The first child of a normal couple was diagnosed to have cystic fibrosis. What is the percentage of the chances for the second child to be affected?

If both father and mother are carriers of an autosomal recessive disorder, each child has a 25% chance of being affected by the disease. For each child, there is a 25% chance that the child will have the disease for an autosomal-recessive disease. Cystic fibrosis is an autosomal-recessive disorder. If husband and wife are clinically normal and the child has developed the disease, then both husband and wife are carriers of cystic fibrosis. Note: There are 2 alleles for this, say for example "A" = normal allele and "a" = abnormal allele. So, if each parent is phenotypically normal and their child is affected, then they are carriers, i.e., "Aa". So, each parent has a 0.5 (50%) probability of passing the affected allele to the next generation. Also, having each child is an independent event, and the chance that the child receives abnormal allele from the father (0.5), as well as mother(0.5), is equal to 0.5*0.5 = 0.25 i.e., 25% Ref: Nelson Textbook of pediatrics 21st edition Pgno: 644