Main Difference – Identical vs Fraternal Twins
Identical and fraternal twins are two types of young produced by the same pregnancy in animals. Identical twins are also called monozygotic twins while fraternal twins are also called dizygotic twins. The main difference between identical and fraternal twins is that identical twins have the same genes whereas fraternal twins have 50% of the same genes. The splitting of a fertilized egg develops two or more fetuses, forming identical twins. Hence, each fetus of the identical twins shares the same placenta. Two separately fertilized eggs develop fraternal twins. Each fetus of the fraternal twins has a separate placenta. Though identical twins are alike and have the same sex, fraternal twins are not identical, and both males and females can occur at the same time.
Key Areas Covered
1. What are Identical Twins
– Definition, Formation, Characteristics
2. What are Fraternal Twins
– Definition, Formation, characteristics
3. What are the Similarities Between Identical and Fraternal Twins
– Outline of Common Features
4. What is the Difference Between Identical and Fraternal Twins
– Comparison of Key Differences
Key Terms: Conjoined Twins, Di-Di Twins, Dizygotic Twins, Egg, Fetus, Fraternal Twins, Identical Twins, Mono-Di Twins, Mono-Mono Twins, Monozygotic Twins, Placenta
What are Identical Twins
Identical twins refer to the twins developed from a single-fertilized ovum. They are also called monozygotic twins as they are formed by the splitting of the fertilized egg in order to develop several fetuses. Therefore, identical twins have the same genome. Hence, their appearance and the sex are the same. Identical twins are also called maternal twins as well. The fetus is surrounded by two membranes in the womb known as chorion (outer membrane) and amnion (inner membrane). Identical twins may share various chorion types and amnion types. They are shown in figure 1.
Figure 1: Different Chorionicity and Amniosity of Identical Twins
One-third of identical twins consist of a separate placenta and an amniotic sac, forming dichorionic-diamniotic twins (Di-Di twins). Di-Di twins are formed by the splitting of the embryo within 2-3 days after fertilization. However, two-third of the identical twins share the same placenta and two separate amniotic sacs, forming monochorionic-diamniotic twins (Mono-Di twins). Mono-Di twins are formed by the splitting of the embryo within 3-8 days after the fertilization. Around 1% of identical twins share their amniotic sac as well, forming monochorionic-Monoamniotic twins (Mono-Mono twins). Mono-Mono twins are formed by the splitting of the embryo within 8-13 days after the fertilization. Conjoined twins are developed by the splitting of the embryo after 13 days of fertilization, creating twins with joined body parts. Identical twins are shown in figure 2.
Figure 2: Identical Twins
The birth of identical twins is quite rare. The rate of birth of identical twins is about 1 in 333 pregnancies.
What are Fraternal Twins
Fraternal twins refer to the twins developed from separate fertilized eggs. Therefore, they are also called dizygotic twins. Since fraternal twins are formed from two separate fertilization events, they are not genetically-identical. They share 50% of genetic similarity as siblings do. Thus, identical twins are also called non-identical twins. The formation of identical and fraternal twins is shown in figure 3.
Figure 3: Formation of Identical and Fraternal Twins
Since fraternal twins are formed in two separate fertilization events, they develop separate chorions, amniotic sacs, and placentas. Thus, they are dichorionic-diamniotic (Di-Di twins). The sex and the other characteristics of fraternal twins also vary as in other siblings. Therefore, fraternal twins can be either girl/boy, boy/boy or girl/girl. Dizygotic twins are caused by hyper-fertilization. They are hereditary. Most twins in the world are fraternal twins. The occurrence of dizygotic twins around the world depends on the population. The highest rate of occurring dizygotic twins is found among African-Americans whereas the lowest rate is found among Asians.
Similarities Between Identical and Fraternal Twins
- Both identical and fraternal twins are produced in the same pregnancy.
- Both identical and fraternal twins share some kind of similarity.
- Fingerprints of both identical and fraternal twins are different from each other.
Difference Between Identical and Fraternal Twins
Definition
Identical Twins: Identical twins refer to the twins developed from a single-fertilized ovum and are alike in all genetic characteristics including the appearance and sex.
Fraternal Twins: Fraternal twins refer to the twins developed from separate fertilized eggs and are not identical in appearance and sex.
Alternative Names
Identical Twins: Identical twins are also known as monozygotic twins.
Fraternal Twins: Fraternal twins are also known as dizygotic twins.
Formation
Identical Twins: Identical twins are formed by the splitting of a fertilized egg, forming several fetuses.
Fraternal Twins: Fraternal twins are formed by the separate fertilization of more than one egg by sperms.
Placenta
Identical Twins: Identical twins share the same placenta.
Fraternal Twins: Fraternal twins have separate placentas.
Chorionicity and Amnionicity
Identical Twins: Identical twins can be either Di-Di, Mono-Di, Mono-Mono or conjoined twins.
Fraternal Twins: Fraternal twins are Di-Di twins.
Genetic Identity
Identical Twins: Identical twins have identical genomes.
Fraternal Twins: Fraternal twins have 50% identical genomes.
Appearance
Identical Twins: Identical twins are alike.
Fraternal Twins: Fraternal twins are like siblings.
Sexes
Identical Twins: Identical twins have the same sex as they are separated from the same fertilized egg.
Fraternal Twins: Fraternal twins may have both sexes as the fertilization occurs separately.
Blood Type
Identical Twins: Identical twins have the same blood type.
Fraternal Twins: Fraternal twins may have different blood types.
Hereditary
Identical Twins: Identical twins are not hereditary.
Fraternal Twins: Fraternal twins are hereditary.
Occurrence
Identical Twins: One-third of twins in the world are identical twins.
Fraternal Twins: Two-third of the twins in the world are fraternal twins.
Risk of Twin-to-Twin Transfusion Syndrome (TTTS)
Identical Twins: Identical twins have a high risk of TTTS.
Fraternal Twins: Fraternal twins have a low risk of TTTS.
Conclusion
Identical and fraternal twins are two types of twins with different types of formation patterns. Identical twins are formed by the splitting of a fertilized egg. Hence, these twins have identical genomes and sexes. Fraternal twins are formed in separate fertilization events. Hence, these twins share the identity similar to siblings. The main difference between identical and fraternal twins is the degree of similarity in each type of twins.
Reference:
1. Delyth Raffell, “Types of Twins: Identical, Fraternal & Unusual Twinning” Twins UK, Available here.
Image Courtesy:
1. “Placentation” By Kevin Dufendach – Own work (CC BY 3.0) via Commons Wikimedia
2. “1099339” (Public Domain) via Pixabay
3. “Identical-fraternal-sperm-egg” By ChristinaT3 at English Wikipedia (CC BY 3.0) via Commons Wikimedia
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