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Using Twin Studies to Determine Heritability

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  • 0:01 Heritability
  • 1:35 Twin Studies
  • 5:58 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

A twin can be more than a built-in buddy for life. Scientists can use monozygotic and dizygotic twin pairs to study the heritability of just about anything, from cancer to behavior.

Heritability

Do you know any identical twins? Maybe you are one yourself. Maybe you grew up with a pair of identical twins in your grade. I did. I remember having one boy in a set of twins in my fourth grade classroom while the other boy was in a different classroom. He and his brother looked, well, pretty identical, but as kids, we could always tell them apart. I'll admit that they made it easier by doing their hair differently.

But one April Fools' Day, the twins dressed alike and they styled their hair the same. They swapped classrooms, hoping to fool their teachers and the rest of us. I just don't really think they fooled anybody for that long. It's a pretty good joke, and only one identical twins can attempt, but that doesn't make it successful. After a few conversations with the 'wrong' twin, you can figure out what's going on.

It's true that identical twins are genetically identical, but genetics aren't everything. They look the same, but not exactly the same. They talk the same, but not exactly the same. They are individuals with different life experiences and interests. You know, it could be a dead giveaway that one kid likes Ninja Turtles while the other one likes Batman. Still, it must be neat to grow up with someone who could always serve as your stunt double.

Behavioral scientists and geneticists have long since relied on identical twins for more than just a good practical joke. Studying two people who are genetically identical can help us figure out what traits are caused purely by genetics and what traits are influenced by the environment. You've learned a little bit about heritability before, the term that describes how a total phenotype, or physical trait, is affected by genetic variation. Twins can be used directly to test the heritability of a trait.

Twin Studies

As you are probably aware, there are two types of twins that can be born. Monozygotic twin pairs, or MZ twins, are the identical twins. They share 100% genetic similarity as they split from one embryo, where a single egg is fertilized by a single sperm. Dizygotic twin pairs, or DZ twin pairs, are non-identical twins. They share roughly 50% genetic similarity, the same as any other sibling pair, because they developed from two separate embryos - two different eggs fertilized by two different sperm. In essence, they are two siblings that just gestated at the same time.

A twin study is an experiment that assesses the genetic and environmental influence on a trait using MZ and DZ twin pairs. Twin studies allow scientists to attack this heritability question. It's an easy question to ask, but a hard one to answer - 'How much of a trait is due to genetic variation opposed to environmental influence?'

So why use both MZ and DZ twin pairs? Let's think about how both MZ and DZ twins grow up. They grow up in the same womb, they are born on the same day, and then they grow up at the same time in the same house with the same family. This means that the two individuals within a twin pair have as close to a similar environmental background as any two people can get.

We can't say that environments are identical because each person has different life experiences, but an MZ or DZ twin pair raised together will have the same family, same socioeconomic status, and the same residence. But even though the environmental influence will be similar between individuals in an MZ or DZ twin pair, only the MZ twin pair shares 100% identical genetics. Therefore, you can pick a trait to follow in hundreds of thousands of MZ and DZ pairs and ask this question: 'How likely is it that a trait is shared between both individuals in a twin pair?' If you see differences between non-identical DZ twins, much of that difference is likely due to genetics rather than environment.

After all, the genetics of DZ twin pairs are only 50% similar, but their environment is more alike. Now let's say there's a trait that has 0% heritability, meaning we know it's not at all influenced by genetics. It's hard to come up with a real example, because in reality, there's a genetic influence on almost everything. But for argument's sake, let's say we looked at bubble gum chewing in several MZ twin pairs and DZ twin pairs to see whether or not they had this trait.

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