Other than providing information about the relative distances that separate genes, map distances also gives us information regarding the proportions of recombinant and nonrecombinant gametes produced in a cross. A map distance of 7.5 m.u. between two genes indicates that 7.5% of the gametes produced by an organism that is heterozygous at both loci will be recombinant.
Double crossovers, as mentioned previously, cause an underestimation of map distances. If we were to theoretically calculate the proportion of double recombinant gametes using the rule of probability (multiplication), and then multiply this theoretical probability by the total number of progeny, we would obtain the expected number of double crossover progeny. In reality, much less are observed in the progeny produced. This is because the calculated number assumes that each crossover is independent of each other. Crossovers are not independent – one crossover may inhibit other crossovers in the nearby vicinity on the chromosome, so double crossovers become less frequent than expected.
The term interference is used to describe the degree to which one crossover interferes with other crossovers in the region at the chromosome in question. We are able to calculate the interference using the following formula:
Interference = 1 – coefficient of coincidence
Now, the coefficient of coincidence can be calculated by the following formula:
Coefficient of coincidence = Number of observed double crossovers / Number of expected double crossovers
Please visit North Dakota State University’s website to read Genetic Linkage, by Phillip McClean (1998), for a worked example of these types of calculations.
Look at the videos below by Catalyst University (2018) and Chegg (2018), for other worked examples on interference and coefficient of coincidence.
Catalyst University. (2018, March 29). Genetics: Linkage problem #1: Map distance, coefficient of coincidence, and interference (video file). YouTube. https://www.youtube.com/watch?v=l7iOQDZsY7c
Chegg. (2016, March 21). Interference and coincidence | Biology | Chegg Tutors (video file). YouTube. https://www.youtube.com/watch?v=sJXLg-YSVlY
McClean, P. (1998). Deriving linkage distance and gene order from three-point crosses. Genetic Linkage/North Dakota State University. https://www.ndsu.edu/pubweb/~mcclean/plsc431/linkage/linkage3.htm