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.

Concept:

• Twins are classified into two main types: monozygotic (identical) and dizygotic (fraternal).
• Monozygotic twins develop from a single fertilized egg that splits into two embryos, leading to identical genetic material. These twins are always of the same sex.
• Dizygotic or fraternal twins occur when two separate eggs are fertilized by two different sperm cells. As a result, they share approximately 50% of their genetic material, similar to regular siblings, and can be of the same or different sexes.

Explanation:

• They are monozygotic twins: This is incorrect because monozygotic twins are genetically identical and always of the same sex. 
• They are fraternal twins: This is correct. Fraternal twins are the result of two separate eggs being fertilized by two different sperm cells. They can be of different sexes, as in this case, where the twins are a boy and a girl.
• They were conceived through in vitro fertilization: This is incorrect. While in vitro fertilization (IVF) can increase the likelihood of twins, there is no indication in the question that IVF was involved. Twins can occur naturally as well.
• They have 75% identical genetic content: This is incorrect. Fraternal twins share 50% of their genetic material, the same as any other siblings.