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Genetics Challenge Problems for Bio SuperStars (Part 2)
Solving Linkage and Crossing-Over Problems
Linked genes are found on the same chromosome, and are therefore usually inherited together. Linked genes
may be separated from one another if crossing over occurs between homologous chromosomes. The closer
together two genes are on a chromosome, the less frequently crossing over will occur between them. In other
words, determining the frequency of cross-over (%CO) gives us information about the relative distance
between two genes.
A typical linkage question will ask (1) whether genes are linked, (2) which genes are linked, and (3) what is the
% cross-over.
Here's a typical problem: Parent 1 (AAbb) is crossed with Parent 2 (aaBB)
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Next, F1 offspring are crossed with a homozygous recessive individual. This will allow us to see the
recessive traits.
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Draw a Punnett square for the above cross. What phenotypic ratio would you predict?
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Genetics Challenge Problems for Bio SuperStars (Part 2)
That's Right! A 1:1:1:1 Ratio!
Punnett squares assume that genes are not linked, and that independent assortment is happening. To
determine whether genes are linked, we compare our observed results with our expected (Punnett square)
results. If your observed results match the 1:1:1:1 ratio you predicted, then chances are good that your genes
of interest are not linked.
BUT....
If your genes of interest are linked, the phenotypic ratios will be altered.
Using our earlier example: AaBb x aabb
Non-Linked Results
Possible linked results
AaBb
25%
10%
Aabb
25%
40%
aaBb
25%
40%
aabb
25%
10%
Aabb and aaBb show up more frequently than expected. We can assume these are our major/expected
products. AaBb and aabb are our minor/mutated products. (They resulted from crossing over.)
To
1.
2.
3.
4.
5.
6.
Summarize:
The Punnett square gives us an expected ratio.
If this is ratio is not present, genes must be linked.
The major products are the two equal and larger numbers.
The minor (mutant) products are the equal and smaller numbers.
Calculate % Cross-over as follows: Sum of mutants/total offspring x 100
1% crossing-over = 1 map unit of distance between genes.
Let's Practice
1. A pure nice but very ugly Drood marries a pure handsome but mean Drood. Droods, of course, produce
hundreds of offspring at a time. The following results were noted when a hybrid was backcrossed:
Nice, very u g l y ^
13
'.h
x
^ k
Nice, handsome
6
Mean, handsome
97
Mean, very ugly
4
On
M
A. Do the expected results match your predicted ratio?
B. Are the genes linked?
n
k)o
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C. If so, what is the % CO? ( ' * / » o o ) >W
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2
Genetics Challenge Problems for Bio SuperStars (Part 2)
2. In Gracklees, the C gene turns on color (is epistatic to) the B gene. The B allele produces a black color,
while the b allele produces no color (white).
Key:
C _ B _ = black
C _ b b = white
cc
= white
P1: CCbb x ccBB produced CcBb (F1). F1 offspring were backcrossed with pure recessive ccbb.
F2:
Black 13
White 40
(Careful! Only 2 phenotypes.)
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f
7
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A. Do the expected results match your predicted ratio?
B. Are the genes linked?
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1
.
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C. If so, what is the % CO?
3. Smoothies have a color gene (C) which turns on a second gene pair. An organism with 2 recessive alleles
for this gene (cc) will be white. The second gene pair can produce either AA (red), Aa(orange), or aa(yellow).
P1: CCaa x ccAA producing F1 (all hybrids)
F1: CcAa were backcrossed with ccaa.
F2: ^Orange
6
— White
40
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34
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— r ^ ^ l —
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A
t^A±:^5-iccA^
ofa^
1^
ujUfc| i
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l 5 w >
1
0
YM' ** • A. Do the expected results match your predicted ratio?
B. Are the genes linked?
C. If so, what is the % CO?
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4. In mice, the following occurs: C (full color) is dominant to c (white). Agouti (A, two-colored) is dominant to
n0n
-
a 9 0 U t i (a)
-
'
i L C ^ c A
_C^
P1: CCaa x ccAA produces F1 (all hybrids)
C <C \p
k \P
\
A
f
j
Ir j ^^ ^- A^* J- VJ- ^
c ^S l^ ^! ^t A
c. a. A
F1: CcAa x ccaa
^ ' <x^ \c \Cc^
F2:
agouti
144
fapM
^
i^uTfcl
albino
290
nonnon-agouti
145
I' I '•
A. Do the expected results match your predicted ratio?
B. Are the genes linked?
S'xA
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C. If so, what is the % CO?
3
Genetics Challenge Problems for Bio SuperStars (Part 2)
5. Results from another series of crosses with the same key as above but in Snuffles produced the following:
agouti
5
albino
(50><
non-agouti
45
r / w v - «o*- ^ w ^ ^
A. Do the expected results match your predicted ratio?
AJQ
B. Are the genes linked?
C. If so, what is the % CO?
( 10 ( jOQ ) * I & °
- I D * |,
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