WHERE DO TWINS COME FROM?

Twins always skip a generation.

Identical twins usually run in families.

Identical twinning is only passed through the mother's side of the family.

People appear to be fascinated with the entire multiple birth experience, from pregnancy aspects to the similar physical, health and mental factors that become apparent after birth. Fertility treatments have spawned a number of well-publicized multiple births in recent years, and two in every 100 sets of parents will see multiple births. However, they are still uncommon enough to cause a stir when seen in public.

Research on the origin of twins and other multiples is dispelling many of the myths from the past; however, some stories about twins and supertwins (triplets or more) maintain their foothold in popular and scientific fact.

For example, many people believe twins of any type may "run" in families. In fact, identical multiples are the result of accidental splits of a single fertilized egg, which makes them a product of chance rather than of heredity. They share everything, from gene pool to amniotic fluid, which often causes them to look, act and often think alike.

Although there may be two sets of seemingly identical multiples in a family, the trend would have to occur several times among immediate family members in order to have any medical basis.

Allison and Amanda Toole, 10, said they are happy to be sisters, but are nothing alike. The identical tandem insisted on dressing differently at a young age, and have never shared a classroom.

"We were born at the same time, but that's all," said Amanda, the oldest by 20 minutes. "I don't like the same things she does."

Allison, the more outgoing of the duo, agrees that their personalities defy their similar looks. "I like to talk to people and do things, but I don't like boys," she said. "She's more quiet."

Fraternal twins -- or multiples who share a womb but are the result of two or more fertilized eggs -- may be the product of heredity and can recur throughout family trees. These multiples share about 50 percent of the same genes, just as regular siblings do.

Also like other siblings, they may look related to each other, but don't share identical features.

Women who have inherited the tendency to ovulate more than once a month have a greater chance of having fraternal multiples, as do those who receive fertility treatments, are over the age of 35, or who have other fraternal sets in their family.

Lawrence and Willetta Brookins are the parents of Marquis and Marquita, 15. They said they were not really surprised by their fraternal set because twins seem to run on both sides of the family.

"It was my husband's anniversary present," said Willetta with a laugh. "I knew before the ultrasound because I was having too much movement, but my husband has twin brothers, and I have three sets of first cousins who are twins."

Willetta said her twins have no health similarities. Marquita has food and other allergies, while Marquis has none, she said. The teenagers are often experiencing different growth patterns.

"Right now, he's going through growing pains bad, but she's done growing, I think," she said. "They do the twin thing, like when they come out of different rooms and they're singing the same song, but that's all I really see.

"I think I just have two cool kids there."