Introduction

When two events are independent, the occurrence of one event does not influence the relative frequency of a second event. We might expect the relative frequency of double crossover events to decrease as flanking loci are located more closely. One chiasma might interfere with the occurrence of a second chiasma for loci located very closely together.

In out previous example, we found that the relative frequency of single crossovers between Lz and Su was 0.107 and between su and Gl was 0.148. If the event of a crossover between Lz and Su is independent of the event of a crossover between Su and Gl then we can find the relative frequency of a double crossover by multiplying the relative freqeuncies of each separate event.

0.107 x 0.148 = 0.0158 expected double crossover relative
0.107 x 0.148 = frequency

0.0158 x 740 = 12 expected double crossover events

We only observed 4+2=6 double crossover events and 12 events were expected. This result suggests that a crossover in one region is not independent of the probability of a single crossover in an adjacent region.

** Interference exists when a single crossover in one region reduces the probability of a single crossover in an adjacent region.

The coeffiient of coincidence is the observed number divided by the expected number of double crossovers. In our example 6 are observed and 12 are expected, so 6/12 = 1/2 = coefficient of coincidence. The interference is defined as:

Interference = 1 - coefficient of coincidence
Interference = 1 - 1/2 = 1/2

C =  Q 
C = QQ

Interference is negative if C>1 and Interference = 1-C<0

Interference is positive if C<1 and Interference = 1-C>0

Interference is absent if C=1 and Interference = 1-C=0

Interference is complete if C=0 and Interference = 1-C=1