Twin Zygosity
The different types of twins are classified by zygosity, or how many zygotes develop into fetuses. Fraternal, or dizygotic twins, form from two separate eggs that are fertilized by two separate sperm. The “di” in dizygotic means two, as in deriving from two zygotes (fertilized eggs). The terms identical and fraternal don’t describe what the twins look like, but actually how they form. Dizygotic twins may be the same sex or different sexes, just like siblings born in separate pregnancies. And they may look very similar, or not very similar, like any siblings.
Identical Twins
Identical (monozygotic) twins come from a single fertilized egg that splits into two. Before it splits, it is either male or female. After it splits, there are either two males or two females. Both parts of the fertilized egg implant in the uterus and each produces one of the twins. Identical twins have the same genetic origin. Monozygotic twins represent about one-third of all twins. They may look remarkably similar, since they share the same DNA, and it may be difficult to tell them apart. But just looking alike isn’t proof that they are identical twins.
Fraternal Twins
Fraternal twins are two genetically unique individuals as they come from two separate eggs that are fertilized by separate sperm. Fraternal twins are the most common form of twins, representing about two-thirds of all twins. Dizygotic twinning can be hereditary and run in families. Also, most twins and multiples that result from fertility-enhancing treatments, such as drugs or procedures like in vitro fertilization, are dizygotic rather than monozygotic.
Exceptions to the Rule
In very, very rare cases, identical twins may be of different sexes. It’s not likely that the average person would ever encounter twins in this situation.
Genetic Mutation
There have been a few reported cases of a genetic mutation in monozygotic male twins. For reasons that are not clear, after the zygote splits, one twin loses a Y chromosome and develops as a female. The female twin would have Turner syndrome, characterized by short stature and lack of ovarian development. This is extremely rare. Less than 10 cases have been confirmed. Given the odds, it’s safe to assume that over 99.9% of all boy/girl twins are fraternal.
Semi-Identical Twins
Even fewer cases of semi-identical, or sesquizygotic, twins have been reported in the medical literature. In these cases, a single egg is fertilized by two sperm. The egg then divides and develops into two babies, who may be of different sexes. These babies share somewhere between 50% of the same DNA (like dizygotic twins do) and 100% (like monozygotic twins do).
Polar Body Twins
Dizygotic twins occur when two eggs are released at ovulation and then fertilized by two sperm. In polar body twinning, a single egg splits into two separate, but unequal, parts before fertilization. Each part is then fertilized by a separate sperm. These twins would also be semi-identical. However, this is just a theory and there have been no documented cases of this type of twinning,
Genetically Dissimilar Twins
While identical twins come from the same fertilized egg and share the same DNA combination, there are more possibilities for fraternal twins to have different genetic makeup. Besides being the offspring of two sperm and egg combinations, they may have different genetic donors.
Eggs From Two Parents
Twins could be conceived in vitro with donor eggs, which could come from two different people. If they are fertilized by sperm from the same parent, the twins would be half-siblings.
Sperm From Two Parents
When two eggs from the same parent are fertilized by sperm from two different parents, fraternal twins may result. This can happen with unassisted conception and with in vitro fertilization. In this case, the twins would be half-siblings.
Both Sperm from Two Parents and Eggs From Two Parents
Donor eggs from two different people could be fertilized by sperm from two different donors. In this case, the twins may have no shared DNA unless the sperm and egg donors were related. They would be similar to step-siblings.
A Word From Verywell
If you are the parent of twins, you are likely to have to answer the same questions about your children repeatedly. Many people do ask if boy-girl twins are identical. Armed with a few facts about twins, you can decide whether to grin and bear it or enlighten the curious about the biology of twins.
