Mendelian Inheritance

Question 1

What is Mendelian inheritance?

Answer 1

This type of inheritance only occurs where the disease is caused by a single abnormal gene on one of the non-sex chromosomes (i.e. not the X or Y chromosomes). These chromosomes are called “autosomes”.

Question 2

I want to illustrate one final scenario for autosomal recessive conditions that is popular in exams as it is quite good at tricking the student so pay close attention. It is usually something like this: both parents are healthy, one sibling has a single gene disease (e.g. cystic fibrosis) disease, a second child does not have the disease, what is the likelihood of the second child being a carrier.

Answer 2

We know that the condition is autosomal recessive, because it is impossible to inherit an autosomal dominant condition without having an affected parent. Both parents must be carriers because two abnormal copies are required for the first child to have the disease and if one parent had two abnormal copies they would also have the disease. Therefore the possible outcomes in future children are the same as in the first example above:

Outcome 1 has two abnormal copies and therefore has the condition.
Outcome 2 has one abnormal copy and one normal copy, therefore becomes a carrier.
Outcome 3 has one abnormal copy and one normal copy, therefore becomes a carrier.
Outcome 4 has two normal copies and therefore is neither has the disease or is a carrier.

So the risk of a future child being a carrier is 2 in 4, or 50%. However this is NOT the answer to the question as we know that the second child in the question does not have the phenotype of the disease, therefore we can exclude Outcome 1. Therefore, the possible genetic outcomes for the second child are 2, 3, or 4, and two of these outcomes (3 and 4) are that the child is a carrier. Therefore the risk of the second child being a carrier is 2 in 3.