According to Hardy–Weinberg principle p2 + 2pq + q2 = 100% = 1,
where, p2 is the frequency of the AA genotype,
2pq - frequency of the genotype Aa,
q2 is the frequency of the aa genotype.
Then q2 = 1 - (p2 + 2pq)
q2 = 1 - (1:8,000) = 0,99
q = 0,99
p = 1 - 0,99 = 0,01
Frequency of disease-causing allele in the population is 0,01.