A normal couple has one daughter affected with Cystic fibrosis. They are now planning to have another child. What is the chance of siblings being affected by the disease -

Ans. (c) 25%(Ref: Robbins 9th/pg 141; 8th/pg 141)Cystic fibrosis is an autosomal recessive condition.If one daughter is affected, genotype will be ccTherefore, both parents must have been carriers of cystic fibrosis.Thus each child (other sibling) has a 1 in 4 (25%) chance of being affected.