How a Single Gene Causes Diabetes

You have two copies of most genes, one from each parent. In most monogenic diabetes, only one of those copies needs to be altered for diabetes to develop. That pattern is called autosomal dominant inheritance.

The genes involved do jobs inside the beta cell — the cell in your pancreas that makes and releases insulin. Some are involved in sensing how much glucose is in your blood. Others control how the beta cell develops or how it releases insulin in response to a meal.

So the fault is not in your immune system and not in your body's response to insulin. It is upstream of both: in the machinery that decides when and how much insulin gets released.

This explains several things about monogenic diabetes. It explains why it appears young — the gene has been there from conception, so nothing has to accumulate first. It explains why it often runs visibly through a family across three generations. And it explains why some subtypes respond to a specific tablet: if the fault is in one step of insulin release, a drug acting on a different step can bypass it.

Because one altered copy is enough, each child of an affected parent has a 50% chance of inheriting it. That is a coin toss per child, not a quota.

Some rarer forms are recessive, needing an altered copy from both parents.