This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics, Subject name: Pathology
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 - 
Select and explain. 
A. 0, B. 50%, C. 25%, D. 75%
C: 25%
Answer: C: 25% 
Explanation: 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.