Question

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?

Answer 1

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%

Answer 2

Answer:

There is 1/4 probability that jim and martha will have cystic fibrocic child.

Explanation:

As the jim's first child was cystic fibrosis and cystic fibrosis is a recessive disease.

Its mean Jim is a carrier and his second wife martha is also carrier. So the offsprings will be:

For example Jim and martha genotype is Cc and Cc then probability of offsprings are:

Cc × Cc = CC, Cc, Cc, cc

There will be three types of kids:

CC = Healthy kid = 25% = 1/4

Cc = Carrier kid = 50% = 1/2

cc = Diseased kid = 25% = 1/4