Affected with Achondroplasia

Mr. and Mrs. B are both affected with achondroplasia, the most common form of dwarfism. Mrs. B is pregnant, and they would like to know whether their child will develop to have normal stature, or will also be affected with achondroplasia.

a) Draw a pedigree for this family based on the above information.

b) What is the probability of the fetus being affected with achondroplasia?

Give the reasoning for your answer.

c) Which gene is responsible for achondroplasia, and where is it located in the human genome? What is the function of the corresponding protein?

d) What technique would you use to test a chorionic villus sample (placental material derived from the embryo) to answer their question definitively? Give the reasoning for your answer.

Mr and Mrs Stephenson have a son, Toby, with cystic fibrosis (CF). Mrs Stephenson is newly pregnant and after much discussion Mr and Mrs Stephenson have decided that Mrs Stephenson will have an elective termination if the fetus is found to have CF.

Mrs Stephenson is a carrier of the common F508 mutation in the CFTR gene. But Mr Stephenson doesn't have this mutation and the laboratory cannot identify the mutation he is carrying, despite screening the 9 most common mutations which cover about 98% of abnormal alleles. There are many hundreds more pathological mutations known in the CFTR gene, however it is impractical to sequence the entire gene due to its large size.

a) Draw a pedigree containing the information given above.

b) Give a brief clinical description of cystic fibrosis. What is the mode of inheritance?