There are two kinds of twins. “Fraternal” or non-identical twins have DNA as different as any two people with the same parent. They may be the same sex or opposite sex. They may be a bit more alike than other siblings just because they grow up at the same time.

Identical twins have the same DNA (mostly). They are formed when one tiny embryo splits in two. That does not mean that the twins are really identical. Their environment is different starting off before they are even born. One twin may get more blood supply, for example. After they are born, they may have different experiences and make different choices.

There may be some differences in some of their DNA because mutations can happen at any time. So even though their DNA starts off identical, some of their cells may have slightly different DNA, you just wouldn’t know it by looking at them.

If you are interested in DNA, you may want to become a geneticist.

There are two types of twins: identical and fraternal. Identical twins, or monozygotic, come from one zygote that splits to form two embryos. Fraternal twins, or dizygotic, are a product of two separate eggs being fertilized. Identical twins are usually both boys, or both girls, but fraternal twins can be both boys, both girls, or even a boy and a girl. Identical twins, since they are split into two from the same zygote, can have the same DNA. The DNA isn't perfectly identical, but it's pretty similar. They look almost exactly the same, but each twin develops and lives their life separates so they are exact mirror images of each other. Fraternal twins, obviously, would have a greater difference in DNA because they are already to completely separate eggs that are being fertilized to become two completely separate babies. So, basically identical twins have "similar" DNA and fraternal twins do not.