Twins
There are two distinct types of twins in humans: identical and fraternal. Identical twins, also called monozygotic twins, result from the splitting of a single zygote into two distinct embryos. Because they arise from a single fertilized egg, monozygotic twins are always of the same sex, and being genetically identical, they tend to bear remarkable similarities in physical appearance. Identical twinning occurs in roughly 1/250 pregnancies worldwide, with relatively little variation between different populations. Evidence indicates that there is no tendency toward identical twin births in families (i.e., it does not "run in families").
Fraternal twins result from the fertilization of two different eggs by two different sperm during the same ovulatory cycle. Fraternal twins, also called dizygotic twins, can be of the same sex, or one of each sex, and are no more alike genetically than other siblings. Fraternal twinning varies in frequency around the world with the highest reported rates in Africa, where twins account for up to one in every eight pregnancies, and the lowest rates reported in Asian populations where twins account for less than one percent of all pregnancies. An increased chance for fraternal twinning can run in families. In these cases, the predisposition toward twinning appears to be due to a higher chance for the woman to produce multiple mature eggs in the same menstrual cycle, and thus the history of twinning tends to fall on the side of the mother.
At birth, both fraternal and identical twins can be very similar in appearance, and identifying which type of twinning has occurred is often difficult. Examination of the placenta and membranes is sometimes helpful. Twins that share a common amnion, chorion, or placenta are always identical twins. The reverse is not always true, however. Identical twins that split into two embryos early in development may develop their own placenta and membranes. At the other end of the spectrum, twins which separate from each other late in development may be conjoined.
Twinning in other mammals sometimes carries unexpected consequences. For example, when male/female fraternal twin pairs occur in cattle, the male hormones produced by the male twin causes the female to remain infertile after birth. These females are called "freemartins". Fortunately, human male/female twin pairs do not appear to interfere with each other's development.
Twins are of special significance in understanding the genetics of complex traits. Comparisons between monozygotic twins and dizygotic twins are the basis for determination of the degree to which these traits are influenced by genes.
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