Hi, Thank you for your comment. It is estimated that, on average, a set of monozygotic twins will have about genetic differences that occurred early in foetal development. Retrieved from " https: December at 7: After the embryo has divided into about 4 cells it can split and become two separate two celled embryos.
In fraternal twins, you have two eggs and two sperm.
Can Identical Twins Be Male and Female?
This case is much more rare, and is different from both identical and fraternal twins. The two zygotes are developing rapidly into two embryos that have the same chromosomes and genes. Monozygotic twins of different sex. Each half goes on to form a separate individual. Identical twins start out as genetically identical: For identical twins, there is only one egg and one sperm.