Twins can be identical or fraternal. There are distinct differences between them, both in physical appearance and in how they are made.
Identical twins are conceived when, during fertilization, only one sperm impregnates one egg, but then the egg divides into two embryos. Called mono-zygotic, these two embryos have the exact same chromosomes, a perfect resemblance and are the same sex. Identical twins can develop and be carried in three different ways. There can be two amniotic sacs fed by one placenta. They can be carried in one amniotic sac, or they can each have their own amniotic sac fed by their own placenta. This instance is similar to the development of fraternal twins. Identical twins ultimately have the same genetic makeup.
Fraternal, or di-zygotic, twins are conceived when two separate eggs are fertilized by two separate sperm. These twins have different chromosome makeups and will not look alike. Unlike identical twins they can be the same sex or different sexes. Ultimately, fraternal twins are the result of two separate pregnancies taking place at the same time. Each embryo is carried in its own sac with its own placenta to feed it.
In extremely rare cases, half twins or semi-identical twins are conceived. this happens when the embryos inherit the exact same genes from their mother but different genes from their father. It is not clearly known how this type of twinning happens but one theory is called polar body twinning, where an extra cell structure within the egg fails to disintegrate and is then fertilized. This is only a theory though, and has not been proven.
Studying the reproductive powers of the body as well as the different ways twins can be formed is an interesting and enjoyable way to learn more about our bodies and personalities. Understanding the relationships between different types of twins can help us continue to learn how and why we interact with each other in the ways we do.
Posted 1440 day ago