Antonio and Anthony are identical twins, and Jonas and Julie are fraternal twins. Both sets of twins want to know why they are identical or fraternal. Explain the events that happen during the germinal period that lead to the formation of identical and fraternal twins.

Question

What will be an ideal response?

Answer 1

The ideal answer should include:
1. To Antonio and Anthony, the following explanation can be offered: Identical twins usually occur at the end of the first week after fertilization, when the blastocyst divides into two identical sets of cells, which develop independently from that point on.
2. They commonly share the chorion and the placenta, but are in two separate amniotic sacs.
3. Less often, identical twins occur at the two- to eight-cell phase and have separate amnions, chorions, and placentas.
4. To Jonas and Julie, the following explanation can be offered: Fraternal twins occur when two ova are present in the fallopian tube and are fertilized by two different sperm cells.
5. They have two amnions, two chorions, and two placentas.
6. Fraternal twins are more common in some families and racial groups and are more common when the mother has taken fertility drugs.