Twins are born to a family that lives next door to you. The twins are a boy and a girl. Which of the following must be true?

The correct answer is: They are fraternal twins.

To understand why this must be true, we can break down the biological differences between the two main types of twins:

1. Monozygotic (Identical) Twins:
Monozygotic twins form when a single egg is fertilized by a single sperm to create one zygote, which subsequently splits into two separate embryos. Because they originate from the same genetic starting point, monozygotic twins share 100% of their genetic material. Consequently, they are almost always of the same sex (either both boys or both girls).

2. Dizygotic (Fraternal) Twins:
Dizygotic twins form when two separate eggs are released during the same ovulation cycle and are fertilized by two different sperm cells. From a genetic perspective, fraternal twins are no more alike than regular siblings, sharing approximately 50% of their DNA. Because they develop from two completely independent fertilization events, they can be of the same sex or different sexes.

Since the twins in this scenario are a boy and a girl (different sexes), they cannot be monozygotic (identical) twins. Therefore, they must be fraternal twins.

Evaluating the other options:
- They are monozygotic twins: Incorrect, as monozygotic twins are genetically identical and must be of the same sex.
- They were conceived through in vitro fertilization: Incorrect, as twins (both fraternal and identical) can be conceived naturally or through assisted reproductive technologies. There is no information to suggest IVF was used.
- They have 75% identical genetic content: Incorrect, because fraternal twins share approximately 50% of their genetic material on average, similar to non-twin siblings.