Achondroplasia, a form of dwarfism, is inherited as an autosomal dominant trait.

Question

Achondroplasia, a form of dwarfism, is inherited as an autosomal dominant trait. Two heterozygote achondroplastics marry and plan a family. Assume a 1:1 sex ratio and that all genotypes are compatible with survival. What is the probability that the couple's first two children will be achondroplastic males?

1/8

3/8

3/16

9/64

none of these choices

Part B:

Refer to the family in question 16. What is the probability that their first five children will consist of at least 3 achondroplastics?

0.237

0.395

0.551

0.775

0.895

Part A is 9/64 and Part B is 0.895.

**Please show how to do Part B, its the only part I need**

Explanation / Answer

Aa X Aa

3 individuals are affected and produce 8 gametes. The probability of first two children affected is = 3/8 x 3/8 = 9/64.

The probability of first 5 children consisting of 3 affected children is:

5 children are produced by 25 or 32 gametes

The probability of 3 affected children would be 3 x 3 x 3 / 32 or 27/32 = 0.843

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