Twins of different genders are universally accepted as a sound basis for a clinical determination that boy and girl twins are not identical. However, in extremely rare cases, boy and girl twins can stem from the same fertilized egg. 

Twins, who stem from the same fertilized egg, are called monozygotic twins. Washington State Twin Registry stresses that it’s an extremely rare event as only a handful of cases have been reported in the medical literature.

Inaccurate copying of the sex chromosomes in male twins, that began as monozygotic twins, have resulted in a few cases of male/female twin pairs.

A third twin type, called sesquizygotic twins, has been discovered.

Three examples that could result in male/female twins:

• There could be a glitch in copying sex chromosomes, resulting in a male zygote (fertilized egg) that starts out with an extra X chromosome (XXY). This is opposed to the usual XX for a girl or XY for a boy.  Through a complicated series of events, this zygote could result in the birth of monozygotic twins who are male (XY) and female (XX). These twins would be genetically identical except for their sex chromosomes.

• After a male (XY) zygote is formed, it starts to develop and splits into two embryos (identical male twins). Early in this process, inaccurate copying of the sex chromosomes could result in the loss of the Y chromosome in some cells. The embryo that is formed from the cells missing the Y chromosome would develop into a female (XO). The other embryo would still develop into a male (XY). The twin with only one X chromosome would be female, but her cells would have only one copy of the X chromosome. This condition is called Turner Syndrome. Turner Syndrome is a chromosomal condition that affects development in females. These twins would be genetically identical except for their sex chromosomes.

• There is an extremely rare twin type that’s referred to as semi-identical twins or sesquizygotic twins. This occurs when two sperm cells fertilize one egg which then later splits. Semi-identical twins share anywhere from 50 to 100 percent of their DNA. They share 100 percent of their mother’s DNA but only a proportion of their father’s DNA. In 2019 a report was published detailing a newly discovered set of boy-girl semi-identical twins in Australia. They were discovered because they during pregnancy shared a placenta but had different genders (only identical twins can share a placenta, and only fraternal twins can have different genders).

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How are identical twins formed?

A woman ovulates and releases one egg. The egg is fertilized by a sperm cell. The egg splits into two within a few days after fertilization.

The two eggs are developing rapidly into two embryos that have the same chromosomes and genes. This type of twin pregnancy is called monozygotic.

Different kinds of identical twin pregnancies

Identical twin pregnancies are either dichorionic-diamniotic (di-di twins), monochorionic-diamniotic (mo-di) or monochorionic-monoamniotic (mo-mo).

It relates to whether or not the twins share a placenta and sacs (also called membranes). Identical twin pregnancies with separate sacs and placentas are called dichorionic-diamniotic.

Identical twin pregnancies with a shared placenta and a shared outer sac – but separate inner sacs – are called monochorionic-diamniotic.

Monochorionic-monoamniotic is identical twins who share the placenta and both sacs. Mo-mo twin pregnancies are extremely rare.

Are identical twins exactly alike?

Although identical twins share the same genetic characteristics, they are not necessarily exactly alike. Environmental differences in the womb – and when the twins are growing up – can affect the way they look.

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How are semi-identical twins formed?

A woman ovulates and releases one egg. Two sperm cells from two different men enter the same egg and fertilize it. The three sets of chromosomes (as opposed to the usual two) are split into two separate cell sets. 

Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather than 100 percent of the same DNA from the fathers.

They share the same genetic material from their mother. This type of twin pregnancy is called sesquizygotic.

Do semi-identical twins look alike?

They won’t look as alike as identical twins, but they may have more similarities than some fraternal twin pairs.

How are fraternal twins formed?

A woman ovulates and releases two eggs. The eggs are then fertilized by two different sperm cells (from the same man). This will make the twins share about 50 percent of their DNA.

In very rare cases the eggs can be fertilized by sperm from different men as well. That’s called heteropaternal superfecundation. Genetically that would make the twins half-siblings, and they would share around 25 percent of their DNA.

In both scenarios, the twins would be fraternal twins. That type of twin pregnancy is called dizygotic.

What are fraternal twin pregnancies called?

Fraternal twin pregnancies are always dichorionic-diamniotic (di-di). Fraternal twins have their own separate placentas and sacs.

Do fraternal twins look like each other?

From a genetic perspective, fraternal twins are like ordinary siblings with the exception that they have shared the womb and are born at the same time. “Ordinary” siblings share around 50 percent of their DNA, similar to fraternal twins.

We hope we’ve answered your questions about whether or not identical twins can be male and female. Please leave a reply to this article if you have more questions about monozygotic twins with different genders.

Identical twins

This type of twin is formed from one fertilized egg being split in two. The eggs develop into two individual embryos that share almost the same genetic material. When developing in the womb, identical twins can share or have their own individual placentas. The same goes with membranes. This sometimes makes it tricky to determine whether or not twin babies are identical or fraternal. Read more about how identical twins are formed to learn how to distinguish identical and fraternal twins from each other, or use our free Twin Test.

