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Here is how the risk was calculated in scenario 2 based on family history:
- Mary's aunt (Joe's mom) must be a carrier to have had an affected child.
- Therefore, Mary's Dad's chance to carry the variant is 1 in 2 and, if he is a carrier, he would have a 1 in 2 chance to pass the variant down to Mary.
- Multiply 1/2 x 1/2, and the result is 1/4 (1 in 4) or 25 percent, which is Mary's chance to be a cystic fibrosis carrier, if she has a cousin with cystic fibrosis.