Ask Prof: How do twins form?

The end of term is near, and that means we’re about to talk about reproduction. We don’t discuss pregnancy or embryology, but a student asked how twins form?

There are two types of twins: fraternal and identical.

Fraternal twins come from 2 eggs both fertilized by sperm. Since the genetic content can be different, you can have one child looking like mom and the other looking like dad.

  • Fraternal (Non-identical) twins are formed when two egg cells are fertilized; each egg by a different sperm so that two embryos are formed. Fraternal twins can be of the same or opposite sex and they don’t have to look at all alike.
  • Identical twins are formed when one egg after being fertilized by one sperm, divides into two halves. The two halves are genetically identical. Identical twins are usually of the same sex.
  • Conjoined (Siamese) twins are formed exactly like identical twins, but during the split, the embryo does not separate completely to form two separate babies and the twins develop attached to one another.

Identical twins come from one fertilized egg, and this was what my student was asking me. I didn’t have an answer as to why after forming the zygote and it goes from 1 cell to 2 cells why these 2 cells become 2 individuals in this situation. A 2001 USA Today Q &A about this apparently stumped the columnist – More recently it seems that they are saying that identical twins are formed when the 2 cells essentially collapses and goes to opposite sides –

Doctors often call identical twins fraternal –

National Geographic takes a look at identical twins, saying “Discovering why identical twins differ—despite having the same DNA—could reveal a great deal about all of us.” –




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