Cystic fibrosis is inherited as an autosomal recessive condition. A normal couple has one daughter affected with the disease. They are now planning to have another child. What is the chance of her sibling being affected by the disease -

Ans. is. 'c' i.e., 1/4 o The fact that one child of a normal couple is affected by an autosomal recessive condition (cystic fibrosis) indicates that both parents are carriers of this condition. o The recurrence risk of autosomal recessive disorders in siblings is 25% (') for each bih. Both the parents are carrier --> 25% children will be affected, 50% will be carrier and 25% will be normal. o From above flow cha it is clear that with each bih :? (i) One chance in four been affected (25%), i.e. the risk of recurrence is 25% for each bih. (ii) Two in four chance to being normal but a carrier (50%) (iii) One in four chance of being normal and unaffected (25%) (iv) Three in four chances of being phenotypically normal (75%), i.e. not manifesting disease