BioSc 231
General Genetics
Exam 3
Name __________________________________
Multiple Choice. (2 points each)
__C__ Recombination frequencies ___
A. are the same for all genes.
B. arise from specific genetic exchange.
C. are the same for cis and trans heterozygotes.
D. decrease with distance.
__ B__ A plant of genotype C/C;d/d is crossed to c/c;D/D and an F1 testcrossed to c/c;d/d. If the genes are linked and 40 map units
apart, the percentage of c/c;d/d recombinants will be
A. 10%
B. 20%
C. 30%
D. 40%
E. 50%
__ C__ The maximum recombination frequency between two genes is
A. 100%
B. 80%
C. 50%
D. 10%
E. 1%
__ B__ In Drosophila, the two genes w and sn are X-linked and 25 map units apart. A female fly of genotype w+ sn+/w sn is crossed to
a male from a wild-type line. What percentage of male progeny will be w+ sn?
A. 0
B. 12.5%
C. 25%
D. 37.5%
E. 50%
__ C__ The maize genes sh and bz are linked, 40 map units apart. If a plant sh+ bz/sh bz+ is selfed, what proportion of the progeny
will be sh bz/sh bz?
A. 0.04
B. 0.10
C. 0.20
D. 0.40
__ A__ In maize, two plants that are both heterozygous for the recessive alleles a and b are crossed. What frequency of doublemutant progeny will appear if a and b are 7.2 map units apart and both parents carry a and b in trans?
A. 0.036
B. 0.0625
C. 0.001296
D. 0.005184
E. 0.072
__ D__ In crossing over
A. Genetic exchange occurs before chromosome replication
B. The probability of its occurrence decreases with increasing distance between the genes exchanged
C. Occurs between two loci very close together
D. The reciprocal exchange between homologous chromosomes is random
__ A__ The F, G and H loci are linked in the order written. There are 30 map units between F and G, and 30 map units between G
and H. If a plant F G h/f g H is testcrossed, what proportion of the progeny plants will be f g h/f g h if there is complete interference?
A. 0.15
B. 0.21
C. 0.245
D. 0.3
E. 0.7
__ C__ Out of 800 progeny of a three-point testcross there were 24 double crossover recombinants, whereas 30 had been expected on
the basis of no interference. The interference is
A. 0.05
B. 0.10
C. 0.20
D. 0.50
E. 0.80
__ B__ In a four-point testcross the number of phenotypic classes of progeny expected is
A. 4
B. 8
C. 16
D. 32
E. depends on the gene linkage of the tetrahybrid
__ B__ In a cross of two yeast strains of genotypes c+ a+ X c a the progeny were
42 c+ a+
46 c a
5 c+ a
7 c a+
The frequency of recombination is
A. 11%
B. 12%
C. 24%
D. 50%
E. 76%
__ D__ Two genes with a 50% recombination frequency are
A. Present on homologous chromosomes
B. Located close by
C. Linked
D. Far apart on a single chromosome
__ E__ In testcross experiments, the frequency of recombination between genes a and b is 0.3; between b and c, 0.2; between c and d,
0.1; between a and c, 0.5; and between b and d, 0.3. What is the order of the three genes?
A. c-b-d-a
B. d-c-a-b
C. a-d-c-b
D. b-a-c-d
E. a-b-c-d
__ A__ Three-point test crosses
A. Detect double crossing over events
B. Require heterozygous gametes
C. Cannot determine gene order on a chromosome
D. Determine goodness of fit
__ B__ Genes ho and dp are 9.0 map units apart, and genes dp and da are 26.3 map units apart. What is the probability of a double
crossover in these regions?
A. 1.3%
B. 2.3%
C. 3.4%
D. 4.5%
E. 5.6%
__ A__ In Drosophila, the genes r and s are linked. Flies of genotype r+ s+/ r+ s+ and r s/ r s are crossed and an F1 obtained. The F1
allele arrangement is called
A. cis
B. trans
C. recombinant
D. complementary
E. crossover
__ C__ The Drosophila genes for white eyes (w), tan body (t) and vestigial wings (vg) lie at map positions 2, 22, and 52 respectively.
Out of 500 progeny, 21 exhibited double crossovers. What is the degree of interference?
