Another method that geneticists use to analyze the genetics of human characteristics is twin studies. Twins come in two types: dizygotic (nonidentical) twins arise when two separate eggs are fertilized by two different sperm, producing genetically distinct zygotes; monozygotic (identical) twins result when a single egg, fertilized by a single sperm, splits early in development into two separate embryos.
Because monozygotic twins arise from a single egg and sperm (a single, "mono," zygote), except for rare somatic mutations, they're genetically identical, having 100% of their genes in common ( FIGURE 6.11a). Dizygotic twins (< Figure 6.11b), on the other hand, have on average only 50% of their genes in common (the same percentage that any pair of siblings has in common). Like other siblings, dizygotic twins may be of the same or different sexes. The only difference between dizygotic twins and other siblings is that dizygotic twins are the same age and shared a common uterine environment.
The frequency with which dizygotic twins are born varies among populations. Among North American Caucasians, about 7 dizygotic twin pairs are born per 1000 births but, among Japanese, the rate is only about 3 pairs per 1000 births; among Nigerians, about 40 dizygotic
6.11 Monozygotic twins (a) are identical; dizygotic twins (b) are nonidentical. (Part a, Joe Carini/Index Stock Imagery/Picture Quest; Part b, Bruce Roberts/ Photo Researchers.)
twin pairs are born per 1000 births. The rate of dizygotic twinning also varies with maternal age ( FIGURE 6.12), and dizygotic twinning tends to run in families. In contrast, monozygotic twinning is relatively constant. The frequency of monozygotic twinning in most ethnic groups is about 4 twin pairs per 1000 births, and there is relatively little tendency for monozygotic twins to run in families.
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