Genders of Twins Explained- Can Identical Twins Have Different Genders?

Genders of Twins Explained

Twin babies are one of the most beautiful phenomena of nature. Only 3% percent of the pregnancies result in twin pregnancies. Moreover, it is even rarer to have identical twins. Mostly we see the twins being of the same gender. This might raise the question, can identical twins be different genders? For a short answer, according to Twins Research Australia, 99.9% of boy/girl twins are non-identical. However, in some rare cases, identical twins can also be of different genders due to genetic mutation.

What Are The Different Types of Twins?

There are two main types when it comes to twin babies. They are maternal/identical twins and fraternal twins. In most cases, the twins are fraternal, or non-identical. These are the twins that have developed from two separate eggs that were fertilized by two different sperms. Non-identical twins are called fraternal twins also.

What Are The Different Types of Twins

Occasionally, we find that the twin pregnancy consists of identical twins, which means the twins were developed from one fertilized egg that split into two, giving birth to two embryos later on. Identical twins are extremely rare and they share all the same physical features.

Even though identical and fraternal are the primary two types of human twins, there are some less common subtypes of twins. One of them is semi-identical twins, meaning twins that develop through one egg being fertilized by two different sperm. Then they form two separate embryos. They are also known as sesquizygotic twins. These twins share 100% of their mother’s DNA but only 50% of their father’s DNA, as per the case study discussed in UNSW Sydney. This type of twin is the rarest type of twin pregnancy. There have been only two records of semi-identical twin pregnancies as of now.

These types of twins are categorized depending on which stage of development the zygote divides or splits. 

Another classification of twins is done on the basis of the presence and number of chorionic and amniotic sacs. An article titled “Twins, Chorionicity, and Zygosity” published in ResearchGate explains this classification. According to the article, firstly, Mo/Mo (monochorionic monoamniotic) twins have the same placenta and chorionic and amniotic sacs. They are formed if the split happens between eight to twelve days after fertilization.

Secondly, mo/di (monochorionic diamniotic) twins share the same placenta and chorion but have separate amniotic sacs.  They are developed if the divide occurs between four to eight days after fertilization.

Again, di/di  (dichorionic diamniotic) twins have their own placenta, chorionic sacs, and amniotic sacs. These twins are formed when the split occurs at the earliest stage after fertilization, usually within 1 to 4 days. Most fraternal twins are di/di twins.

What is The Difference Between Twins and Identical Twins?

The word “twin” is a relatively broad term. It can refer to any type of twins, such as identical and non-identical twins, mono and dizygotic twins, mirror twins, conjoined twins,semi-identical twins, etc.

On the other hand, when we talk about identical twins, we specifically talk about twins that were formed by the splitting of one fertilized egg. Identical twins have essentially the same DNA. This means there is virtually very little difference between twins when it comes to their physical features. They even have the same blood group.

Are Identical Twins the Same Gender?

In some twin pregnancies, the twins are identical. Identical twins share the same physical features, hair color, eye color, blood group, etc. Along with all that, they also have the same biological sex. So, the answer to the question “are identical twins always the same sex?” is yes, for the most part.

Why Are Identical Twins Always The Same Gender?

Identical twins are monozygotic twins, which means they came from one fertilized zygote that split into two. Because the egg had previously been fertilized by one singular sperm and already had formed a zygote, the gender of the zygote was determined before the secondary split. As a result, both the twins share the same genome and therefore, share the same sex.

Can fraternal twins be different genders?

Can fraternal twins be different genders

When comparing identical vs fraternal twins, the primary distinction is in the manner in which their eggs were fertilized. Contrary to identical twins, fraternal twins are born when two separate eggs are fertilized by two separate sperm.  Therefore, even the most identical fraternal twins share only 50% of their DNA, the same amount that is shared by regular siblings, as MCTFR concludes.

If both of the sperm that fertilized the eggs contained either X or Y chromosomes, then the fraternal twins will be either girls or boys. In the case of fraternal twins, this indicates that having the same gender is possible. If, on the other hand, one of the sperm has a Y chromosome and the other includes an X chromosome, then the pregnancy will naturally result in the delivery of twins who are a boy and a girl, respectively. 

In conclusion, it is very much possible to have twins of different gender if the pregnancy is a fraternal twin pregnancy.

