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

Interference = 1- Coefficient of coincidence

=1-(observed Double crossover/Expected double crossover)

-0.17 =1-0.086/Expected DCO

Expected double crossover= 0.073 =7.3%

The expected 2CO's are found from the equation:

exp 2CO = (interval I distance/100) * (interval 2 distance/100) * (total offspring)

If we take the DCO in percent then we can say that the expected % of DCO= 0.073X100= 7.3%

Hence,

from the formula of Expected DCO,

And the information given in the question, that distance between first two gene A and B is double that of distance between B and C

let say B and C are X distance apart, then A and B will be 2X distance apart.Since we are taking the frequency in percent therefore total offspring would be 100

Therefore putting the values in the formula of Expected frequence

7.3=2X/100*X/100*100

X=19.104

2X= 19.104*2= 38.209

So distance between B and C= 19.104+-0.04

and B and A = 38.209+-0.04

therefore the genetic map will be like

A---------38.209+-0.04--------B-----19.104+-0.04----C

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

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