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Modern Genetic Analysis Chapter 5. Recombination of Genes
Problems

1. | If the genes assort independently, what will be the progeny genotypes and their proportions in a cross A/a ; B/b × A/a ; b/b?

2.

In mice, black coat is dominant to brown and intense pigment is dominant to diluted. One mouse with dilute black pigment is crossed to another with intense brown pigment. The progeny are

a. What are the genotypes of parents and progeny?
b. Draw the alleles involved on their respective chromosomes.
See answer

3.

Look at the grid in Figure 5-5.
a. How many genotypes are there in the 16 squares of the grid?
b. What is the genotypic ratio underlying the 9:3:3:1 phenotypic ratio?
c. Can you devise a simple formula for the calculation of the number of progeny genotypes in dihybrid, trihybrid, etc., crosses? Repeat for phenotypes.
d. Mendel predicted that within all but one of the phenotypic classes in the Punnett square, there should be several different genotypes. In particular, he performed many crosses to identify the underlying genotypes of the round, yellow phenotype. Show two different ways that could be used to identify the various genotypes underlying the round, yellow phenotype. (Remember, all the round, yellow peas look identical.)

See answer

4. | The following testcross of a plant is made: If the two loci are 10 m.u. apart, what proportion of progeny will be A B/a b?

5. | The A locus and the D locus are so tightly linked that no recombination is ever observed between them. If A d/A d is crossed to a D/a D, and the F1 is intercrossed, what phenotypes will be seen in the F2 and in what proportions?

6. | A strain of Neurospora with the genotype H.I is crossed with a strain with the genotype h.i. Half the progeny are H.I and half are h.i. Explain how this is possible.

7. | A female animal with genotype A/a.B/b is crossed with a double-recessive male a/a.b/b. Their progeny include 442 A/a.B/b, 458 a/a.b/b, 46 A/a.b/b, and 54 a/a.B/b. Explain these proportions and draw the chromosomes of the dihybrid parent showing the positions of the genes and alleles. See answer

8. | If A/A.b/b is crossed to a/a.B/B, and the F1 is testcrossed, what percent of the testcross progeny will be a/a.b/b if the two genes are (a) unlinked, (b) completely linked (no crossing-over at all), (c) 12 map units apart, (d) 24 map units apart?

9. | A fruit fly of genotype B R/b r is testcrossed to b r/b r. In 84 percent of the meioses, there are no chiasmata between the linked genes; in 16 percent of the meioses, there is one chiasma between the genes. Is the proportion of the progeny that will be B r/b r (a) 50 percent, (b) 4 percent, (c) 84 percent, (d) 25 percent, (e) 16 percent? See answer

10.

In dogs, dark coat color is dominant over albino and short hair is dominant over long hair. Assuming these effects are caused by two independently assorting genes, write the genotypes of the parents in each of the crosses shown below, where D and A stand for the dark and albino phenotypes, respectively, and S and L stand for the short-hair and long-hair phenotypes.

Use the symbols C and c for the dark and albino coat-color alleles and the symbols S and s for the short-hair and long-hair alleles, respectively. Assume homozygosity unless there is evidence otherwise.

(Problem 10 reprinted by permission of Macmillan Publishing Co., Inc., from Genetics by M. Strickberger. Copyright © 1968 by Monroe W. Strickberger.)

11.

In tomatoes, two alleles of one gene determine the phenotypes purple (P) versus green (G) stems and two alleles of a separate, independent gene determine the phenotypes "cut" (C) versus "potato" (Po) leaves. The results for five matings of tomato plant phenotypes are shown below:

a. Determine which phenotypes are dominant.
b. What are the most probable genotypes for the parents in each cross?

(Problem 11 modified from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2d ed. Copyright © 1965 by W. H. Freeman and Company.)

See answer

12.

We have dealt mainly with independent assortment of only two genes, but the same principles hold for more than two genes. Consider the cross

a. What proportion of progeny will phenotypically resemble (1) the first parent, (2) the second parent, (3) either parent, and (4) neither parent?
b. What proportion of progeny will be genotypically the same as (1) the first parent, (2) the second parent, (3) either parent, and (4) neither parent?
See answer

13. | From several crosses of the general type A / A. B / B × a / a.b / b the F1 individuals of type A / a.B / b were testcrossed to a / a.b / b. The results are shown below: For each set of progeny, use the c 2 test to decide if there is evidence of linkage. See answer

14.

