Question

A three-point testcross is performed to obtain data for gene mapping. A coefficient of coincidence of 2.0 is obtained. A coefficient of coincidence of 2.0 means that

a)no parental progeny were obtained in the offspring.
b)twice the number of double crossover progeny were obtained in the offspring compared to the number expected based upon probability.
c)no double crossover progeny were obtained at all in the progeny.
d)crossover interference was high and a crossover in one gene region suppressed the frequency of a crossover occurring in the adjacent region.
e)only half the number of double crossover progeny were obtained in the offspring compared with the number expected based upon probability.

Answer 1

A coefficient of coincidence of 2.0 in a three-point testcross means that twice as many double crossover progeny were observed compared to the expected number based on probability, indicating negative interference.

Step-by-step explanation:

When performing a three-point testcross for gene mapping, a coefficient of coincidence (COC) of 2.0 indicates that twice the number of double crossover progeny were obtained in the offspring compared to the number expected based upon probability. This calculation is derived from comparing the expected double crossovers based on independent probabilities of separate crossover events to the actual observed number of double crossovers. A COC greater than 1 indicates negative interference, where crossovers in one region of the chromosome do not inhibit crossovers in an adjacent region; hence, more double crossovers occur than expected if the events were entirely independent.