Identical twins facts for kids

Identical twins are like natural copies of each other. They start with the exact same genetic information, meaning they have the same alleles (which are different versions of genes). This happens when one fertilized egg splits into two separate embryos very early on. Identical twins are always the same sex. They are also called monozygotic (MZ) twins because "mono" means one and "zygote" means fertilized egg.

This is different from fraternal twins. Fraternal twins come from two separate eggs that are fertilized at the same time. They are not always the same sex. They are called dizygotic (DZ) twins because "di" means two. Both types of twin grow inside the same uterus at the same time. This means their birth environment is the same.

About 1 in every 240 births results in identical twins. Fraternal twins are twice as common as identical twins.

How Twin Studies Help Us Learn

Identical twins are like natural clones. Since they begin with the same genes, scientists can use them to study how much our genes affect who we are. This helps answer the big question of nature versus nurture. It asks whether our traits come more from our genes (nature) or from our surroundings and experiences (nurture).

Studies with twins have shown some interesting things. When we look at different traits (characteristics) people have, we find that some are more influenced by genes than others. For example:

• Eye colour: This is completely decided by your genes.
• Weight and height: These are partly decided by your genes and partly by your environment (like what you eat).
• Which language you speak: This is entirely learned from your environment.

Scientists do these studies by comparing groups of identical twins, fraternal twins, and other siblings. They measure different traits in these groups. Then, they use statistics to see how similar the twins are for each trait. If a trait is partly inherited, identical twins will be much more similar for that trait.

These studies can go even further. Scientists compare identical twins who grew up together with identical twins who grew up in different homes. This helps them understand how much different life situations can change people who started with the same genes. Modern research clearly shows that genes affect not just our bodies, but also how we think and behave.

A Brief History of Twin Research

The first person to study twins was Francis Galton. He was Darwin's half-cousin and helped create the field of statistics. Galton followed twins throughout their lives, taking many measurements. However, he didn't fully understand the genetic differences between identical and fraternal twins. Modern twin studies, as we know them today, didn't start until the 1920s.

Wilhelm Weinberg was the first to estimate how often twins are born. He realized that identical twins must be the same sex. But fraternal twins could be the same sex or different sexes. Weinberg created a formula to figure out how many identical and fraternal twins there were based on this. He also found that the ability to have twins is not something you inherit from your parents.

Why Identical Twins Are Not Exactly Identical

Monozygotic (identical) twins are almost genetically identical. They are always the same sex, unless a very rare change (a mutation) happens during development. However, identical twins always look or act a little different from each other. These differences are called phenotypes. For example, twins might have different sexual phenotypes if one has Klinefelter syndrome (a genetic condition).

Even though identical twins start with almost the same genes, a study in 2012 looked at 92 pairs of identical twins. It found that they develop hundreds of small genetic differences very early, even before birth. These differences happen because of tiny mistakes (mutations or copy errors) in their DNA after the embryo splits. On average, a pair of identical twins will have about 360 genetic differences that happened early in their development.

Another reason for differences between identical twins is something called epigenetic modification. This means that different things in their environment throughout their lives can "switch" genes on or off. A study of 80 pairs of identical twins, from ages three to 74, showed that younger twins have fewer epigenetic differences. But the number of these differences grows as they get older. Fifty-year-old twins had more than three times the epigenetic differences of three-year-old twins. Twins who grew up apart (like those adopted by different families) had the biggest differences.

However, some traits, like IQ and personality, can become more similar as twins age. This shows how much our genes influence many parts of our characteristics and behaviour.

Twins in the Animal Kingdom

Having multiple babies at once is normal for many mammals, like rats, cats, and dogs. But identical twins are rare in most animal species. One interesting example is the nine-banded armadillo, Dasypus novemcinctus. It always gives birth to identical quadruplets (four babies at once).

Animals born without sexual reproduction, like through parthenogenesis, are also identical. This is true unless a process called crossing over happens when the eggs are made.

Images for kids

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  Identical twins Mark and Scott Kelly, both former NASA astronauts.

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  Adult fraternal twins.

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  A pair of female ere ibeji twin figures from the early 20th century. The Yoruba people have the highest rate of fraternal twins in the world.

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  Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

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  Twin calves of the Hereford breed in Miles City, Montana.

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  Dichorionic-diamniotic twins at 8 weeks and 5 days. The twin on the left is shown with its head pointing up and left. The twin on the right is shown with its head pointing right.