George & Amal Clooney's Twins: How Can You Get 1 Boy and 1 Girl?

George and Amal Clooney are now the parents of twins: Amal Clooney gave birth to a boy and a girl today (June 6), People magazine reported. 

Amal Clooney, age 39, and the fraternal twins, named Ella and Alexander, are "healthy, happy and doing fine," according to a statement from the family that was given to People magazine. "George [age 55] is sedated and should recover in a few days," the statement continued, in jest.

Fraternal, or nonidentical, twins occur when two eggs are released from a woman's ovaries at the same time, and each of those eggs is fertilized by a different sperm cell. [Beyoncé Expecting 2: Here Are 5 Fun Facts About Twins]

Fraternal twins can be different sexes because two different sperm cells are involved, and it is the sperm cell that carries the "sex chromosome" that determines the sex of the child. Although both the egg from the mother and the sperm from the father contain sex chromosomes, the egg always contains an X chromosome, whereas the sperm can have either an X or a Y chromosome.

If the sperm that fertilizes the egg has an X chromosome, the baby will be a girl. If the sperm is carrying a Y chromosome, the baby will be a boy.

In the case of fraternal twins in which one is a boy and the other is a girl, one egg was fertilized by a sperm cell carrying an X chromosome, and the other egg was fertilized by a sperm cell carrying a Y chromosome. The eggs can come from the same ovary or from two different ovaries.

In contrast, identical twins occur when a single egg that has been fertilized by a single sperm cell splits into two separate, but identical, embryos early in its development.

Because identical twins come from the same embryo, these twins share 100 percent of their DNA. Fraternal twins, however, share about 50 percent of their DNA.

The chances of having twins increases as a woman ages, according to the Mayo Clinic. 

Originally published on Live Science.