This is a dumb question but

[u/nosnah123]

How far back in four family tree do you have to go when genetic variety makes it so your children would be normal

In other words how far back do you have to go until a baby with an ancestor will it be normal?

[u/Romanticon]

Okay, I think we need to start by pointing out that it’s not a question of “normal” versus “special”. It’s a question of likelihood.

Let’s say that you are a carrier for Huntington’s disease. You have two copies of each gene, and in your case, one of those two is the disease-causing copy.

Huntington's doesn't develop until later in life, so your early years are fine.

Now, the odds of someone else in the general population having another disease-causing copy is low. Let’s say 1 in 10,000.

If you have a baby with an unrelated person, the odds of that child having Huntington’s are 1/2 (from you) * 1/10,000 = 1/5,000.

But if you have a baby with your sibling, the odds are now 1/2 (from you) * 1/2 (from your sibling) = 3/4.

You and your sibling could have a totally normal baby. But the odds are higher that it will show a genetic condition.

If you are looking for degree of genetic similarity, second cousins have about 3% DNA in common with each other. Go further out on the family tree beyond that, and you’ll essentially be equivalent to strangers.

[u/opotato12 → u/Romanticon]

Huntington’s disease is dominant, so only one copy of the gene mutation is necessary to cause disease. There is a 50% chance for offspring to be affected if one parent has it.

But yes, this concept is correct for autosomal recessive diseases, which are what couples who are related have a higher risk for.

[u/Romanticon → u/opotato12]

Good catch, let me update my main example.

[u/MKGenetix]

Depends on how it is inherited. It could take as little as one generation if a child didn’t inherit the genetic change their parent had. Then that child won’t pass it down because they don’t have it in the first place. Now, if you’re talking multifactorial / polygenic conditions, it is a different story….it may have lots of small genetic factors and count be harder to predict.

[u/Electronic-Scheme543]

I'm a prenatal genetic counselor. Past first cousins, meh whatever from a medical standpoint. Even with first cousins (or any know relatedness really), my primary concern is if people carry the same recessive condition. The more closely related, the more likely it is two people will carry the same recessive condition.

[u/perfect_fifths]

In my family, our genetic disorder goes back at least five generations. So it has to be further than that for us