BLY302
FALL 2010
PRACTICE PROBLEMS FOR EXAM 2
Q1
If the recombination frequency between genes (
A
) and (
B
) is 5.3%, what is
the distance between the genes in map units on the linkage map?
ANSWER: This recombination frequency is small, which indicates these
gene loci are fairly close together on the chromosome.
Because map
distance is defined as the recombination frequency between two loci, the
genetic distance between these genes is 5.3 cM.
Q2
Assume that the coefficient of coincidence (CC) for a map distance of 24 cM
= 0.25.
Consider the following 3factor (trihybrid) testcross:
+
+
c
(x)
abc/abc
a
b
+
10.6 cM
13.4 cM
Calculate the number of progeny in each phenotype class (parentals, SCOs, and
DCOs) resulting from this cross.
Base your calculations on a total of 1000
progeny.
Hint:
The actual frequency of
doublecrossover progeny
can be determined
by multiplying the frequencies of recombination for each single crossover
together (to calculate expected DCOs), and then multiplying that product by the
coefficient of coincidence for the known map distance between the alleles (to
calculate the actual, observed DCOs).
Remember that CC = DCO
obs
/ DCO
exp
; and crossover Interference (
I
) = 1 –
CC.
These parameters are not just obscure concepts, they are closely
related and can have very practical applications such as predicting the
percentage of doublecrossover progeny in a given cross.
(1) The recombination frequency between genes
A—B
= 10.6% = 0.106
(2) The recombination frequency between genes
B—C
= 13.4% = 0.134
(3) Calculate the expected number of doublecrossover progeny:
(0.106)(0.134) = 0.0142
= expected DCO frequency
(0.0142)(1000) = DCO
exp
= 14.2 individuals
(4) CC = DCO
obs
/ DCO
exp
.
Therefore, DCO
obs
= (0.25)(14.2) = 3.55
≈
4.
This
means that of 1000 progeny, you will see four doublecrossover
phenotypes.
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FALL 2010
(5) Now you need to determine the numbers of progeny for each single
crossover.
Number of progeny (phenotypes) displaying a SCO between
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 Fall '10
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 Genetic linkage, Chromosomal crossover, recombination frequency, DCOs

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