In the following pedigree, the vertical lines stand for protan color blindness, and the horizontal lines stand for deutan color blindness. These are separate conditions causing different misperceptions of colors; each is determined by a separate gene.

a. Does the pedigree show any evidence that the genes are linked?
b. If there is linkage, does the pedigree show any evidence of crossing-over? Explain both your answers with the aid of the diagram.
c. Can you calculate a value for the recombination between these genes? Does this recombination arise from independent assortment or crossing-over?

15.

A three-point testcross was made in corn. The results and a partial recombination analysis are shown in the following display, which is typical of three-point testcrosses (p = purple leaves, + = green; v = virus-resistant seedlings, + = sensitive; b = brown midriff to seed, + = plain). Study the display and answer the questions below.

a. Determine which genes are linked.
b. Draw a map that shows distances in map units.
c. Calculate interference if appropriate.

Unpacking the Problem

a. Sketch cartoon drawings of the parent (P), F1, and tester corn plants, and use arrows to show exactly how you would perform this experiment. Show where seeds are collected.
b. Why do all the +'s look the same, even for different genes? Why does this not cause confusion?
c. How can a phenotype be purple and brown (for example) at the same time?
d. Is it significant that the genes are written in the order p-v-b in the problem?
e. What is a tester and why is it used in this analysis?
f. What does the column marked "progeny phenotypes" represent? In class 1, for example, state exactly what "gre sen pla" means.
g. What does the line marked "gametes" represent, and how is this different from the column marked "F1 gametes"? In what way is comparison of these two types of gametes relevant to recombination?
h. Which meiosis is the main focus of study? Label it on your drawing.
i. Why are the gametes from the tester not shown?
j. Why are there only eight phenotypic classes? Are there any classes missing?
k. What classes (and in what proportions) would be expected if all the genes are on separate chromosomes?
l. What do the four class sizes (two very big, two intermediate, two intermediate, two very small) correspond to?
m. What can you tell about gene order from inspecting the phenotypic classes and their frequencies?
n. What would be the expected phenotypic class distribution if only two genes are linked?
o. What does the word point refer to in a three-point testcross? Does this word usage imply linkage? What would a four-point testcross be like?
p. What is the definition of recombinant, and how is it applied here?
q. What do the "recombinant for" columns mean?
r. Why are there only three "recombinant for" columns?
s. What do the R's mean, and how are they determined?
t. What do the column totals signify? How are they used?
u. What is the diagnostic test for linkage?
v. What is a map unit? Is it the same as a centimorgan?
w. In a three-point testcross such as this one, why aren't the F1 and the tester considered to be parental in calculating recombination? (They are parents in one sense.)
x. What is the formula for interference? How are the "expected" frequencies calculated in the coefficient of coincidence formula?
y. Why does part c of the problem say "if appropriate"?
z. How much work is it to obtain such a large progeny size in corn? Approximately how many progeny are represented by one corn cob?

16.

R. A. Emerson crossed two different pure-breeding lines of corn and obtained a phenotypically wild-type F1 that was heterozygous for alleles of three different genes that determine recessive phenotypes: an determines the mutant phenotype anther; br, brachytic; and f, fine. He testcrossed the F1 to a tester that was homozygous recessive for the three genes and obtained these progeny phenotypes: 355 anther; 339 brachytic, fine; 88 completely wild-type; 55 anther, brachytic, fine; 21 fine; 17 anther, brachytic; 2 brachytic; 2 anther, fine.
a. What were the genotypes of the parental lines?
b. Draw a linkage map for the three genes (include map distances).
c. Calculate the interference value.

17.

From the five sets of data given in the following table determine the order of genes by inspectionthat is, without calculating recombination values. Recessive phenotypes are symbolized by lowercase letters, and dominant phenotypes by pluses.

See answer

18.

In the plant Arabidopsis thaliana the loci for pod length (L = long, l = short) and fruit hairs (H = hairy, h = smooth) are linked 16 map units apart on the same chromosome. The following crosses were made:

If the F1s from above are crossed,
a. What proportion of the progeny are expected to be l h/l h?
b. What proportion of the progeny are expected to be L h/l h?

