The question is:

if a mutation in 1 in 15 genes will cause a disease, what is the probability that if you pick a gene at random from the genome (30,000), it will cause the disease?

My advisor said that it is 15/30,000, but I don't understand how he got this. I would have thought it is (1/15) / 30,000.

Your help would be much appreciated as I've been driving myself crazy for the last hour.

Thanks