Read this bit again: "If a mutation in 1 in 15 genes will cause a disease"
The "from the genome (30,000)" bit is irrelevant.
All that matters is that you are picking ONE GENE randomly with a 1/15 probability of it being the disease. Note that we do not have any additional information on the genes in the genome. That is, we are not told anything specific about the 30000-pool we are picking from. We must assume that the 30000 are in their randomly as well (since we don't know).