Inuvi

Hello, I have some problems with the following pedigree: AR disease with population frequency of 1/62500. What is the prob. that IV-1 will be affected? Especially how to use the Hardy Weinberg Law in that case is my problem. According to my professor the solution should be 1/3 x 1/4 x 1/125. My problem is how to get the 1/125. According to my calculations it's 1/250. Thank you in advance for your help Inuvi