Phenylketonuria (PKU) is caused by homozygosity for a recessive allele and can cause a severe form of mental retardation if not detected early. The prevalence of PKU (genotype aa) is 1 per 15,000 newborns in the United States according to the National Institutes of Health (2001). 

Given the above data, what is the expected allele frequency of the a allele? Write your answer using three decimal points and include the first zero. For example, 0.050 or 0.123. Do not write .050 or .123. Failing to follow these instructions will result in no credit for this question, irrespective of correctnes. 

freq a = [x]