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Linked Genes and
Recombination
*1. In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (C) is recessive to the allele
for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here,
yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous unbanded, brown snail. The F1 are then crossed to banded, yellow snails (a testcross).
a. What will be the results of the testcross if the loci that control banding and color are linked with
no crossing over?
b. What will be the results of the testcross if the loci assort independently?
c. What will be the results of the testcross if the loci are linked and 20 map units apart?
*2. In silkmoths (Bombyx mori) red eyes (re) and white-banded wing (wb) are two mutant alleles that
are recessive to those that produce wild-type traits (re+ and wb+); these two genes occur on the same
chromosome. A moth homozygous for red eyes and white-banded wings is crossed with a moth
homozygous for the wild-type traits. The F1 have normal eyes and normal wings. The F1 are crossed
with moths that have red eyes and white-banded wings in a testcross. The progeny of this testcross are
shown below.
wild-type eyes, wild-type wings
418
red eyes, wild-type wings
19
wild-type eyes, white-banded wings
16
red eyes, white-banded wings
426
a. What phenotypic proportions would be expected if the genes for red eyes and white-banded wings
were located on different chromosomes?
b. What is the genetic distance between the genes for red eyes and white-banded wings?
*3. In cucumbers, heart-shaped leaves (hl) is recessive to normal leaves (Hl), and many fruit spines (ns)
is recessive to few fruit spines (Nl). The genes for leaf shape and number of spines are located on the
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Pierce, Genetics: A Conceptual Approach
Chapter 7 Linked Genes and Recombination
same chromosome; previous mapping experiments indicate they are 32.6 map units apart. A cucumber
plant with heart-shaped leaves and many spines is crossed with a plant that is homozygous for normal
leaves and few spines. The F1 are crossed with plants that have heart-shaped leaves and many spines.
What phenotypes and proportions are expected in the progeny of this cross?
**4. In Drosophila melanogaster, ebony body (e) and rough eyes (ro) are autosomal recessive genes found
on chromosome 3; they are separated by 20 map units. Forked bristles (f) is an X-linked recessive gene
that assorts independently of e and ro. Give the phenotypes of progeny and their expected proportions
when each of the following genotypes is testcrossed.
+
a. e
e
+
ro + f
crossed with ebony body, rough eyes, and forked bristles male.
ro
f
+
b. e
e
ro
ro+
f
f
crossed with ebony body, rough eyes, and forked bristles male.
*5. A series of two-point crosses among fruit flies are carried out between genes for brown eyes (bw), arc
wings (a), vestigial wings (vg), ebony body color (e), and curved wings (cv). For each of the following
crosses, the number of nonrecombinant (parental) and recombinant progeny obtained is shown below.
Genes in cross
Number of
nonrecombinant
(parental) progeny
Number of
recombinant
progeny
Recombination
frequency
a, bw
2224
117
117/2341 = 5%
a, cv
2609
823
823/3432 = 24%
a, e
3200
3200
3200/6400 = 50%
a, vg
5172
2433
2433/7605 = 32%
bw, cv
4614
1706
1706/6320 = 27%
bw, e
4150
4150
4150/8300 = 50%
bw, vg
2796
1440
1440/4236 = 34%
cv, e
3116
3117
3117/6233 = 50%
cv, vg
2102
208
208/2310 = 9%
Using these data from two-point crosses, create a genetic map for these genes, providing the order of
the genes and the best estimates of the distances separating them.
*6. Waxy endosperm (wx), shrunken endosperm (sh), and yellow seedling (v) are three recessive genes in
corn that are linked on chromosome 5. A corn plant homozygous for all three recessive genes is crossed
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Pierce, Genetics: A Conceptual Approach
Chapter 7 Linked Genes and Recombination
with a plant homozygous for all the dominant genes. The resulting F1 are then crossed with a plant
homozygous for the recessive genes in a three-point testcross. The progeny of the testcross are given below.
wx sh V
87
Wx Sh v
94
Wx Sh V
3479
wx sh v
3478
Wx sh V
1515
wx Sh v
1531
wx Sh V
292
Wx sh v
280
Total
10,756
a. Determine the order of these genes on the chromosome.
b. Calculate the map distances between the genes.
c. Determine the coefficient of coincidence and the interference among these genes.
*7. Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers whose genes are linked on the same chromosome. A cucumber plant heterozygous for all three
traits is used in a testcross, and the following progeny are produced from this testcross.
S U Tu
2
s u Tu
70
S u Tu
21
s u tu
4
S U tu
82
s U tu
21
s U Tu
13
S u tu
17
Total
230
a. Determine the order of these genes on the chromosome.
b. Calculate the map distances between the genes.
c. Determine the coefficient of coincidence and the interference among these genes.
d. List the genes found on each chromosome in the parents used in the testcross.
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Pierce, Genetics: A Conceptual Approach
Chapter 7 Linked Genes and Recombination
*8. In Drosophila melanogaster, black body (b) is recessive to gray body (b+), purple eyes (pr) is recessive to red eyes (pr+), and vestigial wings (vg) is recessive to normal wings (vg+). The loci coding for
these traits are linked, with the map distances shown below.
b
pr
vg
6
13
The interference among these genes is 0.5. A fly with black body, purple eyes, and vestigial wings is
crossed with a fly homozygous for gray body, red eyes, and normal wings. The female progeny are then
crossed with males that have black body, purple eyes, and vestigial wings. If 1000 progeny are produced
from this testcross, what will the phenotypes and proportions of the progeny be?
**9. The locations of six deletions have been mapped to the Drosophila chromosome shown below.
Recessive mutations a, b, c, d, e, and f are known to be located in the same region as the deletions, but
the order of the mutations on the chromosome is not known. When flies homozygous for the recessive
mutations are crossed to flies homozygous for the deletions, the following results are obtained, where
m represents a mutant phenotype and + represents the wild type. On the basis of these data, determine
the relative order of the seven mutant genes on the chromosome.
chromosome
deletion 1
deletion 2
deletion 3
deletion 4
deletion 5
deletion 6
Mutations
Deletion
a
b
c
d
e
f
1
m
+
m
+
+
m
2
m
+
+
+
+
+
3
+
m
m
m
m
+
4
5
+
+
+
+
m
+
m
m
m
m
+
+
6
+
m
+
m
+
+
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Pierce, Genetics: A Conceptual Approach
Chapter 7 Linked Genes and Recombination
**10. A panel of cell lines was created from mouse-human somatic cell fusions. Each line was examined for the presence of human chromosomes and for the production of three enzymes. The following
results were obtained.
Human chromosomes
Cell line
Enzyme 1
Enzyme 2
Enzyme 3
4
8
9
12
15
16
17
22
X
A
B
C
D
+
+
–
–
–
–
+
+
+
+
+
–
–
+
+
–
–
–
+
+
+
–
–
–
–
–
–
+
–
–
–
+
+
–
+
–
+
+
–
–
–
–
–
+
–
+
+
Based on these results, give the chromosome locations of enzyme 1, enzyme 2, and enzyme 3.
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