Tay-Sachs disease is an inherited disease that is caused by a recessive allele. If a man and his wife, who are both carriers, have three children, what is the probability of each of the following? Show your calculations and give your answers as fractions.

a) All their three children do not have the disease.

d) At least one child is phenotypically normal without the disease.

b) One or more of their three children have Tay-Sachs disease.

c) All three children have the Tay-Sachs disease.