The Genetics-Molecular Biology Cluster
No single topic cluster in NEET Biology matches the mark yield of Genetics + Molecular Biology. Together across their chapters — Principles of Inheritance, Molecular Basis of Inheritance, and Evolution — you can expect 12 to 16 questions in every NEET paper.
This is not a chapter to understand superficially. It's a chapter cluster to master completely.
Part 1: Mendelian Genetics
The Laws and Their Applications
Law of Segregation: Alleles separate during gamete formation — each gamete carries only one allele for each trait.
Law of Independent Assortment: Genes for different traits on different chromosomes are inherited independently.
JEE application: Every genetics ratio question flows from these two laws. The classic ratios:
- Monohybrid cross: 3:1 (phenotype), 1:2:1 (genotype)
- Dihybrid cross: 9:3:3:1 (phenotype)
- Test cross: 1:1 (heterozygous parent) or all dominant (homozygous)
When ratios deviate — incomplete dominance: Ratio changes to 1:2:1 in phenotype (not 3:1). Classic example: Antirrhinum (snapdragon) flower colour. Red × White → Pink (F1), Pink × Pink → 1 Red: 2 Pink: 1 White.
Codominance: Both alleles express simultaneously. ABO blood group system:
- Iᴬ (A antigen), Iᴮ (B antigen), i (no antigen)
- Iᴬ and Iᴮ are codominant; i is recessive to both
- Blood group O: ii genotype; AB: IᴬIᴮ genotype
Blood group transfusion compatibility and cross questions appear almost every year. Know all possible donor-recipient combinations.
Sex-Linked Inheritance
X-linked traits appear more frequently in males (XY) because they have only one X chromosome — no second allele to mask a recessive condition.
Haemophilia: X-linked recessive. Carrier female × Normal male → 50% sons affected, 50% daughters carrier.
Colour blindness: X-linked recessive. Same inheritance pattern as haemophilia.
Key test: If a daughter shows the trait, the father must also have it (she got one X from father). If a son shows the trait, the mother must be at least a carrier.
NEET regularly gives a pedigree and asks you to determine the mode of inheritance. Rules:
- Autosomal dominant: trait appears in every generation, affects both sexes equally
- Autosomal recessive: can skip generations, parents may be unaffected carriers
- X-linked recessive: more males affected; carrier females are unaffected
- X-linked dominant: affected fathers pass to all daughters, not to sons
Linkage and Crossing Over
Genes on the same chromosome do not assort independently (violates Mendel's second law). They are inherited together — this is linkage.
Crossing over (exchange of segments between homologous chromosomes during meiosis I prophase) produces recombinants. Recombination frequency = (recombinant offspring / total offspring) × 100%.
1 centimorgan (cM) = 1% recombination frequency. Used to construct genetic maps.
Part 2: Molecular Biology
DNA Structure
Watson-Crick model (1953):
- Double helix, antiparallel strands (5'→3' and 3'→5')
- Base pairing: A=T (2 hydrogen bonds), G≡C (3 hydrogen bonds)
- 10 base pairs per turn; pitch (one full turn) = 3.4 nm; distance between bases = 0.34 nm
- Human genome: 3.2 × 10⁹ base pairs, 23 pairs of chromosomes
NEET regularly tests numerical relationships: given number of base pairs → calculate number of hydrogen bonds, or total coils in DNA.
DNA Replication
Semi-conservative replication (Meselson-Stahl experiment, 1958): Each new DNA molecule has one original strand and one newly synthesised strand.
Key enzymes and their roles:
- Helicase: Unwinds and separates the two strands at the replication fork
- DNA Polymerase III: Synthesises new DNA in 5'→3' direction only
- Primase: Synthesises RNA primers (DNA polymerase cannot initiate without a primer)
- DNA ligase: Joins Okazaki fragments on the lagging strand
- Topoisomerase: Relieves torsional strain ahead of the replication fork
Leading vs lagging strand:
- Leading strand: synthesised continuously toward the replication fork
- Lagging strand: synthesised in fragments (Okazaki fragments) away from the fork; requires multiple primers
NEET frequently asks: "Which enzyme joins Okazaki fragments?" (Ligase) or "Which enzyme adds nucleotides to the growing chain?" (DNA Polymerase III).
Transcription and Translation
Transcription: Template strand (3'→5') is read; mRNA is synthesised in 5'→3' direction. RNA polymerase doesn't need a primer. Sigma factor (in prokaryotes) recognises promoter. Three stages: Initiation (RNA polymerase binds promoter), Elongation, Termination.
In eukaryotes: primary transcript (pre-mRNA) is processed by:
- 5' capping: 7-methylguanosine cap added
- Poly-A tail: added at 3' end; increases stability
- Splicing: Introns removed; exons joined
Translation: mRNA codon → tRNA anticodon → amino acid chain Ribosomes have A site (aminoacyl, accepts incoming tRNA), P site (peptidyl, holds growing chain), E site (exit, releases spent tRNA).
Start codon: AUG (methionine). Stop codons: UAA, UAG, UGA.
The genetic code is:
- Triplet (3 nucleotides per codon)
- Non-overlapping
- Degenerate/redundant (more than one codon per amino acid; 64 codons, 20 amino acids)
- Universal (same in almost all organisms — exception: mitochondria, some protozoa)
Common NEET Questions from This Cluster
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If an organism has 40% cytosine in its DNA, what percentage is adenine? (20% — because C=G=40%, so A=T=10% each... wait: 40% C means 40% G (total 80%), leaving 20% for A+T, so A=T=10%)
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"Which of the following is not a feature of genetic code?" — often tests understanding of 'ambiguous' (incorrect) vs 'degenerate' (correct).
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Pedigree analysis: "If both parents are unaffected but child is affected, which mode of inheritance is possible?" — autosomal recessive.
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Transcription and translation occur simultaneously in — prokaryotes (no nuclear membrane separates the two processes).