Twins
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Twins
Twins (/twɪnz/) are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.
Etymology
The term "twin" comes from the Old Norse word "tvinr", meaning "double".
Types of Twins
- Monozygotic twins - Also known as identical twins, these twins originate from a single fertilized egg that splits into two. Monozygotic twins share nearly 100% of their genetic material and are always the same sex.
- Dizygotic twins - Also known as fraternal twins, these twins originate from two separate fertilized eggs. Dizygotic twins share about 50% of their genetic material, the same as any other siblings, and can be either the same sex or different sexes.
Related Terms
- Conjoined twins - Twins that are physically connected to each other. This occurs when the zygote of identical twins fails to completely separate.
- Twin-to-twin transfusion syndrome - A condition that can occur in pregnancies when identical twins share a placenta. Abnormal blood vessel connections form in the placenta and allow blood to flow unevenly between the twins.
- Vanishing twin syndrome - A condition in which one twin dies in utero and is partially or completely reabsorbed by the mother's body, the other twin, or the placenta.
- Twin studies - Research studies that assess the influence of genetics versus environment on various traits by comparing the similarity of identical (monozygotic) twins to that of fraternal (dizygotic) twins.
External links
- Medical encyclopedia article on Twins
- Wikipedia's article - Twins
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