Gene mapping and linkage

Question

In dodders, wild type plants are completely parasitic (CP) whereas a mutation in gene R makes them partially parasitic (PP). Another mutation in gene F causes the plant to produce large flowers, and another mutation in gene L decreases the size of the stigma. A scientist crossed a wildtype plant to a plant that is partially parasitic with large flowers and small stigma. All the F1 plants are completely parasitic with small flowers and normal sized stigma. You cross an F1 plant back to the parent plant with large flowers and small stigma and you see the following progeny:

1346	CP, small flowers, normal stigma
1320	PP, large flowers, small stigma
102	PP, large flowers, normal stigma
90	CP, small flowers, small stigma
42	PP, small flowers, normal stigma
48	CP, large flowers, small stigma
2	PP, small flowers, small stigma
1	CP, large flowers, normal stigma

a) Draw a map showing the linkage relationships of these three loci including the map distances. Show all calculations.

b) Is positive interference occurring? What percentage of double crossovers did not occur due to interference?

Answer 1

a) To create the linkage map, we first need to determine the recombination frequencies between the genes:

CP and PP: 1346 + 90 / 2000 = 0.718

CP and LF: 48 + 1320 / 2000 = 0.684

PP and LF: 102 + 42 / 2000 = 0.072

Next, we order the genes based on their recombination frequencies:

CP - LF - PP

Finally, we convert the recombination frequencies to map distances by multiplying by 100:

CP - 68.4 cM - LF - 7.2 cM - PP

b) To determine if positive interference is occurring, we calculate the coefficient of coincidence (C) and the interference (I):

C = observed DCOs / expected DCOs = (102 + 42) / [(2000 x 0.072) + (2000 x 0.684 x 0.072)] = 0.95

I = 1 - C = 1 - 0.95 = 0.05

A coefficient of coincidence close to 1 and an interference close to 0 indicate that there is no interference. Therefore, positive interference is not occurring. The percentage of double crossovers that did not occur due to interference is 5%