Twins come in two main types: identical (monozygotic) and fraternal (dizygotic). While the concept of twins is widely understood, the intricate differences between these two types often remain a subject of curiosity. Let us talk about Identical and Fraternal twins from their genetic origins to physical characteristics, formation processes and even their implications for medical research.
What are Identical Twins?
Identical twins come from one fertilized egg called a zygote. The zygote, which usually develops into one child, instead grows and splits early in development to form two embryos. Because the twins come from one egg and one sperm, they have exactly the same DNA and are, therefore, identical twins.
There are different types of identical twins, depending on what they share in the womb.
- Almost one third of identical twins have their own placenta, inner membrane, and outer membrane. The medical term for these twins is ‘dichorionic diamniotic’ or DCDA twins.
- Almost two-thirds of identical twins share the same placenta and chorion, but have their own amnion. These are ‘monochorionic diamniotic’ or MCDA twins.
- The rest — only about 4% of identical twins — share everything, and are called ‘monochorionic monoamniotic’ (MCMA) twins.
Although identical twins are the same sex and are genetically identical, they can develop quite different personalities.
What are Fraternal Twins?
On the other hand, Fraternal twins, also known dizygotic (DZ) or nonidentical twins, are twins that result from two separate eggs released from the ovary and fertilized by two different sperm cells within the same menstrual cycle. The two fertilized eggs implant in the uterus and develop during one pregnancy. Fraternal twins will not have the exact same DNA, although, like other siblings, they can be expected to share some of the DNA they inherit from both parents.
All dizygotic or fraternal twins are dichorionic in nature whereas some monozygotic (identical) twins can be dichorionic at times. When it comes to the placenta, the dichorionic twins have two individual placentas, but sometimes the placentas are seen to fuse.
Key Differences: Identical And Fraternal Twins
- Genetic Origin
- Identical Twins: Form when a single fertilized egg (zygote) splits into two embryos early in development. These embryos carry the exact same genetic material.
- Fraternal Twins: Develop when two separate eggs are fertilized by two different sperm cells, resulting in two embryos with varying genetic makeup.
- Genetic Similarity
- Identical Twins: Have nearly identical DNA sequences because they originate from the same zygote. They are essentially clones of each other.
- Fraternal Twins: Share about 50% of their DNA on average, just like regular siblings.
- Chromosomal Composition
- Identical Twins: Have the same sex and genetic sex characteristics due to the identical splitting of the zygote.
- Fraternal Twins: Can be of the same sex or different sexes, just like any two siblings born at different times.
- Embryo Splitting Timing
- Identical Twins: The zygote splits into two embryos within the first few days after fertilization, resulting in two separate embryos.
- Fraternal Twins: Result from the simultaneous release and fertilization of two separate eggs during ovulation.
- Number of Placentas
- Identical Twins: May share a single placenta or have separate placentas, depending on when the zygote splits.
- Fraternal Twins: Always have separate placentas, as they develop from two different fertilized eggs.
- Amnionicity
- Identical Twins: Can share the same amniotic sac (monochorionic-monoamniotic) or have separate sacs (monochorionic-diamniotic) depending on when the split occurs.
- Fraternal Twins: Always have separate amniotic sacs (dichorionic-diamniotic).
- Physical Resemblance
- Identical Twins: Tend to have strikingly similar physical appearances, as they share the same genetic makeup.
- Fraternal Twins: Resemble each other to varying degrees, just like any two siblings.
- Genetic Diversity
- Identical Twins: Lack genetic diversity since they come from a single zygote.
- Fraternal Twins: Exhibit the same genetic diversity as regular siblings due to their distinct genetic origins.
- Heritability Studies
- Identical Twins: Valuable for studying the relative contributions of genetics and environment to traits because they share nearly identical genes.
- Fraternal Twins: Also used in heritability studies, but with less genetic control compared to identical twins.
- Incidence Rate
- Identical Twins: Less common, accounting for about 1 in 250 pregnancies.
- Fraternal Twins: More common, occurring in about 1 in 30 pregnancies.
Fraternal vs Identical Twins: Key Takeaways
| Elements of Comparison | Fraternal Twins | Identical Twins |
| Development | Two eggs are fertilized by two different sperms. | A single zygote splits into two parts and develops into different embryos. |
| Genetic Traits | The twins share 50% of their genetic traits. | The genetic origin is the same and has the same DNA. |
| Sex | The sexes may be the same or different. | They are of the same sex. |
| Blood Type | The blood types may or may not be the same. | They share the same blood type. |
| Pregnancy Type | They are dichorionic and diamniotic. | They may or may not be dichorionic and diamniotic. |
| Personalities | Their personalities may be similar or different. | Their personalities are complementary and dependent. |
| Finger Print | Different | Different |
| Risk For TTT | Low risk of twin to twin transfusion. | High. |
| Causes | Not Know. | Certain fertility drugs, Hereditary predisposition and IVF. |
| Number | In the world, two-third of twins are fraternal. | In the world, a third of the twins are identical. |
Twin-twin transfusion syndrome
Identical twins who share the same placenta and chorion can sometimes share a condition called twin–twin transfusion syndrome (TTTS). In this condition, blood flows from one twin to the other, resulting in one baby getting too much blood and the other baby not getting enough. This affects the health of both babies, sometimes severely.
Most identical twins don’t get TTTS. But if they do, it is more likely to happen to MCDA twins than to MCMA twins.
If your twins have TTTS, there are many different ways to treat it – ask your doctor for advice.
More Takeaways
- Fraternal twins (also called dizygotic twins) result from the fertilization of two separate eggs with two different sperm during the same pregnancy. Fraternal twins may be the same gender, they may have many of the same characteristics, but also may be very different from each other and, in fact, share half of their genes just like their sisters and brothers.
- Identical twins (or monozygotic twins) result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two. As a result, identical twins share the same genomes and are always the same sex.
- Identical twins may share an amniotic sac and a placenta, or they may just share one of the two, or neither! Fraternal twins are more likely to have two amniotic sacs and two placentas.
- Fraternal twins may grow at different rates, but identical twins will follow the same pattern of growth.
- If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA. A boy has XY chromosomes and a girl has XX chromosomes. Girl-boy twins occur when one X egg is fertilized with an X sperm, and a Y sperm fertilizes the other X egg.
- When two foet uses are growing separately in separate sacs, it is surrounded by two separate chorions, and they are termed as dichorionic.The prefix “di” is used to indicate two.