The rarest type of twins, semi-identical twins, have been identified for the second time ever in a report released this week. These twins, also known as sesquizygous twins, are extremely rare and are believed to share anywhere from 50 to 100% of their genomes.
Identical twins are the most common type of twins, and they occur when a single egg is fertilized by a single sperm and then splits in two. This results in two babies that have the same genetic makeup. Fraternal twins, on the other hand, occur when two eggs are fertilized by two different sperm and then both embryos implant in the uterus. These twins share about 50% of their DNA, just like any other siblings.
Semi-identical twins, however, are much rarer. They occur when a single egg is fertilized by two different sperm, resulting in two embryos that share anywhere from 50 to 100% of their genomes. The first case of semi-identical twins was reported in 2007, and the second case was reported this week.
The second case of semi-identical twins was discovered when the parents of two boys noticed that the boys had different physical features. The parents took the boys to the doctor, who then performed genetic testing and discovered that the boys were semi-identical twins.
The rarity of semi-identical twins means that there is still much to be learned about them. However, researchers believe that they could provide valuable insight into how genes are expressed and inherited.
In conclusion, semi-identical twins are the rarest type of twins, and only two cases have been reported. They occur when a single egg is fertilized by two different sperm, resulting in two embryos that share anywhere from 50 to 100% of their genomes. While there is still much to be learned about these rare twins, they could provide valuable insight into how genes are expressed and inherited.