Cystic fibrosis in humans is caused by recessive mutations in a single gene located in an autosome and follows mendelian laws. a normal couple has two children. the first child has cystic fibrosis, and the second child is unaffected. what is the probability that the second child is a carrier (heterozygous) for the mutation that causes the disease? (hint: we already know that the second child is not homozygous recessive)