Fraternal twins

This type of twin is formed from two eggs being fertilized by two different sperm cells. Fraternal twins are genetically speaking like siblings. They may feel a bit closer, though, as they share a womb and are born at about the same time. As with identical twins there are some patterns of fraternal twinning that are exceedingly rare – read about the different types of twins.

How are twins formed? We hope we’ve answered your questions. Otherwise post a reply below this article and we’ll do our best to assist you. 

Identical twins / one-egg twins

Identical twins are also referred to as monozygotic twins or one-egg twins. They are the result of a single egg, fertilized by a single sperm cell.

Very early in development, the egg divides, resulting in two individuals with the same DNA. Even though they may look identical, they have their own unique traits. This is due to various growth conditions in the womb.

About 25 percent of all identical twins may be mirror twins. They are a subset of identical twins and are identical twins with opposite features. These mirrors are reflections of each other which means that the left side of one twin, matches the right side of the other twin.

They may possess matching or almost matching fingerprints and share the same DNA. Organs and birthmarks can be placed on opposite sides of their bodies and one twin may be right-handed, whereas the other is left-handed. They result from a late split of the fertilized egg.

There are some patterns of identical twinning that are exceedingly rare: in extremely rare cases twins that stem from one egg have been born with opposite sexes. The probability of this is extremely small – multiples having different genders is universally accepted as a sound basis for a clinical determination, that multiples do not stem from one egg.

Learn more about identical twins with different genders.

Conjoined twins

Conjoined twins are also referred to as siamese twins. Conjoined twins are identical twins whose bodies are joined together at birth. This occurs when the fertilized egg fails to separate completely because they split very late in development. Most conjoined twins are also mirror twins.

There is a type of conjoined twin that is sometimes referred to as parasitic twin. The condition is called Twin Reversed Arterial Perfusion (TRAP).  Parasitic twins develop asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin.

A variation of parasitic twinning is a fetus in fetu, where an abnormally formed mass of cells grows inside the body of its identical twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin.

Fraternal twins / two-egg twins

Fraternal twins are also referred to as dizygotic twins, non-identical twins or two-egged twins. They develop from two different eggs; each is fertilized by separate sperm cells.

They share approximately 50 percent of their genes, similar to any other siblings born at different times, who have the same biological mother and father. This means, that the chances of them looking alike are the same as the likelihood of any other siblings.

They may look very much alike – or they may look very different. Fraternal twins can be either gender or a combination of boy and girl.

There are some patterns of twinning that are exceedingly rare. In rare cases, a woman’s eggs are fertilized at different times with two or more acts of sexual intercourse. This is known as superfetation and occurs when a woman continues ovulating after becoming pregnant.

There have also been instances of fraternal twins with different fathers. This occurs when a woman releases multiple eggs and has sexual relations with more than one partner.

If an egg is fertilized by sperm from one man, and then another egg is fertilized by sperm from another man, the result is fraternal twins with different fathers.

This phenomenon is termed heteropaternal superfecundation and these fraternal twins are genetically half-siblings and share approximately 25 percent of their DNA.

Semi identical twins

Semi identical twins are also referred to as polar body twins or half identical twins.

Semi identical twins are types of twins, who share half their genes in common from the mother and the other half different from two separate sperm cells. This occurs when two sperm fertilize one egg, which then later splits.

They share some features of identical twins and some features of fraternal twins. These twins will look very much alike but aren’t a 100 percent DNA match.

An embryo created this way doesn’t usually survive, but a few cases are known.

Mixed chromosome twins

Mixed chromosome twins are also referred to as chimeras.

In human biology, a chimera is an organism with at least two genetically distinct types of cells – or, in other words, someone meant to be a twin. But while in the mother’s womb, two fertilized eggs fuse, becoming one fetus that carries two distinct genetic codes – two separate strands of DNA.

Some individuals have been identified to have more than one distinct red blood cell type and individuals have been born, who are both female and male.

One way to tell the difference is using a twin zygosity test, due to the fact that identical twins share almost identical genetic information, while fraternal twins share around half. The twin test is done by swabbing to gently remove cheek (buccal) cells and can be done from home. The procedure is noninvasive and works on twins of any age, newborns to adults. A twin test can be ordered online and is being offered by numerous private companies. However, if you are not up for paying for a DNA-test, you may find your answer by reading the below.

Answer these twin test questions

• Do you have a boy and a girl? Then they are fraternal (but read this).

• Do they have the same sex, AND do you know for sure that they shared a placenta? Then they are identical.

• Do they have the same sex and individual placentas, or is there doubt whether the placentas have fused together? Then they can be either fraternal or identical. In this situation, choosing to be patient may do the trick. According to the Danish Twin Registry you should wait until the newborns are a bit older. If they look so much alike that friends and family can’t tell them apart, they’re likely to be identical. This method works in more than 95 percent of cases, they write on their website.