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Difference Between Identical and Fraternal Twins

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How are Identical and Fraternal Twins Formed?

A single pregnancy resulting in two offspring is termed as twins. Twins are of two types; when both the offspring are identical, it’s termed as monozygotic (identical), which means they are developed from a single zygote splitting to form two embryos; and when they are not identical, it’s termed as dizygotic (fraternal), which means developed from separate eggs.

In most cases, a singleton takes birth which means an offspring developed from a single egg, alone in a womb. When two biologically unrelated individuals look-alike, they are termed as doppelgangers.

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Identical Twins

Identical Twins appear from the fertilisation of a single egg that splits in two. They are also known as monozygotic twins. Identical twins are always of the same sex and share each common genes. About one in three sets of twins are identical. This happens because the fertilised egg splits in two while it is still a tiny gathering of cells. The self-sustaining halves then form into two babies, with the equivalent genetic information. Twins conceived from one sperm and one egg are termed as (one-cell) identical or ‘monozygotic’ twins. The biological mechanisms that indicate the single fertilised egg to split in two remain a mystery. About one-quarter of identical twins are mirror images of one another, which suggests that the right side of one child resembles the left side of their twin. 

Fraternal Twins

Fraternal twins emerge from the fertilisation of two separate eggs during the same pregnancy. Fraternal twins have single DNA and can change. Fraternal twins are basically two completely individual siblings with no distinct physiological similarity, however, due to their varied developmental rates they pose challenges for health care providers. Fraternal twins are also known as dizygotic twins, indicating two fertilised eggs. They’re the outcome of the mother releasing two eggs at the same time with each egg being fertilised by a different sperm.

As they come from different sperm and eggs, they only bestow about 50 per cent of their chromosomes similar to other siblings. This implies they can be identical or different sexes and aren’t indistinguishable.

Fraternal Twins or Identical Twins

Similar sex twins with separate placentas can be identical or fraternal. It is essential to know whether your twins are identical and fraternal for health reasons. To discover whether twins are identical or fraternal, one can ask for a genetic test after the babies are born. This test is known as the zygosity test. This test doesn’t hurt and includes sample collection of cheek cells of babies by coating the inside cheeks with a soft applicator. 

Identical twins are more likely to get the same illness, unlike fraternal twins. If one of a pair of identical twins is diagnosed with a health condition, the other twin should be monitored often for early symptoms.

Some of the reasons why twins and parents of twins require to recognise whether they’re identical or fraternal includes:

  • possibility of having more twins in future pregnancies 

  • Being sure that they have accurate knowledge about their genetic make-up

  • Being able to explain their family, friends and others

  • Being involved in twin research

Some identical twins are also identified as mirror twins. For example, they are handed oppositely, their hair parts on opposite sides or they have birthmarks on opposite sides of their body. There are also instances of mirrored internal organs among identical twins.

Let us glance at the difference between identical twins and fraternal twins presented in a tabular column below.

Difference Between Identical Twins and Fraternal Twins




The same egg splits up into two 

Two  different sperm cells and two different eggs fertilized

Genetic Code

Nearly identical

Like any other sibling; not identical.


Always the same

Usually different


Uniform around the world

Varies in different countries. In the world two-thirds of all twins are fraternal.

Blood Type

Always the same

May or may not be same 


Not known

Certain fertility drugs,  Hereditary predisposition,  IVF


Remarkably similar, although may not be accurately identical due to environmental circumstances

As similar as any other sibling

Risk for TTTS

Higher risk 

Low risk for twin-to-twin transfusion syndrome




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FAQs on Difference Between Identical and Fraternal Twins

Q1. Define the Term - Conjoined Twins.

Answer: Identical twins who are sometimes physically attached are known as conjoined twins. There are two chances for the creation of conjoined twins- either the single fertilised egg is not divided entirely during the production of identical twins, or two fertilised eggs bounded together during the growth. Conjoined twins emerge when an early embryo only partially departs to form two individuals. Although two fetuses will evolve from this embryo, they will remain bodily connected — mostly at the abdomen, chest or pelvis. They may also share one or more physical organs. Conjoined twins can be diagnosed beforehand in the pregnancy applying standard ultrasound.

Q2. What are the Statistical Possibilities of Identical and Fraternal Twins?

Answer: The probability of identical twins is equal all over the globe: around 3 in every 1000 births. The probability of fraternal twins depends on various factors. They are mainly prevalent in mothers over 35, and in fertilisation that includes IVF treatment. Fraternal twins transpire due to hyper-ovulation when the mother releases more than one egg per cycle. Women who previously had fraternal twins are four times more prone to have fraternal twins in their subsequent pregnancy. Certain drugs like clomiphene develop the possibility of fraternal twins to around 10% because they manage to bring about hyper-ovulation.

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