21.

  1. Two.
  2. Once.
  3. One and half.
  4. Randomly.

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22.

  1. One-half.
  2. One-fourth.
  3. One-third.
  4. Two-thirds.

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23.

  1. They always have the same blood type.
  2. They occur at random in the population.
  3. They can be of different sexes.
  4. They have the same genes.

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24.

  1. A secret language.
  2. The same genes.
  3. The same careers.
  4. A different bone structure.

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25.

  1. How twins are born.
  2. Fraternal twins and their parents.
  3. Characteristics of identical twins.
  4. The marriages of twins.

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Although twins have always been a source of curiosity, they are not so uncommon statistically, since they occur once in every 86 birth.

About one-third of all twins are identical, or single-egg twins. Identical twins have the same genes and, hence, the same sex, hair, eyes, blood type, and bone and tooth structure. Some identical twins are mirror images of each other. For example, one may be left-handed, the other right-handed. As young children, some identical twins may develop their own private language. Identical twins have a special intuition, and they often seem to think and dress alike even they are away from each other. In fact, even when they are separated at birth and raised apart, identical twins develop startling similarities. They may pursue the same careers, have the same interests, or die within days of each other.

In contrast to identical twins,- fraternal twins inherit a separated assortment of genes and are not necessarily of the same sex. In some families there is a hereditary tendency to produce fraternal twins, but identical twins occur at random in the population.