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Unformatted text preview: from vg/b and have map distance between
pr and b Why are double crossover events so rare?
Why
Product of separate independent probabilities:
Probability of recombinants from crossover in region 1 alone
= 10% or 10 m.u.
10
Probability of recombinants from crossover in region 2 alone
= 20% or 20 m.u.
20 The probability of a double crossover is 0.10 x 0.20 = 2%
0.10
1.56 m.u y w 4.06 m.u ec 5.5 m.u For our working example:
probability of single crossover in region 1 = 0.0156
probability of single crossover in region 2 = 0.0406
Expected probability double crossover is .0156 x .0406 = .063
.063
Observed double crossovers are 3+3 = 6/10,000 = .06
.06 Interference
Interference
The number of observed double crossovers may
The
observed
be less than expected
be
expected
Why ? Because sometimes a crossover in
one region of the chromosome will reduce
the likelihood of a crossover in an adjacent
part of the chromosome
A smaller observed value than expected would
indicate interference Interference
Interference
It is not uniform, and may vary for different regions of the
It
chromosome
chromosome
A quantitative measure of the amount of interference in a
quantitative
particular chromosomal region is first obtained by
calculating the coefficient of coincidence
coefficient
Coefficient of coincidence = Frequency observed (DCO)
Frequency expected (DCO) Interference = 1 coefficient of coincidence
Interference
If there is no interference (interference = 0), then the observed
frequency of double crossovers is equal to the expected
frequency.
If interference is complete (interference = 1), then there are no
double crossovers observed. Interference
Interference
Sample problem: The probability of a crossover
in Region 1 is 20%, and in Region 2 is 6%.
in Region
Region
The observed rate of double crossovers for the
observed
area encompassing Regions 1 and 2 is 0.9%.
area
0.9%.
Calculate the interference in this region.
Expected double crossover frequency = (0.2)(0.06)
= 0.012
= 1.2%
1.2%
0.9
0.9
Coeffic...
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This note was uploaded on 10/04/2011 for the course BIOL 139 taught by Professor Christinedupont during the Spring '10 term at Waterloo.
 Spring '10
 christineDupont
 Genetics

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