Genes a, b, and c are known to be in that linear order in an organism. A testcross is done that yields 20 double crossover progeny out of a total of 1000 offspring. Assume that there is no interference.

(a) Type a genetic map showing the distances between these genes. Is there more than one possible map?
(b) What would the distances be if there was 10% interference?
(c) What number of double crossover progeny would be expected if there was interference of 15%?

Explanation:

The observed frequency of the double crossovers (DCOs) is 20/1000, or 0.02. If there is no interference then this will be the same as the expected, which is just the product of the single crossover frequencies, that is, SCO frequency (a-b) x SCO freq. (b-c) = 0.02.

(a) Any combination of map distances whose product (a-b) x (b-c) = 0.02 is possible. However, three are most likely:

a 20 mu b 10 mu c

a 10 mu b 20 mu c

a 14.14 mu b 14.14 mu c

(b) The distances would be exactly the same.

(c) The expected number of DCO progeny would be 17.

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