If individual 1116 and IV4 have a child together what is the

If individual 111-6 and IV-4 have a child together, what is the probability that the child will have the disorder? 2/3 1/8 1/2 1/4 1/3

Solution

I tried doing 2/3 * 2/3 * 2/3 *1/4 and got 2/27 which was wrong. I thought that the probability of III 6 being a carrier is 2/3. The probability that IV 4 is a carrier is also 2/3. The probability that III 6 is a carrier is 2/3 and IV 5 must be a carrier. So IV 4 and III 6 must be carriers. The probability that they have a child with the trait is 1/4. But I can\'t able to get right answer.

The right answer is 1/4.