Why Do Twins Look Alike?

While walking through the Wonderopolis field, we overheard an interesting conversation between a cow and a couple of sheep:

Cow: Hello there, little sheep! I’m Buttercup. What is your name?

Sheep 1: I’m Dolly. Nice to meet you, Buttercup! This is my sister Polly.

Sheep 2: Hi Buttercup!

Cow: Hi Polly — wait a second! You resemble Dolly! What is happening here? Are you one of those cloned sheep?

Sheep 2: No, silly! We are twins!

Cow: Twins? Well, I never…

Sheep 1: That’s correct! Even our parents cannot tell us apart!

We heard a disturbance near the chicken coop, so we did not get to hear the rest of their conversation. However, Dolly and Polly made us start WONDERing. Why exactly do twins look alike?

If you have ever seen a pair of identical twins, then you know that they usually look…well…identical! This is because identical twins occur when one fertilized egg splits into two during pregnancy. When they are born, identical twins share the same DNA, which is why they usually look almost exactly alike.

However, identical twins are not the only type of twins. Fraternal twins occur when two separate eggs are fertilized during pregnancy. Fraternal twins do not share the same DNA. In fact, they only share about half of the same genes.

This is why fraternal twins often do not look any more alike than regular brothers and sisters. Unlike identical twins, fraternal twins can consist of both a boy and a girl born at the same time.

Although identical twins share the same DNA, you may have noticed that there are sometimes differences between identical twins, especially as they grow older. Scientists explain that after an egg divides in two, cells continue to divide as a natural part of the baby’s development process. This results in trillions of cells.

Each time a cell divides, the DNA needs to be copied. Over millions and millions of cell divisions, mistakes can occur, leading to mutations in the DNA. Consequently, even identical twins do not necessarily have the exact same DNA by the time they are born.

DNA is extremely large and complex. Mutations that occur are statistically unlikely to affect the small section of DNA that determines one’s appearance. Thus, identical twins usually look identical. However, if mutations do occur in the genes related to appearance, differences between twins can be observed.

These differences can become more apparent over time. The variations that change with age do not necessarily have anything to do with DNA. Instead, they are likely a result of environmental differences.

As twins grow up, they will not do everything in the same way. Differences in eating habits, exercise, and sleep can lead to differences in appearance, as well as overall health.

If it seems like you have been seeing more twins and triplets in recent years, you are not mistaken. The number of multiple births has been increasing for a few decades now. Scientists believe that new fertility drugs, which assist women in conceiving, often increase the likelihood of multiple births.

Try It Out

Are you ready to double your fun? Explore the following activities with a friend or family member:

In your family or school, how common are twins? With the help of an adult friend or family member, create a family tree for a few generations. How many sets of twins can you find? Consider all the children you know at school. Do you know any twins or triplets? If so, do they resemble each other a lot? Why or why not?

What do you think it would be like to grow up with a twin? You would always have a companion to play with. You could share clothes. It might be fun to occasionally switch identities to trick people. On the other hand, it might be challenging to develop your own individual identity. Make a list of the advantages and disadvantages of being a twin. Considering all the factors you can think of, would you like to be a twin?

Do you have a vivid imagination? Prove it! Write a short story about twins who are separated at birth. They are raised by different parents in different cities. One day, they accidentally meet. It’s like looking at a mirror…almost. They look quite similar, but there are differences. How are they alike? In what ways are they different? How did their different upbringings influence the people they became? Share your story with a friend or family member.

Wonder Sources:

– http://kidshealth.org/en/kids/twin-triplet.html

– http://genetics.thetech.org/ask/ask142

– https://ww2.kqed.org/futureofyou/2016/03/29/why-identical-twins-dont-always-look-the-same/

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