When someone talks about twins, most of us automatically think of identical twins, a pair of brothers or sisters that share so many of the same characteristics it can be difficult to tell them apart. When we are first introduced to the twins that were talked of previously, we are often taken aback when we discover they look completely different or that they are of different sexes. Twins of mixed sex and twins that do not look alike are fraternal twins. The fact of the matter is that identical twins make up a minority of all the twins in the world. Most are of the fraternal sort.
The major difference between the two types of twins, and the major reason for all the differences in characteristics, has to do with how they developed in their mother’s womb from the first moment of conception.
Identical twins develop from a single zygote, which is a female egg that has been fertilized by a male sperm. The zygote cell begins to divide into new cells that form an embryo, first by splitting into two, then those two into four, then those four into eight, etc. In a normal conception, the cells divide as part of a single embryo that develops into a fetus and is born as a human baby. In the case of identical twins, the zygote’s first cellular division, instead of becoming two separate cells of a single embryo, divides into two separate, but identical embryos. The two babies develop next to each other from a shared placenta. Another name for identical twins is monozygotic twins.
Monozygotic twins share the same DNA, so they look close to identical, barring any large differences in environment of the two. They are also always of the same sex. Identical twins are born at a rate of three times out of every 1000 pregnancies.
Fraternal twins develop from two separate zygotes. This occurs when the mother releases more than one egg during her cycle. The two eggs are fertilized by two different spermatozoa. Because of this, fraternal twins are known as dizygotic twins. Because they are developed from two different zygotes, they have different DNA. This means they will have similar appearances only in as much as brothers and sisters born of separate pregnancies. Fraternal twins develop each with their own placentas and can be of mixed sexes since they have different sets of DNA. Worldwide, fraternal twins are born in 10 out of every 1000 pregnancies.
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