Short answer: They don’t. Grandma can’t give you a gene without giving it to your mom first. So why is the idea of skipping generations so common? Here’s the long answer:

Genes are complicated. They don’t act in a vacuum. They interact with other genes, your environment, and a whole host of things we don’t fully understand yet. While there are some pretty cut and dry scenarios (this gene mutation + you = specific disease), a lot of things in genetics are a lot more nuanced. Here’s a few scenarios that may give the illusion of generation skipping.

Reduced Penetrance

Sometimes a gene by itself isn’t “strong” enough to cause an effect 100% of the time. Two people could have the exact same genetic mutation, but one might show no sign of it, while the other might be born with a cleft lip because of it. We call that “reduced penetrance,” or “incomplete penetrance.” In these kind of cases, grandpa, dad and baby all have the same gene, but dad’s one of the lucky few that isn’t affected by it for some reason.

Variable Expressivity

Some genetic diseases have a long list of features they can cause, and most people with the condition will only have a fraction of the possible symptoms. Some of these symptoms are obvious medical conditions, but others might be more subtle, like the color of your eyes or how tall you are. You could have a group of relatives that all have the same gene, but some are severely affected by it, and others end up having only a couple, subtle findings, and therefore go undiagnosed. We call this “variable expressivity” because the way the gene expresses itself varies from person to person.

X-Linked Disorders

When it comes to chromosomes, women have two X’s and men have an X and a Y (usually). If you look at a photo of these chromosomes, X looks like a giant next to Y. In other words, Y’s inventory is pretty lackluster compared to X’s (sorry guys).

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If women have a problem in a gene on their X chromosome, the other X can usually handle it on her own. But in a man, if his X chromosome has a problem, it’s got no back-up: just that poor little stumpy Y chromosome. So if you and your grandpa are red-green colorblind, that means your mom has to have the gene, too. She’s what we call a “silent carrier,” since her one working X-chromosome is enough to keep her seeing christmas colors. No “skipping” has really taken place.

So there you have it. Myth debunked. Or at least more accurately explained.


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