Question

A couple, Franco and Ethyl have come to you for genetic counseling.  They are expecting a child and are concerned that it may have cystic fibrosis. Ethyl’s father had an older sister with cystic fibrosis.   Ethyl’s mother has no signs of disease. Ethyl has no siblings. Ethyl and all her family are white from North America.   Franco, who is younger than Ethyl, has two younger brothers and a sister (the youngest sibling) who has cystic fibrosis. He and all his family are Hispanic.  

a. Draw a pedigree of this family.

b. What is the probability that Franco and Ethyl’s first child will have cystic fibrosis?

c. Explain to the couple, who are not scientists, how you derived that probability.

d. The couple asks if there is anything else that they can do to learn more about their child’s chances of having cystic fibrosis.  What would you recommend?

Answer 1

a.

b. There is 25 % probability that their first child will have cystic fibrosis.

C.

• Genetic information is stored in chromosomes , may be termed as genetic encyclopaedia . Humans have 23 pairs of chromosomes and each pair is made up of one copy of chromosome from mother and other one from the father .
• Cystic fibrosis is an example of Autosomal recessive disease.
• This means a person must have a mutation in both copies of CFTR gene to have Cystic fibrosis.
• If someone has mutation only in a single Copy of CFTR then the person doesn't have CF rather he / she is a potential carrier of CF .
• These carriers can pass their mutated copy of CFTR gene to their children.
• In this case as evident from the pedigree both Ethyl and Franco is a carrier of Cystic fibrosis ( CF ) .
• Each time two CF carrier have a child together the chances are as following :

1. 25 percent of the child will have Cystic fibrosis .
2. 50 percent of the child will be carrier for Cystic fibrosis ( not having CF disease ).
3. 25 percent of child will neither be the carrier of CF gene nor will have the CF disease.

D. The couple may try the Amniocentesis process In this process the amniotic fluid from the mother is collected and tested in the laboratory to identify if their is any genetic abnormalities ( like cystic fibrosis ) in the foetus .