Unformatted text preview: from vg/b and have map distance between
pr and b Why are double crossover events so rare?
Product of separate independent probabilities:
Probability of recombinants from crossover in region 1 alone
= 10% or 10 m.u.
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%
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
Observed double crossovers are 3+3 = 6/10,000 = .06
The number of observed double crossovers may
be less than 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
It is not uniform, and may vary for different regions of the
A quantitative measure of the amount of interference in a
particular chromosomal region is first obtained by
calculating the coefficient of coincidence
Coefficient of coincidence = Frequency observed (DCO)
Frequency expected (DCO) Interference = 1- coefficient of coincidence
If there is no interference (interference = 0), then the observed
frequency of double crossovers is equal to the expected
If interference is complete (interference = 1), then there are no
double crossovers observed. Interference
Sample problem: The probability of a crossover
in Region 1 is 20%, and in Region 2 is 6%.
The observed rate of double crossovers for the
area encompassing Regions 1 and 2 is 0.9%.
Calculate the interference in this region.
Expected double crossover frequency = (0.2)(0.06)
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- Spring '10
- Genetics, Recombinants