You know how identical twins are formed: a single fertilized egg splits into two and grows as two separate babies. But the exact opposite of this process can also happen in rare cases. Two separate fertilized eggs which would normally grow as non-identical twins, sometimes merge into one and grow as a single baby. This merge happens at the embryonic stage. In this case, some cells of the baby’s body would come from one embryo and others from the other. Essentially, it is two babies merged as one.
This concept is called Chimerism. While it can happen in normal births also, the chances are higher in in-vitro fertilization because doctors usually place multiple embryos in the womb, hoping at least one would catch on. You could’ve watched this concept explained in the series House M.D. in season 3 episode 2.
Chimera is originally a Greek mythological term meaning a monster with the body of a goat and the head of a lion. Unlike the mythology, chimeric babies can grow up normally and lead a normal life, without even realizing they are Chimeras. There are no statistics available on how many Chimeras are born but it is very rare. You could be a chimera and you wouldn’t know it unless DNAs from different parts of your body are compared.
DNA Test
If you are a chimera, a DNA test between you and your child may indicate that you are not the child’s parent because the DNA in your sperm or egg may be different from those in your hair or saliva from where the test sample is taken. Here, the test result will say that you are an uncle or aunt of the child, because those cells come from your unborn twin who merged with you.
To Think About
What if one of the merged embryos was a male and the other was a female?
Reference Links
https://www.healthline.com/health/chimerism
https://www.webmd.com/children/what-is-chimerism
https://en.wikipedia.org/wiki/Human_chimera