# What is the probability that exactly one sibling is affected

**3.31 What is the probability that exactly one sibling is affected?**

A/ Let

A = &lbrace older child is affected &rbrace,

B = &lbrace younger child is affected &rbrace

P(A) = 0.5 |
P = 1- 0.5= 0.5 |

P(B) = 0.5 |
P() = 1- 0.5 = 0.5 |

Let P(C1) represents probability that exactly A has affected;

P(C1) = P(A ) = (0.5 × 0.5) = 0.25

P(C2) represents probability that exactly B has affected;

P(C2) = P( B ) = (0.5 × 0.5) = 0.25

Let P(C) represents probability of exactly one sibling is affected, then:

P(C) = P(C1) + P(C2) = 0.25 + 0.25 = 0.5 = **50%**

**3.32 What is the probability that neither sibling is affected?**

A/

Let P(D) represents probability that neither sibling is affected

P(D) = P( ) = (0.5 × 0.5) = 0.25 = **25%**

**3.33 Suppose the older child is affected. What is the probability that the younger child is affected?**

A/

P (B I A) = P (B A) / P (A)

= P (B) * P(A) / P (A)

= 0.5 × 0.5 / 0.5

= .5

= **50%**