twinning
biology
Quick Summary
Twinning refers to the birth of two offspring from the same mother at one time. There are two primary biological types of twins: identical (monozygotic) and fraternal (dizygotic).1
- Identical twins develop from a single fertilized egg (zygote) that splits into two embryos. These twins are genetically identical and always the same sex.2
- Fraternal twins develop from two separate eggs fertilized by two different sperm. They are genetically no more alike than ordinary siblings and can be of the same or different sexes.23
Studies of identical twins are valuable for understanding the relative influences of genetics and environment on human traits.14
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