Answer:
1/4
Step-by-step explanation:
By saying that both parents have the recessive allele, but don't have it, it tells us that both parents are Heterozygous for this trait. They have one dominant trait and one recessive trait. This is demonstrated in the in the punnett square. We can see that 1/4 of the offspring will have two recessive alleles, 1/2 of the offspring will be Heterozygous, and 1/4 of the offspring will have both dominant alleles. In order to have the disease, the offspring needs to have both recessive alleles. That is 1/4 of the the offspring.