YES – you’ve read that right. There are actually 7 different types of twins. Apart from the commonly known identical and fraternal twins, there are half-identical, mirror image twins, mixed chromosome twins, superfecundation, and superfetation.
Without further ado, let’s take a look at their characteristics:
1. Identical twins
Identical twins start out as genetically identical as they have the same genes. They are formed by a fertilized egg dividing into two separate individuals. They are always of the same sex.
2. Fraternal twins
Twins that are male and female or have different blood types can easily be determined as fraternal. Fraternal twins can share many similarities in appearance just like any brother or sister of the same family.
3. Half-identical twins
These are the type of twins who share half their genes in common (from the mother) and the other half different (from the two sperms). They share some features of identical twins and some features of fraternal twins, hence the name half-identical twins.
4. Mirror image twins
These twins will often have “mirror images” of their features. For example, one twin has a birthmark on the right shoulder while the other has a birth mark one left shoulder.
5. Mixed chromosome twins
This occurs if two separate sperms fertilize two separate eggs. This produces individuals with different sets of chromosomes. Some have been identified to have more than one distinct red blood cell type and individuals who are both XX and XY (the sex chromosomes – XX being female and XY being male.) It is very rare, and less than 25 cases have been identified as yet.
Twins can have different fathers. One well known case was described in 1810 in the United States. A woman had both a white and a black lover, and she became pregnant and gave birth to twins, one white and the other black. Each twin had a different father. This is called superfecundation.
Superfetation is the occurrence of another pregnancy during an ongoing pregnancy. It happens when there is fertilization of an ovule by a sperm cell while a conceptus is already present in the womb.
Source: Mind Blowing Facts.