daughters are homozygous normal - proteus

[ArchivalUser]

A husband and wife have 1 son with cystic fibrosis and 3 healthy daughters. What is the probability that all three daughters are homozygous normal?

A. 1/8
B. 1/9
C. 1/16
D. 1/27
E. 1/32

[ArchivalUser]

Choice (D) is the correct answer. Each daughter is healthy, so there is a 1/3 chance that each daughter is homozygous normal, and a 2/3 chance that each daughter is a heterozygous carrier for cystic fibrosis. The chance of all three daughter being homozygous normal is (1/3) x (1/3) x (1/3) = 1/27. The other choices are incorrect.

[ArchivalUser]

thanks for the explanation
i allways get confused in these calculations.