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, this is not true for all genes. There are many genetic conditions where if one gene of the pair is not working, the other one cannot compensate, and the person develops the condition with just one non-working copy of the gene. This illustrates a common pattern of heredity called autosomal dominant 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).
Dominant refers to the above explanation that we have two copies of each gene, one from mom and one from dad, and in order to have an autosomal dominant condition a person only has to have one copy of the gene not working. They can inherit this copy from mom or dad (either would also usually have the condition).
Often autosomal dominant conditions can be seen in multiple generations within the family. If one looks back through their family history they notice their mother, grandfather, aunt/uncle, etc., all had the same condition. In cases where the autosomal dominant condition does run in the family, the chance for a person with the condition to have a child with the condition, regardless of whether it is a boy or a girl, is 50%. This chance is the same with each pregnancy.
There are cases of autosomal dominant gene changes, or mutations, where no one in the family has it and that it has first happened in the individual who has the condition. This is called a de novo mutation. For the individual with the condition, the chance of their future children inheriting it will be 50%. However, other family members are generally not likely to be at increased risk.
The diagram below illustrates autosomal dominant inheritance. The example below shows what happens when dad has the condition, but the chances of having a child with the condition would be the same if mom had the condition.