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Genetics questions in NEET reward a method more than memory. Many mistakes come from skipped steps: a gamete written wrongly, a genotype ratio given where a phenotype ratio was asked, or a fraction taken from the wrong total. The topic sits in Unit 7 of the NMC syllabus for NEET (UG) 2026, which lists Mendelian inheritance, incomplete dominance, co-dominance, multiple alleles and blood groups, and linkage and crossing over. NCERT covers it in Principles of Inheritance and Variation.
What did Mendel actually count?
NCERT notes that Mendel ran his pea experiments for seven years (1856 to 1863) and chose 14 true-breeding pea lines. His real counts were not neat. In the English translation of his 1865 paper, he reports 5,474 round seeds and 1,850 wrinkled ones, a ratio of 2.96 to 1. For seed colour he counted 6,022 yellow and 2,001 green, a ratio of 3.01 to 1.
So a 3:1 ratio is an expectation over many offspring. When a question gives real counts, look for the ratio they are close to.
A five-step method for any cross
Write the key. Which allele is dominant, and what each letter means. For example, T for tall and t for dwarf.
Write the parents' genotypes. Turn words like "true-breeding" into TT or tt, and "heterozygous" into Tt.
List the gametes. Each gamete carries one allele of each gene. NCERT shows that an RrYy plant makes four kinds of gametes (RY, Ry, rY and ry), each a quarter of the total.
Combine them. Use a Punnett square, or multiply probabilities gene by gene.
Answer the exact question. Genotype ratio, phenotype ratio, a fraction, or a fraction of a subgroup.
Worked example 1: the subgroup trap
Cross Tt with Tt. NCERT gives the result: a genotype ratio of 1 TT : 2 Tt : 1 tt and a phenotype ratio of 3 tall : 1 dwarf. Now the question: "What fraction of the tall plants are heterozygous?"
The tempting answer is 1/2, because half of all offspring are Tt. But the question asks about tall plants only. Tall plants are 3/4 of the total, and Tt plants are 1/2 of the total. So the answer is (1/2) divided by (3/4), which is 2/3.
Worked example 2: multiply instead of drawing 16 boxes
Cross RrYy with RrYy and ask for the fraction of round, green seeds. NCERT shows the dihybrid ratio 9:3:3:1 is just (3 round : 1 wrinkled) multiplied by (3 yellow : 1 green). So treat each gene separately. Round (R_) is 3/4. Green (yy) is 1/4. Multiply: 3/16.
For a test cross, RrYy with rryy, the second parent only gives ry gametes. The four gamete types of the first parent show up directly, so the offspring come out in a 1:1:1:1 ratio.
When the usual ratios change
Incomplete dominance. In NCERT's snapdragon example, red (RR) crossed with white (rr) gives pink (Rr). The F2 phenotype ratio is 1 red : 2 pink : 1 white. The genotype ratio has not changed; the phenotypes now show it.
Co-dominance and multiple alleles. The ABO gene has three alleles (IA, IB and i), which give six genotypes. Cross IAi with IBi and you get IAIB, IAi, IBi and ii in equal shares: blood groups AB, A, B and O, 1:1:1:1.
Linkage. NCERT describes Morgan's Drosophila crosses, where the F2 ratio moved far from 9:3:3:1 because the genes sat on the same chromosome. One gene pair showed only 1.3 per cent recombination, another 37.2 per cent. If a question mentions linked genes, do not use 9:3:3:1.
Common errors and the habit that fixes each
Two alleles of one gene in a single gamete (writing "Tt" as a gamete). Fix: circle each gamete and check it has one letter per gene.
Giving the genotype ratio when the phenotype ratio was asked. Fix: underline the question's key word first.
Using the whole population when the question names a subgroup. Fix: write "out of what?" next to every fraction.
Forgetting that i is recessive to both IA and IB. Fix: write the dominance rule in your key every time.
Drawing a 16-box square under time pressure. Fix: multiply gene by gene for any question with two or more genes.
A practice plan for one week
Do ten mixed problems a day: three monohybrid, three dihybrid, two on incomplete dominance or ABO, two on linkage or sex-linked traits. Write the key and gametes every time, and tag each error with its type from the list above.
When your method is right but the answer still goes wrong, compare your steps with a full solution. The Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
For the probability side of genetics, see common reasoning mistakes in probability. On when to study solved problems and when to work from scratch, read worked examples or solving from scratch.
Start tonight: explain the 2/3 answer aloud without looking at this page.
