Question

In a small animal, P, E and B are found on a chromosome at map positions 5, 25, and 55, respectively. A three-factor cross was performed that yielded 1,000 progeny, including 42 double crossover recombinants. What is the value for interference?

a) 0.15
b) 0.30
c) 0.45
d) 0.60

Answer 1

The value for interference in the genetic mapping question is 0.30, calculated by taking 1 minus the ratio of observed to expected double crossovers, which gives the result that matches option (b).

Step-by-step explanation:

The question is asking about the calculation of interference in genetic mapping, which is a measure of how often observed double crossovers do not occur as frequently as would be expected from the product of single crossover frequencies. To find the value of interference, we have to subtract the observed double crossover proportion from the expected double crossover proportion, then divide by the expected double crossover proportion, and finally, subtract from 1.

First, we note the map positions of genes on the chromosome: P at 5, E at 25, and B at 55. The distance between P and E is 20 map units, and the distance between E and B is 30 map units. The number of offspring is 1,000, and there are 42 double crossover recombinants. We calculate single crossovers as 20% for P to E (20/100) and 30% for E to B (30/100), then multiply these to find the expected double crossover frequency: 0.20 x 0.30 = 0.06, or 6%. This would lead to an expected 60 double crossovers in 1,000 offspring (0.06 x 1000).

The observed double crossovers are 42, so we calculate interference using the following formula: Interference = 1 - (observed double crossovers / expected double crossovers) = 1 - (42 / 60) = 1 - 0.7 = 0.3.

Therefore, the value for interference is 0.30, making the correct answer (b) 0.30.