A. 0.10
B. 0.20
C. 0.30
D. 0.40
__ A__ One Drosophila genotype P m/P m is crossed with another genotype p m/p m. If the genes are closely linked
A. All progeny will be phenotypically Pm
B. All progeny will be phenotypically pm
C. 50% of the progeny will be phenotypically Pm and 50% phenotypically pm
D. None of the above
__ B__ A plant of genotype C/C; D/D is crossed to c/c; d/d and an F1 testcrossed to c/c;d/d. If the genes are unlinked, the percentage
of c/c;d/d recombinants will be
A. 10%
B. 25%
C. 30%
D. 40%
E. 50%
__ D__ Which of the following statements is true regarding recombination between genes on homologous chromosomes?
A. Equivalent ratios (1:1:1:1) are found in progeny from homozygous dihybrid crosses
B. Recombination frequencies are greater than 50%
C. 25% of each genotype will appear among the progeny from dihybrid crosses
D. Exchange of the middle pair of alleles occurs in double crossovers
Problems (variable points each)
Two-point testcrosses revealed the following map results:
am___________pm 23 map units
pm___________nn 16 map units
A. Draw the two possible maps for these loci. (2)
am 23
pm 16 nn
or
am 7
nn
16
pm
B. What cross would resolve the two possible maps and what are the possible outcomes of that cross? (1)
A two-point testcross to map the distance between am and nn. If the first map is correct the distance would be 39
mu and if the second map is correct the distance would be 7 mu
You could also do a three factor test cross and low recombination freq. would indicate which gene is in the middle.
Consider a fully heterozygous flowering plant with normal blue flowers and round seeds. This plant, when crossed with one that has
red flowers and oval seeds, results in the following progeny:
91 blue, round
94 red, round
93 blue, oval
92 red, oval
A. What is the recombination frequency? (2)
50%
B. Are these genes linked? (1)
NO
What gametes are produced by an individual with the genotype kL/Kl if the genes are A) linked (no crossing over) and B) unlinked?
(2)
Linked - kL and Kl
unlinked - kL, kl, KL, Kl
Several testcrosses have revealed the rate of recombination for the following genes:
k-m 0.06
k-l 0.08
l-m 0.13
A. What is the order of the three genes? (2)
l-k-m
B. Why is the frequency of recombination between the two closest pairs of genes not equal to the recombination rate between
the two genes farthest apart? (1)
Double crossover events lead to reduced recombination frequency between the two outside genes.
An organism heterozygous for three genes AaBbCc was mated to a homozygote aabbcc, resulting in the progeny distribution below.
ABC
aBC
AbC
ABc
abC
aBc
Abc
abc
327
44
30
3
4
45
28
319
----800
Determine the gene order (1) and calculate recombination frequencies between each gene (3) and interference. (1)
Gene Order - A - C - B
Recombination frequency -
A - C = 9.8%
C - B = 10.2%
A - B = 18.4%
Interference -
coefficient of coincidence = 7/8
I = 1 - 7/8 = 1/8
An Arabidopsis thaliana flowering mutation has been mapped very close to an RFLP marker on chromosome 3 (so close in fact that
no recombinants are detected between the mutation and the RFLP marker). The RFLP is due to the presence of a VNTR (variable
number tandem repeat) located between two restriction endonuclease cleavage sites. The RFLP allele from the plant that carries the
flowering mutation contains 3 copies of the VNTR and the RFLP allele from the wild-type plant contains 5 copies of the VNTR. Use
any of the methods used in class (i.e. linear diagram of chromosome 3 or diagram of how the RFLP would appear using gel
electrophoresis, etc.) to illustrate the difference between the chromosome that contains the wild-type allele and the flowering mutant
allele. (4)
5 point bonus question
A cross was made between an aaddGG female and an AADDgg male. The resulting F1 females were testcrossed and 1000 offspring
were produced. The F2 phenotypes and numbers were:
ADg
aDG
adg
aDg
AdG
ADG
Adg
adG
197
201
45
51
49
55
199
203
Which genes are linked and what is the recombination frequency?
A and G are linked and D is either on a separate chromosome or else linked but far away from the other genes.
A series of two-point mapping crosses indicated the following map distances involving A, D, and G and the additional genes B, C, E
and F.
C-A
A-E
B-F
D-F
C-E
C-G
D-B
7 mu
5 mu
20 mu
10 mu
12 mu
27 mu
30 mu
Draw a map for all the genes showing the correct gene sequences and map units between the loci.
C -7- A -5- E -15-G
D -10- F -20- B