Ask a Geneticist

by Cecilia Sedano, Stanford University

My son, daughter and husband have Long QT Syndrome. How was this passed to them?

-A curious adult from California

July 22, 2010

People with long QT Syndrome have spells where their hearts beat irregularly. This can cause problems like palpitations, fainting or seizures. Sometimes if it goes on too long, it can even lead to death.

Unfortunately I can’t tell you exactly where your son, daughter and husband got their LQTS from. There are lots of ways to end up with the condition. Some are genetic and some aren’t.

You can get it from certain medicines. This is called drug-induced LQTS. So if your son, daughter and husband all happened to be taking the same medicine, then that might be how they all ended up with LQTS.

Your genes can also cause LQTS. But there isn’t just one gene that can cause the condition. Many different genes can lead to LQTS.

And they aren’t all passed down in the same way. Some can be hidden in a person’s genes and appear in a child. Others need to be directly passed from parent to child. Your case could very well be either one of these. This all makes it very difficult to say what exactly happened in your family.

What I’ll do for the rest of the answer is talk about how genes can lead to LQTS. Then we can go into two of the ways it can be passed down from parent to child. As you’ll see, which form your husband and kids have will affect the chances of your grandchildren having it as well.

Genes and L QT Syndrome

Genes have the instructions for making and running a person. And each gene has a very specific job.

So there is a gene that makes the protein insulin that helps our cells use the sugar we eat. And a gene that makes a protein that lets us taste a certain bitter chemical called PTC found in broccoli and brussel sprouts. And so on.

Many things need more than one gene to happen. Your eye color is one. So is your heart.

You need lots of genes to set up and keep a heart beating. These genes work to give the heart four chambers, set up the valves, keep a regular heartbeat, etc.

Even keeping a regular heartbeat is pretty complicated on its own. Here’s how it works:

Prolonged QT interval: abnormality
seen on an electrocardiogram
(ECG/EKG).

After a heart beats, its electrical system needs to recharge for the next beat. This time of recharging is called Q-T (see the image on the right for why this is). So people with Long Q-T syndrome have a heart that takes longer to recharge.

There are lots of steps involved in this recharging and each one is controlled by a specific gene. People with genetic LQTS inherit a version of one of these genes that can’t quite do its job right. Now their heart has a longer time recharging. And this sometimes leads to an irregular heartbeat.

Because there are many genes involved in this process, there are many ways for it to go wrong. Not all of these will be passed on to the next generation the same way.

Genes Come From Our Parents

We have two copies of most of our genes: one from mom and one from dad. And of course, our parents have two copies of most of their genes too.

Which of a parent’s two gene copies a child inherits is totally random. Like getting heads or tails when you flip a coin.

As you might have guessed from our previous discussion about LQTS, genes come in different versions. Which means it is possible for someone to have two different versions of the same gene at the same time.

Let’s take red hair as an example. Some people do or do not have red hair because of a single gene that comes in two different versions. People with red hair have two copies of the red version. People without red hair have one or two copies of the not-red version.

So the red hair version is weaker than the not-red version. In genetics-speak, the red version is recessive. And the not-red version is dominant.

Gene versions that lead to LQTS can be either dominant or recessive. In each case, LQTS will be passed down differently. And how it is passed down can influence the chances that the next generation will get it.

Dominant LQTS

Dominant LQTS: The disease
gene copy is dominant
over the other.

As I said in the last section, sometimes a version of a gene that causes a disease is dominant. In these cases, one or both parents usually have the disease too. This certainly fits your family’s situation.

Your husband has LQTS and so do your two kids. If this is how it happened in your family, then your kids may pass it on to their kids too. Let’s go over an example to show how this might have happened.

Before getting started, one more bit of bookkeeping. To make things easier, scientists have a bit of shorthand to describe genes. They make the dominant version a capital letter and the recessive one a little letter.

So in this case, our gene comes in two versions, L and l. L is the dominant form that causes LQTS and l is the “normal” form.

Since you don’t have LQTS, we know you are ll—you have two copies of the l version of the gene. Let’s say your husband is Ll and so has one copy of each.

We know you can only pass an l down to your kids—that is all you have to give. Your husband, though, can pass either L or l.

Remember, which one he passes down is totally random, sort of like flipping a coin. And just like you have a 50% chance of getting heads and a 50% chance of getting tails when you flip a coin, each child has a 50% chance of getting an L and a 50% chance of getting an l. When a child gets an L, he or she will get LQTS too.

In this case, each child had a 1 in 2 chance of having LQTS. The chance that 2 out of 2 children would have LQTS is 1 in 4.

This means that if this is how LQTS was passed down in your family, then each child of your children has at least a 50% chance of having it too. In other words, your grandkids have a 50-50 shot of having LQTS as well.

There is also a chance that your grandkids would have pretty close to a zero chance of ending up with LQTS. This would be true if your family has the recessive kind.

Recessive LQTS

Recessive LQTS: Both recessive
gene copies are needed to
have the disease.

This whole situation changes if the form of the gene that causes the disease is recessive.
As I mentioned earlier, recessive means that both disease gene forms must be present in order to have the disease. In this case, an individual must be ll to have LQTS.

Since your husband has the disease, he must be ll. He has two versions of the l copy of the “disease” gene.

The same is true for your son and daughter, they are both ll. In this situation, you must be Ll – a disease carrier.

A disease carrier is a person that doesn’t develop the disease because he or she has only one copy of the recessive disease gene form. A carrier can pass down either the “normal” or the disease gene form.

So just like dominant LQTS, recessive LQTS seems to fit your family situation too. The way it would have worked is that your husband passed an l since that is all he had to give. You happened to pass your l to each child so they ended up with LQTS.

The chances of this happening were the same as in the dominant case—1 in 2. And the chances that both kids would end up with LQTS were 1 in 4.

In recessive LQTS, your children will pass down the disease gene form to their children 100% of the time – they can only pass l. But their children will have LQTS only if the person they are having children with is a carrier (Ll) or has the disease as well (ll).

There you go! If your children have dominant LQTS, they got it from their dad. If they have recessive LQTS they got from mom and dad – where mom is a carrier.

Cecilia Sedano

Ask a Geneticist

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