A twin half sister refers to a twin who shares one biological parent, typically the mother, but has a different biological father from her co-twin. This unique genetic relationship occurs when two separate eggs from the same mother are fertilized by sperm from two different fathers during the same menstrual cycle.
Understanding Heteropaternal Superfecundation
The phenomenon that results in twin half sisters is known as heteropaternal superfecundation. This is a rare form of atypical twinning where twins, despite being conceived and born at the same time from the same pregnancy, are genetically half siblings.
In contrast, typical fraternal (dizygotic) twins are conceived when two eggs are fertilized by sperm from the same father, making them full siblings. Identical (monozygotic) twins result from a single fertilized egg splitting, making them genetically identical full siblings.
| Type of Twin Relationship | Shared Biological Parents | Genetic Relationship | Conception Mechanism |
|---|---|---|---|
| Twin Half Sisters | Mother Only | Half Siblings | Heteropaternal superfecundation |
| Fraternal Twins | Mother & Father | Full Siblings | Homopaternal superfecundation |
| Identical Twins | Mother & Father | Full Siblings | Single egg splits |
How It Occurs
Heteropaternal superfecundation happens when:
- A woman ovulates two eggs in the same menstrual cycle.
- She has sexual intercourse with two different partners within a very short timeframe (usually hours to a few days) around her ovulation period.
- Each egg is then fertilized by sperm from a different father.
While rare, cases of twin half sisters have been confirmed through genetic testing, often when paternity is disputed or suspected. This highlights the complex and fascinating aspects of human reproduction and genetics.
