Mendelian genetics accounts for 8–12 marks in NEET, spread across MCQs, numerical problems, and diagram-based questions. Yet most students stumble on monohybrid and dihybrid crosses, mixing up phenotype ratios or failing to interpret Punnett squares under exam pressure. The problem isn't the concepts—they're straightforward. The problem is that students memorize instead of understanding the mechanism. This guide walks you through Mendel's three laws using the exact approach that toppers use, so you can solve any cross in under two minutes and eliminate wrong answers with confidence.

Understanding Mendel's First Law: Law of Segregation

Mendel's first law, also called the law of segregation, states that allele pairs separate during gamete formation, and each offspring receives one allele from each parent. This is covered in NCERT Chapter 5 (Heredity and Variation) and is the foundation for all inheritance problems.

Here's what students must grasp: an organism with alleles Aa produces two types of gametes—50% carrying A and 50% carrying a. When these gametes fuse randomly during fertilization, the offspring genotypes follow a predictable 1:2:1 ratio (AA:Aa:aa) and phenotypes follow a 3:1 ratio (dominant:recessive) in a monohybrid cross.

The Monohybrid Cross Setup

For a cross between Aa Ă— Aa (heterozygote Ă— heterozygote), always construct a Punnett square with gametes on both axes. Write A and a along the top, A and a down the left side. Fill each cell by combining the gamete alleles. The result: 1 AA : 2 Aa : 1 aa genotypically, and 3 dominant : 1 recessive phenotypically. NEET examiners test this repeatedly with questions like "In a cross between two heterozygous tall plants, what fraction of offspring will be dwarf?" Answer: 1/4, derived from the aa genotype.

A common mistake: students confuse the 3:1 ratio with the 1:2:1 ratio. Remember—3:1 is phenotypic (what you see), 1:2:1 is genotypic (what alleles they carry). Questions asking "how many will show dominant phenotype" expect 3/4 or 75%, not the genotypic breakdown.

Decoding the Second Law: Law of Independent Assortment

Mendel's second law applies when tracking two traits simultaneously. The law of independent assortment states that allele pairs for different traits assort independently during gamete formation. This is also in NCERT Chapter 5 and is essential for dihybrid crosses.

Consider a cross AaBb Ă— AaBb (heterozygote for both traits). Instead of memorizing "the ratio is 9:3:3:1," understand why. The heterozygote AaBb produces four types of gametes: AB, Ab, aB, and ab, each at 25% frequency. When these combine, you get 16 possible offspring combinations. The phenotypic ratio is 9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive.

Building a 4Ă—4 Punnett Square

For a dihybrid cross, write AB, Ab, aB, ab across the top and down the left side. Fill all 16 cells systematically. Count the phenotypes: 9 with both dominant traits (A_B_), 3 with A_ but bb, 3 with aa but B_, and 1 with aabb. This method is slower than probability formulas but eliminates errors on exam day.

NEET questions on independent assortment often ask: "What is the probability of obtaining an offspring with genotype aaBb from the cross AaBb Ă— aaBb?" Use the probability method: for the aa genotype from Aa Ă— aa, it's 1/2. For the Bb genotype from Bb Ă— Bb, it's 1/2. So the answer is 1/2 Ă— 1/2 = 1/4. This shortcut saves minutes on longer crosses.

🎯 Common Exam Trap: Backcross vs Test Cross

A test cross means crossing the dominant phenotype individual with a recessive homozygote (e.g., Aa Ă— aa), which produces a 1:1 ratio. A backcross means crossing the F1 with either parent, which may produce different ratios depending on which parent. NEET questions often slip "backcross" into the stem to confuse students. Always identify the exact cross before calculating.

The Third Law: Law of Dominance and Gene Interactions

Mendel's law of dominance is simpler than the first two: in a heterozygote, the dominant allele masks the recessive allele phenotypically, but both are inherited. However, real genetics often involves non-Mendelian patterns—incomplete dominance, codominance, and epistasis—which NEET covers alongside classical Mendelian inheritance in Chapter 5.

Incomplete dominance (like red Ă— white flowers producing pink flowers) changes the phenotypic ratio in a monohybrid cross from 3:1 to 1:2:1 (phenotypically equals genotypically). Codominance (like human ABO blood groups) causes both alleles to be expressed equally. Epistasis involves one gene masking the effect of another, producing modified dihybrid ratios like 9:7, 13:3, or 12:3:1 instead of the classic 9:3:3:1.

Recognizing Non-Mendelian Ratios in Exams

If a dihybrid cross doesn't yield a 9:3:3:1 ratio, suspect gene interaction. For example, if the ratio is 9:3:4 (combining 3:1), epistasis is at play. NEET rarely asks you to predict which genes interact; instead, they give you the observed ratio and ask you to identify the inheritance pattern. A 1:1:1:1 ratio in a dihybrid cross signals that both genes are heterozygous and equally expressed without dominance.

Solving Numerical Problems: The Strategy Toppers Use

NEET genetics problems often look complex but follow a formula. Here's the step-by-step approach:

  1. Identify the cross: Write down both parental genotypes clearly. If given phenotypes, infer genotypes using dominance rules.
  2. Determine gamete types: Heterozygous loci (Aa) produce two gamete types; homozygous loci (AA or aa) produce one. For a dihybrid AaBb, there are 2 Ă— 2 = 4 gamete types.
  3. Calculate probability: Multiply the probabilities for each locus independently. For Aa Ă— Aa, the probability of Aa is 1/2. For Bb Ă— Bb, the probability of Bb is 1/2. For AaBb from