Main Difference – Monozygotic vs Dizygotic Twins
Monozygotic and dizygotic twins are two types of offspring produced by the same pregnancy. The main difference between monozygotic and dizygotic twins is that monozygotic twins are developed from one embryo, splitting into two embryos whereas dizygotic twins are developed from two different eggs, which are fertilized by sperms separately.
This article looks at,
1. What are Monozygotic Twins
– Cause, Development, Features
2. What are Dizygotic Twins
– Cause, Development, Features
3. What is the difference between Monozygotic and Dizygotic Twins
What are Monozygotic Twins
Monozygotic twins are developed by the splitting of a fertilized embryo. The embryo is developed from the zygote, which is formed by the fusion of one egg with one sperm. Since monozygotic twins are developed by the splitting of an embryo, both individuals share the same chromosomes. Therefore, the genetic code of monozygotic twins is considered as the same. Hence, monozygotic twins are called as identical twins or maternal twins as well. Both individuals have the same gender in monozygotic twins. The birth of monozygotic twins are quite rare. The rate of birth of monozygotic twins is about 1 in 333 pregnancies.
Figure 1: Monozygotic Twins
The degree of separation of twins inside the uterus depends on the time. That means the separation of both chorion (Chorionicity) and amniotic sac (amniosity) depend on the time in which the separation occurs. One-third of the monozygotic twins consist of a separate placenta and the amniotic sac inside the uterus. This situation is called dichorionic-diamniotic twins) (Di-Di twins). Di-Di twins are formed by the splitting of the embryo within 2-3 days after fertilization. In contrast, two-third of the monozygotic twins share the same placenta and two separate amniotic sacs (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 monozygotic twins share their amniotic sac as well (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. Different types of chorionicity and amniosity are shown in figure 2.
Figure 2: Various chorionicity and amniosity in monozygotic twins
What are Dizygotic Twins
Dizygotic twins are developed by separate fertilizations of two eggs by two sperms. Since dizygotic twins are developed by two separate fertilization events, they are genetically varied as any two siblings. Hence, their gender is also different from each other. They can be either girl/boy, boy/boy or girl/girl. Therefore, they are called non-identical twins or fraternal twins. Dizygotic twins are caused by hyper-fertilization. They are hereditary. 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. All the dizygotic twins are dichorionic-diamniotic. The development of both monozygotic and dizygotic twins are shown in figure 3.
Figure 3: Development of Monozygotic and Dizygotic Twins
Difference Between Monozygotic and Dizygotic Twins
Development
Monozygotic Twins: Monozygotic twins are developed by the splitting of a fertilized embryo into two.
Dizygotic Twins: Dizygotic twins are developed by two separate fertilization events occurring at the same time.
Causes
Monozygotic Twins: The cause for monozygotic twins is not known.
Dizygotic Twins: Dizygotic twins is caused either by IVF, certain fertility drugs or hereditary predisposition due to the hyper-ovulation.
Called as
Monozygotic Twins: Monozygotic twins are called identical twins.
Dizygotic Twins: Dizygotic twins are called fraternal twins.
Genetic Code
Monozygotic Twins: The genetic codes of the monozygotic twins are nearly identical.
Dizygotic Twins: The genetic code of the dizygotic twins are same as any other sibling.
Gender of Twins
Monozygotic Twins: The gender of monozygotic twins are same.
Dizygotic Twins: The gender of dizygotic twins are different.
Blood Type
Monozygotic Twins: The blood type of monozygotic twins are the same.
Dizygotic Twins: Dizygotic twins may have different blood types.
Appearance
Monozygotic Twins: Monozygotic twins are extremely similar. But, they may vary depending on the environmental factors.
Dizygotic Twins: The appearance of dizygotic twins is similar as any other sibling.
Likelihood
Monozygotic Twins: The likelihood of the monozygotic twins is uniform around the world.
Dizygotic Twins: The likelihood of the dizygotic twins varies by country.
Occurrence
Monozygotic Twins: One-third of the twins in the world are monozygotic twins.
Dizygotic Twins: Two-thirds of the twins in the world are dizygotic twins.
Hereditary
Monozygotic Twins: Monozygotic twins are not hereditary.
Dizygotic Twins: Dizygotic twins are hereditary.
Inside the Uterus
Monozygotic Twins: Monozygotic twins can be either Di-Di, Mono-Di or Mono-Mono twins.
Dizygotic Twins: Dizygotic twins are only Di-Di twins.
Risk of Twin-to-Twin Transfusion Syndrome (TTTS)
Monozygotic Twins: Monozygotic twins bear high risk for TTTS.
Dizygotic Twins: Dizygotic twins bear a low risk for TTTS compared to monozygotic twins.
Conclusion
Monozygotic twins and dizygotic twins are the two major types of twins found in the world. Monozygotic twins are developed by the splitting of a fertilized egg. Therefore, they are genetically identical. The gender of the monozygotic twins is also the same since they are originated from the same embryo. Usually, monozygotic twins exhibit the same phenotypical characters as well. Dizygotic twins are formed by the separate fertilization of two eggs at the same time. Therefore, they are considered as two siblings genetically varied to each other. Dizygotic twins occur due to hyper-ovulation. However, the main difference between monozygotic and dizygotic twins is the development of twins inside the uterus.
Reference:
1.” Types of Twins: Dizygotic, Monozygotic, Dichorionic, & Monochorionic.” BabyMed.com. N.p., 31 July 2016. Web. 04 May 2017. [http://www.babymed.com/twins/twins-monozygotic-vs-dizygotic-and-monochorionic-vs-dichorionic] 2. “What does Monozygotic and Dizygotic mean?” Innovateus.net. N.p., n.d. Web. 04 May 2017. [http://www.innovateus.net/health/what-does-monozygotic-and-dizygotic-mean]
Image Courtesy:
1. “Twin boys” By Raul Carabeo – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Placentation” By Kevin Dufendach – Own work (CC BY 3.0) via Commons Wikimedia
3. “Identical twins lg” By National Human Genome Research Institute – National Institutes of Health. National Human Genome Research Institute. “Talking Glossary of Genetic Terms.”Retrieved November 17, 2016, from (Public Domain) via Commons Wikimedia
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