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NEET UG · Biology · chapter
Genetics questions in NEET come to roughly eight to eleven of the ninety Biology questions across Principles of Inheritance and Variation and Molecular Basis of Inheritance, and they are the reasoning-heaviest marks in the paper. This guide covers the ratios worth knowing cold, the disorders and their inheritance patterns, the molecular facts NEET returns to, the mistakes that cost the most marks, and a free quiz with worked examples.
Quick answer
Questions per paper
8–11
Of 90 Biology questions · BrainStride analysis of past papers
Chapters covered
2
Inheritance and variation · molecular basis of inheritance
Question style
Reasoning + numericals
Crosses, probabilities and experiment recall
Class
12
Part of the Genetics and Evolution unit (10–14 Q)
BrainStride analysis of past papers
Dominance, segregation and independent assortment, demonstrated on the garden pea and its seven contrasting characters. Learn the monohybrid and dihybrid ratios with their genotypic versions, and the test cross that tells a homozygote from a heterozygote.
Incomplete dominance gives 1:2:1 for both genotype and phenotype; codominance shows both alleles, as in the AB blood group; multiple alleles, pleiotropy and polygenic inheritance each bend the expected numbers. Questions usually describe the situation and expect you to name it.
Morgan's work on Drosophila showed that genes on the same chromosome do not assort independently. Recombination frequency measures how often they separate, and that frequency became the basis for mapping genes along a chromosome.
XX–XY in humans, XX–XO in grasshoppers, ZZ–ZW in birds and haplodiploidy in honeybees. Pedigree questions almost always reduce to one decision: is the trait autosomal or X-linked, dominant or recessive.
Haemophilia and colour blindness are X-linked recessive; sickle-cell anaemia, thalassaemia and phenylketonuria are autosomal recessive. Chromosomal disorders come from aneuploidy: trisomy 21 in Down syndrome, XXY in Klinefelter syndrome, XO in Turner syndrome.
Griffith's transforming principle, the Avery–MacLeod–McCarty follow-up and the Hershey–Chase experiment with radioactively labelled bacteriophages. Know what each experiment could and could not conclude — that distinction is the question.
Semiconservative replication, proved by Meselson and Stahl in E. coli; the transcription unit and its promoter and terminator; the genetic code and its properties; and the lac operon as the standard example of regulation.
3 : 1 phenotypic · 1 : 2 : 1 genotypic
From a heterozygote selfed. The genotypic ratio is the one questions hide behind a phenotype count.
9 : 3 : 3 : 1
Only when the two genes assort independently and both show complete dominance.
1 : 1 for one gene · 1 : 1 : 1 : 1 for two
Crossing with the homozygous recessive reveals the unknown genotype directly.
1 : 2 : 1 phenotypic = genotypic
The heterozygote has its own intermediate phenotype, so phenotype and genotype ratios coincide.
2ⁿ for n heterozygous gene pairs
AaBb gives 4 gamete types; AaBbCc gives 8. A quick check before drawing any square.
A = T · G = C · purines = pyrimidines
So knowing the share of one base fixes the share of its partner.
10 base pairs per turn · pitch 3.4 nm · 0.34 nm per base pair
The three numbers are related: 10 × 0.34 nm = 3.4 nm.
64 codons · 61 code for amino acids · UAA, UAG, UGA stop
AUG is both the initiator codon and the codon for methionine. The code is degenerate but unambiguous.
Human XX/XY · grasshopper XX/XO · birds ZZ/ZW
In birds the female is the heterogametic sex — the reversal is the whole question.
Trisomy 21 → 47 · XXY → 47 · XO → 45
Down, Klinefelter and Turner syndromes respectively, all from non-disjunction.
Glu → Val at the sixth position of the β-globin chain
A single base substitution, inherited as an autosomal recessive trait.
i gene repressor · z, y, a structural genes
Lactose acts as the inducer; z gives β-galactosidase, y permease and a transacetylase.
