Case Study 1: Some introductory information
This first story is about an inherited condition called Cystic Fibrosis.
CF is caused by altered CFTR genes. The CFTR protein controls the movement of salt and water in and out of cells. People with CF experience a build-up of thick, sticky mucus in the lungs, digestive system and other organs, causing a wide range of challenging symptoms affecting the entire body.
This condition is inherited in an autosomal recessive pattern (see Glossary). Genetic counsellors do risk calculations to figure out the chances of a couple having an affected child. We will walk you through the risk calculation we would do in this case.
To work out the risks of a couple having an affected pregnancy you first need to work out the chance that each parent is a carrier of CF.
Ths chance Genevieve is a carrier is 1 in 2 (also written as 1/2). We know this because of her family history. The chance Sam is a carrier is 1 in 25 (1/25). This is the population risk for people with white northern European heritage.
You then multiply these figures together. This gives you the probability that both parents are carriers. This chance is 1 in 50 (1/50).
This figure is then multiplied by 1 in 4. This is the chance that, if parents are carriers, they will both pass on a copy of the gene with a disease-causing alteration.
If one parent is affected with CF (has two copies of CFTR with an alteration) then the risk calculation is different. This can be complicated as there are mild forms of CF. In some rare cases people will not be affected until they are adults. For this calculation, and our purposes here, we are thinking about ‘classic’ CF. This presents in childhood.
For a further exploration of recessive inheritance you might like to watch the following video.
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