Correct Answer: A. Fingerprint pattern
Fingerprint patterns are not identical in monozygotic (identical) twins despite their genetic identity. This is the key discriminating fact. While identical twins share 100% of their DNA, fingerprints are determined by a combination of genetic factors and random developmental/environmental influences during fetal life—specifically, the mechanical forces, amniotic fluid pressure, and fetal movements in utero between weeks 8–24 of gestation. These stochastic (random) factors are independent between twins and create unique ridge patterns for each individual, even in genetically identical siblings. This principle is foundational to forensic identification: fingerprints remain the gold standard for personal identification precisely because they are unique to each individual, including identical twins. In contrast, blood groups, DNA sequences, and iris colour are determined purely by genetic inheritance and are therefore identical in monozygotic twins. This distinction is critical in forensic medicine and is tested frequently in NEET PG to assess understanding of the interplay between genetics and developmental randomness.
Why the other options are wrong
B. Blood group — Blood group is determined entirely by genetic inheritance (ABO, Rh, and other blood group antigens are coded by alleles). Identical twins inherit the same alleles from both parents and therefore have identical blood groups. This is a purely genetic trait with no environmental variation. C. DNA fingerprinting — DNA fingerprinting (VNTR/STR analysis) detects genetic polymorphisms that are inherited. Identical twins share 100% of their DNA sequence, so their DNA fingerprints are indistinguishable. This is why DNA fingerprinting cannot differentiate between monozygotic twins—a key limitation in forensic casework. D. Iris colour — Iris colour is determined by melanin deposition, which is controlled by genetic factors (polygenic inheritance). Identical twins inherit the same genes governing melanin production and distribution in the iris, resulting in identical iris colours. This is purely genetically determined with no developmental randomness.
High-Yield Facts
- Fingerprints are unique even in identical twins due to random developmental factors (amniotic pressure, fetal movements) during weeks 8–24 of gestation, not purely genetic determination.
- Blood groups, DNA, and iris colour are genetically determined and therefore identical in monozygotic twins.
- Fingerprints remain the gold standard for personal identification in forensic medicine because they are unique to each individual and do not change throughout life.
- DNA fingerprinting cannot differentiate between identical twins because they share 100% of their DNA sequence.
- Epigenetic and developmental randomness (not genetic) account for fingerprint variation in genetically identical individuals.
Mnemonics
GRIT for Identical Twin Similarities Genetic traits (Blood group, DNA, iris) = Identical in twins. Random developmental traits (fingerprints, Ridge patterns) = Individual/unique. Use this to remember that fingerprints are the exception because they depend on random fetal developmental factors, not genetics alone. FIN-PRINT Rule FIN = Fingerprints are INdividual (unique even in identical twins). All other traits listed are genetically inherited and therefore identical. Quick memory hook: fingerprints are the 'exception' in the identical twin rule.
NBE Trap
NBE pairs "identical twins" with "similarities" to lure students into assuming all biological traits are genetically determined. The trap is conflating genetic identity with phenotypic identity—students may incorrectly assume fingerprints must be identical because DNA is identical, forgetting that fingerprint formation involves random developmental forces independent of genetic code.
Clinical Pearl
In Indian forensic casework, when identical twins are suspects in a crime, fingerprints at the scene can definitively identify which twin was present—a critical distinction that DNA fingerprinting alone cannot make. This principle has been used in several high-profile Indian criminal investigations to differentiate between monozygotic twins.
_Reference: Forensic Medicine & Toxicology (Reddy, 34th ed.) Ch. 3 "Personal Identification"; Parikh's Textbook of Medical Jurisprudence, Forensic Medicine & Toxicology Ch. 4 "Identification of Persons"_