We’re Both Healthy. So How Could We Have a Baby With a Genetic Condition?

Okay... so what does being a carrier actually mean?

Let’s keep this simple. For many genes, we have two copies. One came from our mum and one came from our dad. For some genetic conditions, having a change in just one of those copies is not enough to actually cause the condition cause the other copy is still working. So you can carry that genetic change and be completely healthy.

That is essentially what being a carrier means.

You have a genetic variant linked to a condition, but you do not have the condition yourself.

And unless you have been tested for it, there is a very good chance you would have absolutely no idea.

So why has nobody in the family had it?

This was the part I found really interesting when I first learned about recessive inheritance.

A genetic variant can move through a family without necessarily causing disease. Your mum could be a carrier. Her dad could have been a carrier. Someone generations before him could have been a carrier too.

And all of them could have been completely healthy.

So when we say, “There is nothing genetic in my family,” what we usually mean is that nobody we know of has actually had a genetic condition. But that is not necessarily the same thing as nobody carrying a genetic variant linked to one.

For a recessive condition to appear, a child usually needs to inherit a disease-causing variant in the same gene from both parents.

And what if your partner carries the same condition?

Let’s imagine that you are a carrier for the same autosomal recessive condition as your partner. Neither of you has the condition. You probably would never have known you were carriers without testing.

But when you have a child together, there are several possible combinations of the two copies of that gene.

With each pregnancy, there is:

a 25% chance the child inherits both disease-causing copies and has the condition,

a 50% chance the child inherits one and becomes a healthy carrier, just like you,

and a 25% chance the child inherits neither of those disease-causing copies.

Those chances start again with every pregnancy.

Having one healthy child does not mean the next pregnancy cannot be affected and equally, having one affected child does not mean every future child will have the condition.

But how likely is it that we both carry the same thing?

This depends entirely on the condition.

Some disease-causing variants are rare. Others are much more common in certain populations. And all of us carry genetic variants.

That, by itself, is not unusual or something to be frightened of.

The important question in reproductive genetics is whether both people carry variants that could come together in a way that creates a significant risk for a child.

This is one of the reasons carrier screening exists.

So what exactly is carrier screening?

Carrier screening is a genetic test that looks for variants associated with certain inherited conditions.

It can be done before or during pregnancy, although doing it beforehand gives you more time to understand the result and think about what you want to do with that information.

Depending on the test, it might look for a small number of specific conditions or screen many genes at once.

Usually it involves a blood or saliva sample.

And if one partner is found to carry a variant for a particular condition, the other partner may then be tested for that same condition.

A negative result does not mean “zero genetic risk”

Carrier screening is not a test that can look at your DNA and declare that your future baby will definitely be free from genetic disease as it cannot test for every possible genetic condition. It cannot detect every disease-causing variant in every gene.

And many genetic conditions do not follow the simple recessive pattern we have been talking about here at all.

So a negative carrier screen can reduce the chance that you carry the specific conditions included in that test, but it cannot make the risk zero.

And art this point it is crucial to understand. that genetics very rarely gives us a completely clean yes or no.

And recessive conditions are not the whole story

I have focused a lot on recessive conditions because they are probably the easiest example of how two completely healthy people can still have a child with a genetic condition.

Genetics though does not always work like that.

Some conditions are linked to the X chromosome. Others only need one altered copy of a gene to be passed on.

And sometimes, the genetic change was never quietly running through the family in the first place.

It can happen for the first time when an egg or sperm is being made, or very early after an embryo begins to develop.

There are also plenty of conditions that are much more complicated than one gene being “wrong”. Lots of genes can be involved, together with things that have absolutely nothing to do with genetics.

So when a condition is characterised as “genetic”, it does not necessarily mean that mum or dad passed it down.

Sometimes it was inherited and sometimes it was not.

So should everyone be worried about what they carry?

Of course not.

I actually think that would be the worst takeaway from all of this.

Being a carrier is part of normal human genetic variation. Most carriers are healthy.

The point of carrier screening is to give people information about reproductive risk that they otherwise might never have known was there.

And for some couples, that information can matter.