Consider a Drosophila mapping cross performed for 3 genes on the q-arm of a chro

Question

Consider a Drosophila mapping cross performed for 3 genes on the q-arm of a chromosome, with locus A located twice as far away in cM units from locus B as locus C is from B (A and C are even farther apart). Assume that from multiple cross experiments, one observes double cross-over events at a frequency of 0.086±0.04. If there is cross-over interference in the amount of -0.17, draw a genetic map and determine the distance in map units of locus A from B and locus B from C. Now, using the upper and lower bounds of the observed DCO frequency within the experimental error range, what would you expect the upper and lower bounds of the distances between these genes (i.e., A…B and B…C)?

Explanation / Answer

Lack of independence is called interference

Resulted in the observation of fewer double crossover types than expected according to true map distance.

1-cc= interference

Expected = 0.914

Observed = 0.086

Coefficient of coincidence = 8.6/0.914x0.086x1000 = 8.6/78.60=0.109

C.C. values between 0 and 1 indicate partial interference. Generally interference decreases as the
distance between the loci increases.

A--------------------------------------B------------------------------------------C

45.5mu                                                     45.5mu

Expected upper bound = 95.4 mu

Expected lower bound = 87.4 mu   between A----------C

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