How Do Identical and Fraternal Twins Form?
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
FAQs on Identical vs Fraternal Twins: What Sets Them Apart?
1. What is the main biological difference between identical and fraternal twins?
The primary difference lies in their genetic origin. Identical (monozygotic) twins develop from a single fertilised egg that splits into two embryos. In contrast, fraternal (dizygotic) twins develop from two separate eggs, each fertilised by a different sperm. This fundamental difference in origin determines their genetic similarity and physical appearance.
2. How much DNA do identical and fraternal twins share?
Identical twins share virtually 100% of their DNA, which is why they are always the same sex and have very similar physical traits. Fraternal twins, on the other hand, share approximately 50% of their DNA, the same amount as any other pair of siblings born at different times. This is why fraternal twins can be of different sexes and may not look alike.
3. Can doctors determine if twins are identical or fraternal from an ultrasound?
An ultrasound can provide strong clues but not always a definitive answer. Doctors look at the number of placentas (chorionicity) and amniotic sacs (amnionicity).
- A single shared placenta (monochorionic) strongly suggests the twins are identical.
- Two separate placentas (dichorionic) are more common in fraternal twins, but about one-third of identical twins can also have separate placentas.
4. What factors influence the chances of having identical or fraternal twins?
The chances for each type of twin are influenced by different factors:
- Identical Twins: The splitting of a single fertilised egg is considered a spontaneous and random event. The rate is consistent globally, at about 3 to 4 per 1,000 births, and is not affected by heredity, age, or race.
- Fraternal Twins: The chances are influenced by factors that increase hyperovulation (releasing more than one egg in a cycle). These include maternal age (over 35), family history of fraternal twins, certain ethnic backgrounds, and fertility treatments like IVF.
5. From a biological perspective, are there different health risks for identical vs. fraternal twin pregnancies?
Yes, identical twin pregnancies, especially those sharing a single placenta (monochorionic), carry unique and higher risks. A major concern is Twin-to-Twin Transfusion Syndrome (TTTS), where there is an imbalanced blood flow between the twins through placental connections. This condition does not occur in fraternal (dizygotic) twins as they always have separate placentas. Generally, fraternal twin pregnancies have a lower risk of complications compared to monochorionic identical twin pregnancies.
6. How are conjoined twins formed, and are they a type of identical or fraternal twin?
Conjoined twins are a very rare form of identical (monozygotic) twins. They form when a single fertilised egg begins to split into two embryos but fails to separate completely. The two developing fetuses remain physically connected, often at the chest, abdomen, or pelvis, and may even share one or more internal organs. This phenomenon cannot occur with fraternal twins because they originate from two entirely separate eggs.
7. Is it possible for twins to be 'semi-identical'?
Yes, though extremely rare, there is a third type of twinning known as sesquizygotic or 'semi-identical' twins. This occurs when a single egg is fertilised by two different sperm before it splits. The resulting twins share 100% of their mother's DNA but only share about 50% of their father's DNA. They are genetically positioned between identical and fraternal twins, representing a fascinating biological exception.
