Monozygotic vs Dizygotic Twins
Two offsprings produced in the same pregnancy are called twins. Twins can be either monozygotic (identical) or dizygotic (fraternal). In contrast, a fetus developed alone in the womb is called a singleton. Families with a history of fraternal twins have a higher chance of producing more twins than families without twins, because fraternal twins are hereditary, whereas identical twins are not. Fraternal twin is caused by a gene located in the X chromosome. If a man has fraternal twins in his family, he can pass the twin gene to his daughter.
There are five common variations in twinning. Out of them, three variations are dizygotic (fraternal); they are male-female twins, which are the most common type, female-female dizygotic twins, and male-male dizygotic twins. Other two variations are monozygotic twins. They are male-male monozygotic twins(less common) and female –female monozygotic twins. Male-female monozygotic twins are possible but considerably very rare.
Death rate in the uterus is higher for twins, and males are more susceptible to death in the uterus than females.
Dizygotic or fraternal twins are referred to as “non-identical” twins. The two eggs are independently fertilized by two different sperm cells, and the fertilized eggs are implanted in the uterus wall at the same time and become two zygotes; hence the term become dizygotic, and the result is fraternal twins.
Like other siblings, dizygotic twins also have an extremely small chance of having the same chromosome. They may have similar appearances or maybe very different from each other; also, they may be of the same sex or different sex. Particularly, they are of the same age.
Dizygotic twins are more common for older mothers, and above 35 years of age, the twinning rate doubles.
Monozygotic twins are referred to as “identical” twins. It occurs, when a single egg is fertilized to form one zygote (hence it is called “monozygotic”) and then divides into two separate embryos. In natural monozygotic twinning, the twins are formed after a blastocyst collapses splitting the progenitor cells into half, and the genetic material divides into two on opposite sides of the embryo. Eventually the two separate fetuses develop. Spontaneous division of the zygote into two embryos is a spontaneous or random event, which is not a hereditary trait. Monozygotic twins can also be created artificially by embryo splitting.
Almost all monozygotic twins are genetically identical and always they are of the same sex, unless there has been a mutation during the development. However, they don’t have the same finger prints. On rare occasions, monozygotic twins may express different phenotype.
Conjoined twins are monozygotic twins whose bodies are joined together during pregnancy where the single zygote of monozygotic twins fails to separate completely, and the zygote starts to split after 12th day following fertilization.
What is the difference between Monozygotic Twins and Dizygotic Twins?
• Monozygotic twins originate from the same fertilized egg, and fertilized by the same sperm so, they share the same DNA. The dizygotic twins originate from two fertilized egg and separately fertilized by two sperm; therefore, they do not share the same DNA.
• Dizygotic twins have separate placenta, amniotic sac and chorion. In Monozygotic twin, the division occurs in the zygote within first four days, and zygotes only share the outer layer of the amniotic sac and have the two placentas; if it divides within four to eight days, the zygotes will share the amniotic sac and share a single placenta.
• Monozygotic twins almost have the same appearance while the dizygotic twins may have the same appearance or different appearance.
• Monozygotic twins have the chance for same characters, developments and etc, but dizygotic twins don’t have that.