Cystic fibrosis is a hereditary disorder caused by a recessive mutation. in a previous marriage, jim had a child with cystic fibrosis, though neither jim nor his first wife had the disease. jim is now married to martha, who had a brother that died of cystic fibrosis. though martha does not have the disease, she found out that she is a carrier. what is the probability that jim and martha will have a baby with cystic fibrosis?

Jim is clearly a carrier for the disease, as is his new wife. 

The distribution will be as such, with C denoted the dominant, non-disease trait and c denoting the hereditary disease. Only cc results in the disease. 
Martha          Jim                Offspring: 
          C          C   c                Cc, cC, CC, cc
          c

There is a 50% chance that the child is a carrier but does not have the disease (Cc)
There is a 25% chance that the child inherited no disease allele and therefore is not a carrier (CC)
There is a 25% chance that the child has the disease (cc)

The probability of having a baby with cystic fibrosis is 25%

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