Twin
Twin
Twin (/twɪn/) is a term used in medicine and biology to refer to one of two offspring produced in the same pregnancy. Twins can either be monozygotic (identical), meaning that they develop from one zygote, which splits and forms two embryos, or they can be dizygotic (fraternal), meaning that they develop from two different eggs, each fertilized by separate sperm cells.
Etymology
The term "twin" comes from the Old English twinn, meaning "two together", which is related to the German Zwilling, the Dutch tweeling, the Danish tvilling, and the Swedish tvilling.
Types of Twins
- Monozygotic twins - These twins are commonly referred to as identical twins. They occur when a single egg is fertilized to form one zygote which then divides into two separate embryos. Monozygotic twins share the same genetic material, are always the same sex, and look remarkably similar.
- Dizygotic twins - Also known as fraternal twins, these occur when two eggs are independently fertilized by two different sperm cells. Dizygotic twins can be the same sex or different sexes and share about half of their genes, just like any other siblings.
Related Terms
- Conjoined twins - These are identical twins joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births.
- Parasitic twin - A parasitic twin, also known as an asymmetrical or unequal conjoined twin, occurs when one twin is not fully formed, and is totally dependent on the body functions of the complete fetus.
- Vanishing twin - This is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed.
External links
- Medical encyclopedia article on Twin
- Wikipedia's article - Twin
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