People are truly mesmerized by twins. And why not? They're cute, miraculous, and just different enough to be special. Since most people do not have first hand experience with twins, they haven't taken the time to learn some simple facts about them. However, a little bit of basic knowledge would go a long way towards preserving the already-fragile sanity of mothers of twins everywhere.
Identical twins do not simply mean they look alike. Identical twins occur when a single egg splits into two. Therefore, these types of twins are an exact genetic match. This means that boy-girl twins cannot be identical. Think about it!
Twins cannot run in the father's family. Identical twins are still considered a random, freak event with no genetic link. Fraternal twins occur when the mother (for a variety of reasons) releases two eggs that are then fertilized, etc. This "extra" ovulation can be caused by many things, including a genetic predisposition, fertility drugs, and age. It cannot be caused, however, by a man. No man is sexy enough to make a woman ovulate twice. Seriously. Not even Taye Diggs, whose lovely wife Idina Menzel delivered their first child, ONE baby, last fall.
As is widely known, many twins (or more) today were conceived via fertility treatments. This could have occurred in one of a few different ways. If the mother was having difficulty ovulating on her own, which is a common problem, she probably took some form of ovulation inducing medication. These drugs can cause the mother to release more than one egg. Then these eggs are fertilized through "normal" intercourse or artificial insemination. Alternatively, some couples have to escalate to in vitro fertilization for a variety of reasons. In this instance, a reproductive endocrinologist will transfer fertilized embryos into the mother's uterus. More than one such embryo can be transferred to increase the likelihood of success and then can proceed to cause a multiple birth. In addition, even if only one embryo was transferred to the uterus, the process can increase the likelihood of the embryo splitting into two, causing identical twins as described above.
With some same sex twins, only a genetic test done after birth can confirm the identical-fraternal debate, if it matters that much to the parents.