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Unformatted text preview: gion of the chromosome will reduce
the likelihood of a crossover in an adjacent
part of the chromosome
i.e., if the recombination frequency in region I
is 15% and 5% in region II, we would expect
the frequency of double crossovers for regions
I and II together to be (0.15)(0.05) = 0.0075 = 0.75%
A smaller value would indicate interference.
28 Interference
Will occur when examining 3 genes that
are relatively close together
difficult to get doublecross over events 29 Interference
It is not uniform, and may vary for different
regions of the chromosome
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
Frequency expected Interference = 1 coefficient of coincidence
30 Interference
Coefficient of coincidence = Frequency observed
Frequency expected Interference = 1 coefficient of coincidence 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.
31 Interference
Sample problem: The recombination frequency
in region I is 10%, and in region II is 9%. The
observed rate of double crossovers encompassing
both regions is 0.6%. Calculate the interference.
Expected double crossover frequency = (0.1) (0.09)
= 0.009
0.006
Coefficient of coincidence = 0.009 = 0.667
Interference = 1 – 0.667 = 0.33 = 33%
means you are seeing 33% less dco than you should. due to interference. 32 Next topic
DNA
Chapter 9
pp 192204 33...
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This document was uploaded on 02/25/2014.
 Winter '14
 Genetics

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