If you haven’t read Genetics 101, it might be a good place to get some background for this next part. To briefly summarize, our bodies are made up of billions of cells. In our cells there are structures called chromosomes that are made up of individual instructions called genes. We have 46 chromosomes in total that come in 23 pairs, half come from mom and half come from dad. An exception to this is the X chromosome in men, as they have only one copy of the X chromosome and one Y chromosome. The genes that make up these chromosomes tell our bodies how to grow and function, and we also get one set from mom and one set from dad. To grow and function as usual, it is important to have all 46 chromosomes and their genes working.
Typically both copies from the pair are functional, but we all have genes that are not working as they should. In fact, scientists estimate that we each have approximately 20 non-working genes! This usually doesn’t cause a problem because you have two, so there’s a built-in backup system; if one doesn’t work the other one takes over. However, if both copies of a gene that a person inherited do not work, this can lead to a genetic condition that is caused by autosomal recessive inheritance.
Autosomal simply refers to the fact that whatever gene is involved is found on one of the first 22 chromosomes (called the autosomes) and not on the X or Y chromosome (the sex chromosomes).
Recessive refers to the explanation above that we have two copies of each gene, one from mom and one from dad, and in order to have a recessive condition a person has to have BOTH copies of a gene not working. The copy they inherit from mom is not working AND the copy they inherit from dad is not working, resulting in zero working copies of that gene. With recessive conditions, if you have one copy of the non-working gene (called a carrier), you do not have the condition and typically do not have any related symptoms.
It is rare to see any history of autosomal recessive conditions within a family because if someone is a carrier for one of these conditions, they would have to have a child with someone who is also a carrier for the same condition. Autosomal recessive conditions are individually pretty rare, so the chance that you and your partner are carriers for the same recessive genetic condition are likely low. Even if both partners are a carrier for the same condition, there is only a 25% chance that they will both pass down the non-working copy of the gene to the baby, thus causing a genetic condition. This chance is the same with each pregnancy, no matter how many children they have with or without the condition.
That means there’s a 50% chance the baby would be a carrier just like it’s parents, and ask discussed above carriers generally do not have any symptoms. There’s also a 25% chance that the baby would inherit a working copy of the gene from both parents. The diagram below helps explain these chances as well.