Why it happens
The dihybrid ratio is memorised as the answer to "two genes" rather than as the result of independent assortment with complete dominance.The fix
Before writing any ratio, check three things: are the genes linked, is dominance complete, and is either gene lethal or interacting with the other.Why it happens
Revision groups diseases by what they do to the body, but questions ask how they pass through a family.The fix
Keep one list by inheritance pattern: X-linked recessive, autosomal recessive, autosomal dominant, chromosomal. Learn the pattern first and the symptoms second.Why it happens
Both are learnt as "RNA polymerase reads a template", so the differences that carry marks disappear.The fix
Contrast them explicitly: one polymerase against three, a single polycistronic transcript against monocistronic RNA, and the splicing that only eukaryotes need.Why it happens
Griffith, Avery and Hershey–Chase are revised as one story about DNA rather than as three separate steps.The fix
Write what each experiment alone proves: Griffith found transformation without naming the molecule, Avery identified it, and Hershey–Chase confirmed it with labelled phages.Why it happens
A pattern "looks" dominant, so the affected generations are counted instead of tested against the rule.The fix
Test the pedigree: unaffected parents with an affected child means recessive; affected sons of unaffected mothers point to X-linked. Decide those two before assigning genotypes.6 questions
Question 1
A dihybrid test cross (AaBb × aabb) gives which phenotypic ratio, assuming independent assortment?
Question 2
How many types of gametes can an individual of genotype AaBbCc produce, with all three genes unlinked?
Question 3
Which of these is inherited as an X-linked recessive trait?
Question 4
In B-DNA, one complete turn of the helix contains about:
Question 5
Which set lists only stop codons?
Question 6
The Meselson and Stahl experiment on E. coli established that DNA replication is:
A test on one chapter alone tells you little — the exam never asks Genetics in isolation. So this builds a full NEET Biology paper that leans on it. Weighted like the exam, a 90-question paper gives Genetics 9 questions; marked weak, it gets 17, and the rest of the paper keeps the other chapters honest. Fill the plan from previous-year questions or your module.
Your paper
Press Generate paper plan for a chapter-by-chapter blueprint: questions per chapter, numericals, marks and time — ready to print and fill from your PYQs, modules or the BrainStride question bank.
Chapter weights are BrainStride’s reading of past papers, not official data. For any other chapter mix, use the NEET Biology test generator.
Probability · dihybrid cross
Two plants of genotype AaBb are crossed, with both genes unlinked and completely dominant. What fraction of the offspring is homozygous recessive for both genes?
Answer: One in sixteen — the single "1" at the end of the 9:3:3:1 ratio.
Reasoning · multiple alleles
A mother has blood group O and the father has blood group AB. Which blood groups can their children have, and which are impossible?
Answer: The children can be group A or group B only. Neither O nor AB is possible, because every child receives one i from the mother and one dominant allele from the father.
Assertion and reason · sex linkage
Assertion: a colour-blind daughter must have a colour-blind father. Reason: colour blindness is an X-linked recessive condition.
Answer: Both statements are true, and the reason explains the assertion. The mother, by contrast, may be an unaffected carrier.
In recent NEET papers, Principles of Inheritance and Variation with Molecular Basis of Inheritance have carried roughly 8 to 11 of the 90 Biology questions, inside the 10 to 14 we estimate for the whole Genetics and Evolution unit. These are BrainStride estimates from past papers; NTA publishes no chapter-wise weightage.
It is the part that rewards reasoning rather than recall, which makes it feel harder. Crosses, probabilities and pedigrees need working rather than memory, and the molecular chapter carries a dense set of experiments. The advantage is that the question types repeat, so practice moves the score quickly.
Check the assumptions before writing a ratio. A 9:3:3:1 result needs two unlinked genes with complete dominance; incomplete dominance turns a monohybrid into 1:2:1, and linkage removes independent assortment altogether. Writing the parental genotypes and gamete types first takes fifteen seconds and prevents the commonest error in the paper.
The standard ratios and test crosses, the inheritance pattern of each Mendelian disorder, chromosomal disorders and their chromosome counts, the experiments that identified DNA as the genetic material, the properties of the genetic code, and the lac operon. Those seven cover most of what recent papers have asked.
NCERT is the source for the facts, but Genetics also needs practice at applying them. Read the chapters closely, then attempt previous-year questions on crosses, pedigrees and probability until the working is automatic. Extra theory books rarely help here; more solved crosses usually do.
Start with the ratios and the disorders, because they carry the most repeatable marks, then rebuild the molecular chapter experiment by experiment. Keep a written log of which cross type you get wrong — most students lose marks in one or two specific situations, such as linkage or codominance, rather than across the chapter.
The 20-minute diagnostic finds which cross types you get wrong and writes the practice that fixes them. Free, no card.