Imagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Charles was married once before, and he and his first wife had a child with cystic fibrosis. The brother of his current wife, Elaine, died of cystic fibrosis. What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles, Elaine, nor their parents have cystic fibrosis.)

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Determine the genetic status of Charles and Elaine. Since Charles had a child with cystic fibrosis (CF) and CF is a recessive disorder, Charles must be a carrier of the CF gene (Ff). Similarly, since Elaine's brother had CF and neither of her parents had the disease, it is likely that both of her parents were carriers, making Elaine likely to be a carrier as well (Ff).

Set up a Punnett square with both Charles and Elaine as carriers (Ff x Ff). This will help visualize the possible genetic outcomes for their children.

Fill in the Punnett square. The possible genotypes of the offspring will be: FF (normal), Ff (carrier), Ff (carrier), and ff (affected by CF).

Calculate the probability of each genotype. Since there is one ff combination out of four possible outcomes, the probability that Charles and Elaine will have a child with CF is 1/4 or 25%.

Explain the results to the couple, emphasizing that there is a 25% chance with each pregnancy that their child will have cystic fibrosis, a 50% chance the child will be a carrier, and a 25% chance the child will neither have CF nor be a carrier.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cystic Fibrosis Genetics

Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. For a child to be affected by CF, both parents must carry at least one copy of the mutated gene. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two copies of the mutated gene and thus have cystic fibrosis.

Carrier Status

A carrier is an individual who has one copy of a mutated gene but does not exhibit symptoms of the associated disorder. In the case of cystic fibrosis, Charles and Elaine are not affected, but Charles's previous child indicates he is likely a carrier. The family history of CF in Elaine's brother also suggests she may be a carrier, increasing the risk of having a child with the condition.

Punnett Square

A Punnett square is a tool used in genetics to predict the probability of an offspring inheriting particular traits from its parents. By mapping out the possible combinations of alleles from both parents, one can visualize the likelihood of different genetic outcomes, including the probability of having a child with cystic fibrosis based on the carrier status of both parents.

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Punnett Squares

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Textbook Question

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