Twins

Most people are born into this world alone. However, sometimes individuals come into the world with a sibling. When two siblings are born from the same pregnancy, they are referred to as twins. There are two types of twins: fraternal (dizygotic [DZ]) and identical (monozygotic [MZ]). Fraternal twinning occurs when two separate eggs are fertilized by separate sperm cells, thereby forming zygotes that develop in the womb at the same time. Fraternal twins can be of either the same sex or different sex. Essentially, fraternal twins are brothers/sisters born at the same time, sharing about one half of their DNA. In contrast, identical twinning occurs when a single fertilized egg divides into two separate embryos at the beginning of a pregnancy. The resulting identical twins ...