There are two kinds of twins. They may be the same sex or opposite sex. They may be a bit more alike than other siblings just because they grow up at the same time. Identical twins have the same DNA mostly. Every parent of twins faces the question "Are your twins identical or fraternal? Many think that being identical twins refers to how twins look and not how they form. Take a look at the biology that determines the sex combinations that are possible with identical and fraternal twins. 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 sex because they form from a single zygote fertilized egg that contains either male XY or female XX sex chromosomes. Fraternal twins can be either two girls, two boys, or one of each. These are the possible sex combinations:. But, for a woman in New Jersey suing for child support, things are a little more complicated. It turns out the man she thought was the father of her twins was only the father of one of the pair. Here are the different types of multiple births:. In this case, two different sperm impregnate two different eggs. This is what happened to the woman in New Jersey. All pregnancies start when a sperm fertilises an egg. This forms twins. These twins are called fraternal twins, dizygotic twins meaning two zygotes or non-identical twins. The technical name for this is dichorionic.