The Science Behind the Damage of Inbreeding

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To further examine the difficulties with inbreeding, host Andrea Seabrook speaks with Joan Scott. Ms. Scott is a certified genetic counselor and the Deputy Director of the Genetics and Public Policy Center.

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ANDREA SEABROOK, host:

Let's take a closer look now to some of the genetic problems found in this village. Joining me now is Joan Scott. Ms. Scott is a certified genetic counselor and the deputy director of the Genetics and Public Policy Center.

Good morning.

Ms. JOAN SCOTT (Genetics and Public Policy Center): Good morning.

SEABROOK: You've just heard the same story our listeners have. Can you give us an idea of how this interbreeding can cause such extensive genetic problems?

Ms. SCOTT: Well, when individuals marry relatives, they're at a higher risk for having children with genetic disorders. And there's a particular category of genetic disorders that they're at higher risk for, and those are what we call in recessively inherited disorders. So our genes come in pairs. We get one copy from mom and one copy from dad. That's what makes us an equal 50/50 product of our parents. In recessive disorders, both copies of that pair have to be abnormal in order to be infected. So both parents have to be affected.

SEABROOK: So you can have one...

Ms. SCOTT: And be perfectly fine.

SEABROOK: Okay.

Ms. SCOTT: And it isn't unless, until you have a child with someone who's a carrier for the very same recessive disorder that you are that you're at risk to have a child with a double dose and to be affected.

SEABROOK: So in essence, the bad genes, the genes that are responsible for, you know, physical problems, mental problems and so on, get more concentrated over the generations?

Ms. SCOTT: That's right. If you had a child with just someone in the general population, the chances that the two of you would carry the very same recessive gene is pretty low. But when you marry a relative, the probability that you're both carrying the same recessive gene is much higher, and so you're in a much higher risk to have a child with a recessive disorder. And this is, you know, particularly so in those communities where there's been lots of intermarrying. So it's not like you're just first cousins. You're probably first cousins several different ways, because, you know, your families have been intermarrying and related to each other in a number of ways.

The other thing that could be going on is when you have small communities that are sort of isolated, you can get concentrations of some deleterious genes in that community that might be pretty rare in the general population, but they get concentrated in that community.

SEABROOK: One family in the story had three healthy children and four children with serious problems. How does that happen?

Ms. SCOTT: Well, even if you, both you and your partner are carriers for the same recessive disorder, there is a one in four chance that that double dose will come together and the child be affected. But there is also a 75 percent chance that it won't happen. So even in families where you are, you know, related to each other and both members are carriers, there's still a chance to have perfectly normal children.

SEABROOK: What options do these children have if they grow up and have relatively normal lives, if they continue to intermarry? It sounds like, that it may only get worse for them. What if they go outside of their villages?

Ms. SCOTT: Yeah. I think any kind of solution has to take into account the social, the economic, the cultural realities of the situation that the couple lives in. Having children by someone whom you're not related would obviously lower that risk. If the specific disorder is known, it might be possible to do carrier testing and determine for sure if your partner carries the same recessive disorder that you do. And if you don't, then you know you're not at risk to have a child affected. And if you do, then you sort of know what your risks may be, and then you can decide what you want to do about that.

SEABROOK: Some of those options may not be available to people in a small village in Syria, but we should also say that this is not something that only happens in a small village in Syria. This happens all over the world.

Ms. SCOTT: That's right. There are parts of the - around the world where marrying relatives is not uncommon for a variety of reasons. So it's not all that unusual.

SEABROOK: So it really is the generations of this happening that concentrates the risks.

Ms. SCOTT: And you get this concentrated of these genes sort of concentrating in these particular populations, where the risk really becomes much higher.

SEABROOK: Very interesting. Joan Scott is a certified genetic counselor and the deputy director of the Genetics and Public Policy Institute in Washington.

Thank you very much.

Ms. SCOTT: Thank you.

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