How are Twins formed?

There are identical twins and fraternal twins

At the time of conception sperm cell fuses with the ovum to create a new cell called Zygote with 46 (23 pairs) chromosomes

Identical Twins (Monozygotic)

If the fertilized egg (Zygote) splits and forms two eggs with identical genetic code, the off-springs are called identical twins.

Fraternal Twins (Dizygotic)

When two different eggs are released during ovulation, and each egg is fertilized by a different sperm cell, basically it forms two zygotes. The off springs are called Fraternal Twins.

Genetically, fraternal twins are no different that other siblings.

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