Alternatively, write c1j as a sum of indicator random variables and calculate the mean and variance directly. Check that the two methods give the same answer. (Hints: In applying Problem 8, i has two components. Set all but one of the li equal to 0.

Set the remaining one equal to 1 or 2 to get either a first or second factorial moment. The kth indicator random variable indicates whether the kth person is a case and has genotype j.)