Two healthy parents know from blood tests that they each carry the recessive allele for Tay-Sachs. If their first 3 children have the disease, what is the probability that their fourth child will not have the disease? Assuming they have not yet had a child, what is the probability that, if they have 4 children, all 4 will have the disease? If their first 3 children are male, what is the probability that their 4th child will also be male?