Can Identical Twins Be a Boy and A Girl?

Identical twin pregnancies are rare as they are. For the reasons explained above it is almost impossible to have identical male and female twins. In almost every case of identical twins, sex remains the same. 

In exceedingly rare circumstances, a genetic mutation occurs. It causes the zygote to lose a copy of the Y chromosome during the early developmental period when it is creating two embryos. The babies that are born as a consequence of this mutation are male (XY) and female(XO). However, a normal girl has XX chromosomes. Because of the lost chromosome, the resulting baby is only outwardly a girl. This condition is called the Turner Syndrome.

In the instances of boy girl identical twins turner syndrome occurs. This condition only affects female twins of identical different gender twins sets. It contributes to a variety of health problems and developmental issues in a person, including short height, heart defects, and an improper formation of the ovaries. In order for them to have a chance at a happy and healthy life, women who have Turner syndrome need the ongoing assistance of medical professionals.

Can Semi-identical Twins have different Genders?

Even though semi-identical twins are extremely rare, it has happened before. In one such case, the semi-identical twins were of opposite genders, according to the BBC. If the two sperms fertilizing the egg have different sex chromosomes that lead to the birth of two twins with different sexes. However, there is a possibility that the fetus could develop genetic abnormalities as a result of this. Even though sesquizygotic twins are very rare, there are high risks of genetic problems in such cases.

One of the twins also might have ambiguous genitalia. Prenatal care and regular checkups can help in terms of finding out such problems during the early stages of pregnancy.

Can A Brother and Sister Be Identical Twins?

Brother & sisters, or boy/girl twins are called fraternal/dizygotic twins. One of the ways of detecting if twins are fraternal vs identical before their birth is to find out their genders. Because in the matter of identical twins, opposite gender is not possible normally. On the other hand, for identifying fraternal twins, different gender works as a marker. So, can a boy and a girl be identical twins? The straight-cut answer to this question is, no, they cannot be identical if the twins have biologically different genders.

Why Is It Important to Know If The Twins Having different Genders are Identical Or not?

It is always a good idea to know if your twins are identical or not for many reasons. Identical twins often have the same type of genetic and health issues such as diabetes, and high blood pressure. It becomes easier to check the twins for early symptoms of the same health conditions if the parents know for sure that the twins are identical. Moreover, identical twins can easily become organ donors for the other twin as opposed to fraternal twins.

It is difficult to determine whether or not a pair of twins are identical, particularly in their younger years, due to the fact that twins almost always seem very similar to one another, regardless of whether or not they are identical. Moreover, if the twins are of opposite genders, they are automatically assumed to be fraternal. However, as discussed above, identical twins can also have different genders due to genetic mutation.

If such identical twins are of different genders but are misdiagnosed, this may lead to their genetic complications not being treated correctly. As a result, the twins will not be able to enjoy their life in a healthy way. This is why it is always better for you to be sure if your twins are identical or not, even if they seem to have different genders. 
On a lighter note, you can get your opposite gendered twins matching outfits if you find out what their genders are beforehand.

Can You Induce Identical Twins of Different Genders?

Fraternal/ non-identical twins sometimes happen due to twin genes and heretics. Moreover, you can even induce such type of twin pregnancy through IVF( In Vitro Fertilization). Indira IVF asserts that the chances of twins in normal pregnancy are around 6% while the chances of twins with IVF reach up to 25%. On the contrary, the reasons underlying identical twin pregnancies are still unknown to scientists to this day.

No genetic or external factors can influence a pregnancy into being an identical twin pregnancy. In the same vein, can you have identical twins boy and girl in artificial ways? The answer is no.

Final Words

In conclusion, the world of identical twin pregnancy is still full of mysteries and scientists are always looking for ways to understand the factors that lie behind identical twins with questions like can identical twins be different genders? Due to the fertilization process, normally it is not possible to have identical twins of opposite genders. However, due to genetic mutation, some twins can be born with genetic anomalies regarding their sex chromosomes.

In such cases, the twins can appear to have opposite genders even though they are identical in every other way. Such cases are extremely rare and the babies born in such cases are often at high risk of physical and genetic abnormalities and need to remain in close contact with medical health professionals throughout their whole lives.

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