Question: 176. 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?
(1) They are monozygotic twins.
(2) They are fraternal twins.
(3) They were conceived through in vitro fertilization.
(4) They have 75 \% identical genetic content.
Answer: Option (2)
Explanation:
Monozygotic twins arise from the splitting of a single zygote.
They always have the same genetic makeup and are of the same sex.
Since the twins mentioned are a boy and a girl, they cannot be monozygotic.
Fraternal twins, also called dizygotic twins, develop from two different ova fertilized by two different sperms.
They can be of the same or different sexes.
Therefore, a boy and a girl twin pair must be fraternal twins.
In vitro fertilization is not necessary for the birth of fraternal twins.
Fraternal twins share about 50 \% of their genetic material, not 75 \%.
Hence, the correct answer is Option (2).