why do some fraternal twins look identical

We were walking through the Wonderopolis field the other day when we overheard an interesting conversation between a and a couple of Cow: Hey there, little sheep! I'm Buttercup. What's your name? Sheep 1: I'm Dolly. Nice to meet you, Buttercup! This is my sister Polly. Sheep 2: Hi Buttercup! Cow: Hi Polly Б wait a second! You look just like Dolly! What's going on here? Are you one of those? Sheep 2: No, silly! We're twins! Cow: Twins? Well, I neverБ
Sheep 1: That's right! Even our parents can't tell us apart! We heard a over by the chicken, so we didn't get to hear the rest of their conversation. Dolly and Polly did make us start WONDERing, though. Exactly why do twins look alike? If you've ever seen a of identical twins, then you know that they usually lookБwellБidentical! That's because identical twins result from one fertilized egg splitting into two during. When they're born, identical twins share the same DNA, which explains why they usually look almost exactly alike. Identical twins aren't the only type of twins, however. Fraternal twins occur when two eggs are fertilized during. Fraternal twins do not share the same. In fact, they share only about half the same genes. This is why fraternal twins often don't 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 between identical twins, especially as they grow older.


Scientists note that, after an egg divides in two, cells to as a natural part of the of of a baby with trillions of cells. Each time a cell divides, the DNA has to be copied. Over the course of millions and millions of cell divisions, mistakes can occur, leading to in the DNA. As a result, even identical twins don't have the same DNA by the time they're born. DNA is large and. that occur are to the small section of DNA that pertains to one's appearance. Thus, identical twins do usually look identical. To the extent that do occur in the genes dealing with appearance, between twins can be seen. can become even more apparent over time. These that vary with age don't have anything to do with DNA. Instead, they're likely the result of environmental. As twins grow up, they will not do everything identically. in eating habits, and can lead to in appearance, as well as overall health. If it seems like you've seen more twins and triplets in recent years, you're not seeing things. Multiple births have been on the rise for a few decades now. Scientists believe new fertility drugs, which help women conceive, often increase the likelihood of multiple births. Even though this is true for all identical twins, we canБt usually tell because the changes happen in parts of the DNA that donБt affect how identical twins look. Which isnБt surprising. DNA is large and very little of it has to do with, for example, skin and eye color. By chance, the DNA differences for these girls happened to be in the smallish part of their genome that deals with looks.


These two girlsб let us see what goes on in every identical twin. And in every one of us. DNA Differences Identical twins do indeed start with identical DNAБthey are the result of the same sperm from dad and the same egg from mom. Even though identical twins come from the same fertilized egg, in the end each twin has slightly different DNA. ( The original fertilized egg divides one or more times before the resulting clump of cells splits into two. Each clump of cells goes on to become one of the identical twins. In this process of becoming a brand new baby withб trillions of cells, the cells in each clump divide over and over again. DNA differences or mutations can happen any time a cell divides. This is because a cell needs to copy its DNA before it can divide. б And while the cellular machinery is astonishingly good at copying DNA, it isnБt perfect. Every now and then it makes a mistake. All of the new cells that come from the one with the mistake will have that same mistake. One consequence is that if it happens early, the baby will have more cells carryingб that mistake. Think about it like one of those medieval monks patiently copying manuscripts in a monastery somewhere in medieval Europe. Imagine he makes a mistake during copying and the original manuscript is destroyed. Now every new manuscript containsб his mistake. For the English twins, one may have developed a mutation in a gene that affects skin and eye color.


Most likely it would be in the lighter child as it is easier to break something than to fix it. And often traits like blue eyes are the result of a gene not working quite right. But this isnБt the only way this could have happened. Another possibility has to do with how cells read their DNA. Using DNA Differently Imagine Louis C. K. , Meryl Streep and Kevin Hart are all going up for the same role. Even though they all have the exact same lines to read, odds are there will be real differences in how they say them. Just like different actors will read the same script differently, so too will different people's cells read the same genes differently. This is sort of what happens in different peopleБs cells. One person'sб cells will read a gene one way and another personБs cells will read the exact same gene a different way. If that gene controls skin and/or eye color, then it will affect a personБs skin or eye color. Here's how it might work: Near theб genes there little chemical markers that can, for example, tell the cell how often to read a gene. In theб case of the twins, it could be that a set of these БepigeneticБ marks is telling the cells of oneб twin to read her skin and eye color genes just a little bit. That meansб she would make less pigment and so have fair skin and blue eyes. Or her sister might have marks on her DNA telling her cells to read her skin and eye color genes much more often. This would explain her brown eyes and cafe au laitб skin.

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