Difference Between Identical and Fraternal Twins

Key Difference – Identical vs Fraternal Twins
 

Twins are offspring born from the same pregnancy. They can be produced from a single zygote (monozygotic) or from two zygotes (dizygotic). A fertilized egg cell (single zygote) can be split into two and form two embryos to produce two fetuses. The offspring produced from a single zygote which splits into two are known as identical twins. Two different egg cells can also be fertilized with two sperms separately at the same time and produce two different zygotes. The offspring produced from two separate zygotes are known as fraternal twins. This is the key difference between identical and fraternal twins.

CONTENTS
1. Overview and Key Difference
2. What are Identical Twins
3. What are Fraternal Twins
4. Side by Side Comparison – Identical vs Fraternal Twins
5. Summary

What are Identical Twins?

Generally, a fertilized egg develops into an embryo and later become a fetus in order to produce an offspring. After fertilization, there is a rare chance of the zygote being split into two halves and developing into two fetuses. This results in two offspring who are extremely similar in traits. They are known as identical twins. Since they are derived from a single zygote, they are known as monozygotic twins as well. Here, the two fetuses share the same placenta and grow into two individuals.

Identical twins have the same genome and same genetic codes because they share the same set of genes. They share same physical characteristics and look alike. However, due to environmental factors, their physical appearance can slightly differ. Identical twins always have the same gender. Likelihood of occurrence of the identical twins is very rare and is not related to age, race or family history.

What are Fraternal Twins?

When two egg cells are fertilized independently by two sperm cells, two zygotes are produced. These two separate zygotes develop into two separate embryos which result in two different offspring. They are known as fraternal twins or non-identical twins. They are also referred to as dizygotic twins since they are derived from two zygotes. Here, these two fetuses do not share the same placenta. Each fetus has its own placenta for nourishment and they grow independently.

Fraternal twins are not genetically identical since they develop from two different eggs and two different sperms. They share only about 50 % of the genes. They can belong to different sexes or the same sex. Likelihood of the fraternal twins is higher than the likelihood of identical twins.

What is the difference between Identical and Fraternal Twins?

Identical vs Fraternal Twins
Identical twins develop from a single fertilized egg which has split.	Fraternal twins develop when two separate eggs are fertilized with two separate sperms.
Placenta
They share one placenta.	Each twin has his own placenta. They don’t share the same placenta.
Sex
Twins are always of the same sex.	They can belong to different sexes.
Genes
They are genetically identical since they share more or less the same genes.	They share about 50 % of the genes. They are not identical.
Appearance
They look extremely alike but may not be exactly identical due to environmental factors	They do not look alike; they can be brother and sister.
Occurence
The occurrence of identical twins is rare.	The occurrence of fraternal twins is comparatively common.
Zygocity
They are monozygotic	They are dizygotic.

Summary – Identical vs  Fraternal Twins

Twins are the offspring produced from the same pregnancy. They can look alike or different. If the fertilized zygote is split into two and produce two fetuses, it will result in two identical twins who are genetically identical. If the two eggs are independently fertilized with two separate sperms, it will result in two zygotes which develop into two fetuses and later to two offsprings. They are known as fraternal twins. Identical twins are derived from the same zygote and share same genes and same placenta. Fraternal twins are derived from two different zygotes and do not share the same placenta. This is the difference between identical and fraternal twins.

Reference:
1.Lazy Grace, Types of Twins: Identical, Fraternal & Unusual Twinning | Twin Pregnancy & Multiple Births | Twin Tips | Resources Centre | Twins UK. N.p., n.d. Web. 04 May 2017. [http://www.twinsuk.co.uk/twinstips/4/140/twin-pregnancy–multiple-births/types-of-twins–identical-fraternal–unusual-twinning/]
2.”Insights From Identical Twins.” Learn Genetics. N.p., n.d. Web. 04 May 2017 [http://learn.genetics.utah.edu/content/epigenetics/twins/]

Image Courtesy:
1. “Les Twins profile” By Shawn Welling – www.shawnwelling.com (CC BY-SA 3.0) via Commons Wikimedia
2. “Identical-fraternal-sperm-egg” By ChristinaT3 at English Wikipedia (CC BY 3.0) via Commons Wikimedia
3. “Red Hair Twins 2” By Dpulitzer – Own work (CC BY-SA 4.0) via Commons Wikimedia