Answer
Understanding the Nature of Boy-Girl Twins
When twins are of different sexes—specifically, one boy and one girl—they cannot be monozygotic (identical) twins. Monozygotic twins develop from a single fertilized egg that splits into two embryos, resulting in twins that are genetically identical and always of the same sex because they share exactly the same genetic material, including sex chromosomes.
In contrast, fraternal twins, also known as dizygotic twins, occur when two separate eggs are fertilized by two different sperm cells during the same menstrual cycle. These twins are genetically similar to regular siblings, sharing approximately 50% of their genetic material, and they can be of the same or different sexes.
Therefore, the presence of a boy and a girl clearly indicates that the twins developed from two separate fertilized eggs, confirming that they are fraternal twins.
Conclusion: They are fraternal twins.