Expert: Dana Krempels, Ph.D. - 10/28/2008

Question
So.. Im doing a virtual fly lab, and I've found all mapping distances of several genes relative to one another (using three point crosses) and now all i have to put it all together and map all the genes on a single chromosome, but i found that the numbers dont add up. For instance, the distance from gene a and b changed when i mapped the cross for a, b, and c and another cross for a, b, and d. Also... if the correct order was a, b, c, d, and e.. then the distance from c to e should be (c+d) +(d+e), right? My numbers dont add up there as well.

One of the questions was to find the coefficient of coincidence and the interference.. And im wondering if I'm supposed to use that somehow..

The numbers are fairly similar (1-2 cM ranges), but some of them are like.. 8 cM differences, especially for the genes farther apart.

The virtual fly lab programs chance and all.. but i dont know how to put it all together and calculate the distances between two genes (since i have different numbers for some, and large differences for others - as i have stated above).

I've quadruple checked my steps and so I know it wasnt any procedural error or mathematical error.

I can provide more details if my question is confusing ^^;;

Answer
Dear Sungie,

From your description, I'm not sure you're considering the difference between actual and measured map distances.  You will *expect* those numbers to be different if you use an intervening locus.

I can't really check your work without having actual distance values.  But I thought I should mention that first, to be sure you're aware of the actual/measured phenomenon.

Hope that helps.

Dana