Can Boy Girl Twins Be Identical?

Every parent of twins faces the question. "Are your twins identical or fraternal?" The general public doesn't really understand exactly what the terms mean; if they did, they wouldn't ask whether a set of boy/girl twins are identical!

The short answer is No! The terms identical and fraternal don't describe what the twins look like, but actually how they form. Identical (monozygotic) twins are always of the same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosomes. Therefore, boy/girl twins are always fraternal or (dizygotic); they can only form from two separate eggs that are fertilized by two separate sperm. Fraternal twins can be either two girls, two boys, or one of each.

Let's break it down further. Identical=monozygotic. Monozygotic means that the twins formed from a single fertilized egg that splits into two. It starts as one, splits into two. When it splits, it is either male or female. After it splits, there are either two males or two females.

Fraternal twins are dizygotic. Dizygotic = two zygotes. They are two separate eggs that are fertilized by separate sperm. The resulting egg/sperm combination can be male or female. The result is either two male twins, two female twins, or one male and one female.

Therefore:

Two girl twins can be either identical or fraternal (monozygotic or dizygotic).
Two boy twins can be either identical or fraternal (monozygotic or dizygotic).
A set of boy/girl twins can only be fraternal (dizygotic). Boy/girl twins can not be identical (monozygotic).

Except....

However, there have been a few reported cases of a genetic mutation in monozygotic male twins. For some reason, after the zygote splits, one twin loses a Y chromosome and develops as a female. The female twin would be afflicted with Turner Syndrome, characterized by short stature and lack of ovarian development. It's extremely rare; less than ten cases have been confirmed. Given the odds, it's safe to assume that 99.9% of all boy/girl twins are fraternal.

Okay, One More Exception...

Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.