Explain how dizygotic and monozygotic twins originate, and identify the frequency with which each type of twin occurs. Summarize the genetic and environmental factors that increase the chances of giving birth to each type of twins
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Dizygotic, or fraternal, twins are the most common type of multiple offspring. They result from the release and fertilization of two ova. Genetically, they are no more alike than ordinary siblings. Older maternal age, fertility drugs, and in vitro fertilization are major causes of the dramatic rise in fraternal twinning and other multiple births in industrialized nations over the past several decades. Currently, fraternal twins account for 1 in about every 33 births in the United States. Monozygotic, or identical, twins are created when a zygote that has started to duplicate separates into two clusters of cells that develop into two individuals. The frequency of identical twins is the same around the world—about 1 in every 350 to 400 births. Environmental influences that prompt monozygotic twinning include temperature changes, variation in oxygen levels, and late fertilization of the ovum.
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