12. Genetic carriers and recessive genetic conditions

Sometimes only one copy of a gene is changed and may either make no protein at all or a broken protein that doesn’t work properly. If we can make enough fully working protein from the unchanged copy the person may have no symptoms and be described as a ‘carrier’. The genetic change might be described as ‘recessive’. 

When two parents are both ‘carriers’, their children have a 50% chance (1 in 2 - like flipping a coin and getting heads) of inheriting a broken copy from either parent which will make the child a carrier too. They have a 25% chance (1 in 4 – like flipping two coins at the same time and getting heads on both) of inheriting a broken copy from both parents. This will cause symptoms because the child won’t have any of the working protein and they will be born with EB. There’s also the same 25% chance (1 in 4) that a child will inherit the perfectly working copies from both mum and dad and be totally unaffected, not even a carrier. They will not pass on EB to their own children. 

Recessive genetic diseases have unaffected family members who are carriers of the broken gene without having symptoms themselves. 

People with symptoms of recessive EB have two broken copies of the affected gene so will pass on a broken one to all their children. If the other parent has two working copies, all the children will be carriers. If the other parent is a carrier there is a 50:50 (like tossing a coin) chance of the children being affected as they may either inherit the broken copy or the working copy.

 

Image credit: Autorecessive_en_01, by Kuebi (Armin Kübelbeck). Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.