Medical genetics, risk calculation is useful
Prior carrier probability = 2/3 (for a lethal mutation if male and female mutation rates are equal).
In this example, only few non-affected boys reduce significatively the risk.
The pre-test risk for the first cousin is low (1/77 or 1.3 %) and less than 1/1000 for the first cousin once removed.
These low risk are to be compared with the general population risk of being carrier of a DMD mutation, that is about 1/2500.
Risk can be further lowered with a simple (non-DNA) test.
Sources and references
JAMA , 1983, Vol 249, n°13, p 1743-1745
BMJ, 1995, 311, p 619-620
Ann Génét, 1996, vol 39 n°3, p 133-138