Possible Sex Combinations for Identical and Fraternal Twins

Are your boy and girl twins identical or fraternal?

8 month old boy and girl twins
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Every parent of twins faces the question: "Are your twins identical or fraternal?" When your twins are a boy and girl pair, the answer is easy, but people don't always realize it. 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.

Can Boy/Girl Twins Be Identical?

The short answer is no.

They are always fraternal.

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. 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.

These are the possible sex combinations:

  • 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 cannot be identical (monozygotic).

Identical Twins

Identical 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 both produce one of the twins.

Identical twins have the same genetic origin. No direct cause of monozygotic twinning has been determined; it is not hereditary.

Monozygotic twins represent about one-third of all twins. They may look remarkably similar, and it may be difficult to tell them apart.

Fraternal Twins

Fraternal twins are two genetically unique individuals as they come from 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.

The genetic similarity between fraternal twins is the same as any two siblings (about 50 percent if they have the same mother and father). They may look alike, or not, just like any sisters and brothers. 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.

Exception to the Twin Sex Rule

Every rule has exceptions, of course. In this case, they are extremely rare exceptions, and it's not likely that the average person would ever encounter twins in this situation. Note that 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 have Turner syndrome, characterized by short stature and lack of ovarian development. It's extremely rare; less than 10 cases have been confirmed. Given the odds, it's safe to assume that 99.9 percent of all boy/girl twins are fraternal.

Twins That Are Genetically Dissimilar

While identical twins come from the same fertilized egg and share the same DNA combination, there are more possibilities for fraternal twins. Besides being the offspring of two sperm and egg combinations, they may have different genetic donors.

Here are the different combinations:

  • Fraternal twins could be produced when eggs from one woman are fertilized by sperm from two different men. This can happen with unassisted conception and with in-vitro fertilization. In this case, the twins would be half-siblings.
  • A woman may carry twins conceived in-vitro with donor eggs, which could come from two different women. If they are fertilized by sperm from the same man, the twins would be half-siblings.
  • Donor eggs from two different women could be fertilized by sperm from two different men. 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

You are likely to have to answer the same questions about your twins repeatedly. Armed with a few facts about twins, you'll be able just to grin and bear it or enlighten the curious about the biology of twins.

Sources:

Mayo Clinic. Turner Syndrome. http://www.mayoclinic.org/diseases-conditions/turner-syndrome/basics/definition/CON-20032572.

Womenshealth.gov. Twins, Triplets, and Other Multiples. https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/twins-triplets-and-other-multiples.

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