NEET 2025 Zoology Question 176 Solution

Multiple gestations in humans occur via two distinct physiological mechanisms:

• • Monozygotic (Identical) Twins: Arise from the cleavage and separation of a single blastocyst derived from a single fertilization event (one sperm + one ovum). Because they inherit identical nuclear genomes, monozygotic twins are invariably concordant for chromosomal sex (strictly both XX or both XY).
• • Dizygotic (Fraternal) Twins: Arise when two separate primary follicles mature and ovulate simultaneously (polyovulation). Each ovum is fertilized by an independent sperm cell, producing two genetically distinct zygotes.

Analyzing the sex chromosomes of the newborn twins:

• • The boy twin possesses an $XY$ chromosomal constitution (formed by union of an $X$-bearing ovum and a $Y$-bearing sperm).
• • The girl twin possesses an $XX$ chromosomal constitution (formed by union of an $X$-bearing ovum and an $X$-bearing sperm).
• • Because they have different sex chromosome karyotypes, they could not have arisen from the cleavage of a single zygote.
• • They must have developed from two distinct zygotes produced by two separate fertilization events. Thus, they are fraternal twins.

Why the remaining options are incorrect:

• • Option (1): They are monozygotic twins -> Impossible for opposite-sex twins.
• • Option (3): Conceived through IVF -> Fraternal twins occur frequently through natural polyovulation; IVF is not mandatory.
• • Option (4): Have 75% identical genetic content -> Fraternal twins share, on average, 50% of their segregating genetic material, identical to ordinary siblings.

The deduction that must be true is that they are fraternal twins, matching Option (2).