19. | Assume in an ascomycete fungus that heteroduplex DNA occurs at a heterozygous locus A/a (consult Figure 5-18). Assume that an A strand invades an a helix at the same frequency as the invasion of A by a. Assume no branch migration occurs. Also assume heteroduplexes are repaired to A 80 percent of the time, and to a 20 percent of the time. Out of all the cases of heteroduplex DNA at that site, what proportion of heteroduplexes will become converted into aberrant ratios of the types 6:2, 2:6, 5:3, and 3:5?

20.

The plant Haplopappus gracilis is diploid, and 2n = 4. There are one long pair and one short pair of chromosomes. The accompanying diagrams represent anaphases ("pulling-apart" stages) of individual cells during meiosis or mitosis in a plant that is genetically a dihybrid (A/a ; B/b) for genes on different chromosomes. The lines represent chromosomes or chromatids, and the points of the V's represent centromeres. In each case, determine whether the diagram represents a cell in meiosis I, meiosis II, or mitosis. If a diagram shows an impossible situation, indicate this.

See answer

21. | The Neurospora hist-1 gene, which is on the right arm of chromosome 1, is involved in the biochemical pathway for the synthesis of histidine. Two mutations of the hist-1 gene were obtained in separate experiments. Both left the strains auxotrophicthat is, they would not grow unless histidine was added to the growth medium. These mutant strains were stable and never reverted to wild type. The two mutants were crossed together (hist-1i × hist-1ii), and thousands of ascospores were spread onto medium lacking histidine (minimal medium). Most ascospores did not grow, but three colonies were observed. Draw a diagram to account for the origin of these rare prototrophic colonies.

22.

The year is 1868. You are a skilled young lens maker working in Vienna. With your superior new lenses you have just built a microscope that has better resolution than any others available. During your testing of this microscope you have been observing the cells in the testes of grasshoppers, and have been fascinated by the behavior of strange elongated structures you have seen within the dividing cells. One day in the library you read a recent journal paper by G. Mendel on hypothetical "factors" that he claims explain the results of certain crosses in peas. In a flash of revelation you are struck by the parallels between your grasshopper studies and Mendel's ratios (1:1:1:1, 9:3:3:1, etc.), and you resolve to write him a letter. What do you write?

(Problem 22 based on an idea by Ernest Kroeker.)

Pattern Recognition Problems

In Problems 23 through 27 diagrams show phenotypes and the results of breeding analyses. Deduce the genotypes of the individuals shown in each diagram as far as possible.

23. |

24. |

25. |

26. |

27. |

28.

In Drosophila the genes for wing size (phenotypes large or vestigial) and bristle length (phenotypes long or short) are inherited independently. The following crosses are made:

a. Deduce the dominance relations of the phenotypes involved.
b. Deduce the genotypes of the parents in the four crosses.
See answer

29.

In Drosophila the genes for ebony body (e) and stubby bristles (s) are linked on the same arm of chromosome 2. Flies of genotype + s/e + developed predominantly as wild type but occasionally showed two different types of unexpected abnormalities on their bodies. The first abnormality was pairs of adjacent patches, one with stubby bristles and the other with ebony body. The second abnormality was solitary patches of ebony color.
a. Draw diagrams to show the likely origins of these two types of unexpected abnormalities.
b. Explain why there were no single patches that were stubby.

30.

A plant of genotype Q/q.R/r.T/t produces the following gamete genotypes in the proportions shown:

Draw labeled meiosis diagrams to explain clearly how these gametic proportions were produced. Show the chromosomes
a. Before premeiotic S phase (replication phase)
b. After chromatid formation
c. During pairing
d. At anaphase I
e. At anaphase II

31.

In beans, tall (T) is dominant to short (t), red flowers (R) is dominant to white (r), and wide leaves (W) is dominant to narrow (w). The following cross is made and progeny obtained as shown:

a. Explain why these progeny phenotypes were obtained and in the proportions observed (list all genotypes and show chromosomal positions).
b. Under your hypothesis, if the tall, red, wide parent is selfed, what will be the proportion of short, white, wide progeny?
See answer

© 1999 by W. H. Freeman and Company. All rights reserved.