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All biological siblings share the same parents, inheriting chromosomes from each. Despite this genetic similarity, siblings are often quite different from one another. Twins are siblings who share the same womb. Twins occur in about 1 out of every 33 births in the United States (Martin, Hamilton, Osterman, Driscoll, & Drake, 2018).

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Figure 2.3 Sex Determination

The majority of naturally conceived twins are dizygotic (DZ) twins, or fraternal twins, conceived when a woman releases more than one ovum and each is fertilized by a different sperm. DZ twins share about one half of their genes, and like other siblings, most fraternal twins differ in appearance, such as hair color, eye color, and height. In about half of fraternal twin pairs, one twin is a boy and the other a girl. DZ twins tend to run in families, suggesting a genetic component that controls the tendency for a woman to release more than one ovum each month. However, rates of DZ twins also increase with in vitro fertilization, maternal age, and each subsequent birth (Pison, Monden, & Smits, 2015).