Genetics double crossovers help please?

Question

In a chromosome region having the map A---10----B----20----D, and a coefficient of coincidence of 0.5, how many double crossovers will there be among 1000 progeny of the test cross ABD/abd x abd/abd?

Answer 1

Expected double crossovers (ie between A & D) =  0.1 x 0.2 = 0.02.This would be 20 in a 1000. But with a coefficient of coincidence of 0.5 the number observed will be 1/2 this = 10.

See page 35/36 of link.

http://chuma.cas.usf.edu/~garey/labmanfa...