes.

25. Unordered tetrad analysis in yeast gives the following data:

(1)(2)(3)

+ b+ ++ b

+ b+ ++ +

a +a ba b

a +a ba +

3633234

Evaluate these data, and determine the distance between genes if they are linked.26. Explain the origin of each of the tetrad classes in problem 23 through 25 by diagramming the events of meiosis (assortment, crossing over) that were responsible for their reproduction.

27. Unordered tetrad analysis of a cross between a+b+c+ and abc strains in yeast yields the following data. Determine the map distances (genes are linked in the order given).(1)(2)(3)(4)(5)

+++++++++++++++

++++bc++c+b++bc

abca++ab+a+ca+c

abcabcabcabcab+

412244832

(6)(7)(8)(9)

++c++c+bc++c

+b++bc+bc++c

a++a++a++ab+

abcab+a++ab+

3215

28. Unordered tetrad analysis of a cross between d+e+f+ and def strains in yeast gives the following data (genes are linked but not necessarily in the order given). Determine the correct gene order and genetic distances.

(1)(2)(3)(4)(5)

+++++++++++++++

+++d+f++fd++d+f

def+e+de++ef+ef

defdefdefdefde+

29942901011

(6)(7)(8)(9)(10)

++f++fd+f++fd++

d++d+fd+f++fd+f

+e++e++e+de++e+

defde++e+de++ef

10125201

29. Suppose that in a study involving chromosome-mediated gene transfer, human metaphase chromosomes are used to transfer HPRT mouse cells to the HPRT+ condition. Of the transformed HPRT+ cells, 12 percent also received the human gene that codes for glucose 6-phosphate dehydrogenase (G6PD). In a few cases, the HPRT+ transformants received the PGK gene. Using these data on co-transfer frequencies, construct a linkage map of the three loci.

30. Human cells are fused with mouse cells, and the resulting hybrid cells are placed into HAT medium. After extensive chromosome loss, sex different cell lines are selected and tested for the presence of human chromosomes as well as for the presence of four human enzymes, one of which is thymidine kinase (TK), The results are tabulated below. (The presence of a chromosome or enzyme is indicated by +, the absence by -.).

CELL LINES

(a)(b)(c)(d)(e)(f)

Human Enzymes E1++----

Human Enzymes E2-++-++

Human Enzymes E3++---+

Human Enzymes E4++++++

Human Chromo 1++---+

Human Chromo 11++----

Human Chromo17++++++

Human Chromo X-++-++

(a). Identify the chromosome that carries the gene for each enzyme. (b) Which of the enzyme (E1, E2, E3, or E4) is thymidine kinase (TK)? How can you tell? (c) The order enzymes tested for were lactate dehydrogenase (LDH), 6-phosphogluconate dehydrogenase (6PGD) and phosphoglycerate kinase (PGK).