From this paper:
The one man in seven who harbors risk alleles at both 20p11.22 and AR (encoding the androgen receptor) has a sevenfold-increased odds of androgenic alopecia (OR = 7.12, P = 3.7 x 10(-15)).
If the prior probability of a man going bald is say 50%, what is the posterior probability given that he harbors these risk alleles? $7 * 0.5 = 3.5$ is obviously wrong.
Put another way, if I have these risk alleles how do I calculate my overall probability of going bald?