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?
- They were conceived through in vitro fertilization.
- They have 75\% identical genetic content.
- They are monozygotic twins.
- They are fraternal twins.
Answer
(4) They are fraternal twins.
Solution
Monozygotic (identical) twins arise from one zygote and are always of the same sex.
A boy–girl pair must therefore have developed from two separate eggs fertilised by two sperms, i.e. they are dizygotic (fraternal) twins.
Fraternal twins share on average about 50\% of their genes, like ordinary siblings, and need not be conceived by IVF.
Answer: (4)