Genetics for NEET: Two NCERT Chapters, Every Ratio and Rule
Genetics for NEET is the first two parts of Unit 7, "Genetics and Evolution", in the official NEET (UG) syllabus, and it is taught in exactly two NCERT Class 12 Biology chapters: Chapter 4, Principles of Inheritance and Variation, and Chapter 5, Molecular Basis of Inheritance. The third part of the unit, evolution, lives in Chapter 6. This guide matches each syllabus line to the NCERT section that teaches it, lists the ratios, disorders and DNA numbers the chapters print, and ends with a revision plan for Class 11, Class 12 and droppers.
What is genetics for NEET, and which chapters does it cover?
Genetics is the branch of biology that explains how traits pass from parents to children and how DNA stores and uses that information. For the exam, the scope is fixed by the NEET (UG) 2026 syllabus, which the National Medical Commission's Undergraduate Medical Education Board finalised on 22 December 2025. Its Biology section has 10 units, and Unit 7, "Genetics and Evolution", has three bullet points: heredity and variation, molecular basis of inheritance, and evolution. The first two are genetics for NEET in the narrow sense.
Both live in the current NCERT Class 12 Biology book, in PDFs marked "Reprint 2026-27":
- Chapter 4, Principles of Inheritance and Variation: Mendel, deviations from Mendel, linkage, polygenic traits, pleiotropy, sex determination, mutation and genetic disorders.
- Chapter 5, Molecular Basis of Inheritance: DNA and RNA structure, the search for the genetic material, replication, transcription, the genetic code, translation, the lac operon, the Human Genome Project and DNA fingerprinting.
Why bother sorting it so carefully? Because Biology is 90 of the 180 questions and 360 of the 720 marks in the NEET (UG) 2026 Information Bulletin, with plus four for a right answer and minus one for a wrong one. The same bulletin breaks ties on Biology marks first, before Chemistry and Physics. NTA does not publish chapter-wise weightage, so nobody can honestly tell you how many genetics questions will come. What you can control is whether you know every line the two chapters teach.
What does the syllabus wording include, line by line?
Here is the heredity and molecular bullet of Unit 7, split into its phrases, with the NCERT section that teaches each one. The syllabus is a list of headings. The chapter is where the testable detail sits.
| Syllabus phrase (Unit 7) | Where NCERT teaches it | Detail worth a flashcard |
|---|---|---|
| Mendelian inheritance | Ch 4, sections 4.1 to 4.3 | 7 pea characters, 14 true-breeding varieties, experiments from 1856 to 1863 |
| Incomplete dominance | Ch 4, 4.2.2.1 | Snapdragon flower colour, F2 of 1 red : 2 pink : 1 white |
| Co-dominance, multiple alleles, blood groups | Ch 4, 4.2.2.2 | Gene I with alleles IA, IB and i: 6 genotypes |
| Pleiotropy | Ch 4, 4.5 | Phenylketonuria, one gene with several effects |
| Polygenic inheritance (elementary) | Ch 4, 4.4 | Human skin colour with three genes, additive effect |
| Chromosome theory, linkage and crossing over | Ch 4, 4.3.2 and 4.3.3 | Sutton and Boveri; Morgan's 1.3% and 37.2% recombination |
| Sex determination in humans, birds, honey bee | Ch 4, 4.6 | XO, XY, ZW and haplodiploid systems |
| Sex-linked inheritance: haemophilia, colour blindness | Ch 4, 4.8.2 | Carrier mother to son |
| Mendelian disorders: thalassemia | Ch 4, 4.8.2 | NCERT also covers sickle-cell anaemia and phenylketonuria |
| Chromosomal disorders: Down's, Turner's, Klinefelter's | Ch 4, 4.8.3 | Trisomy 21, 45 with XO, 47 XXY |
| Search for genetic material, DNA and RNA structure, packaging | Ch 5, 5.1 to 5.3 | Griffith, Avery, Hershey and Chase; nucleosome of 200 bp |
| Replication, central dogma, transcription, code, translation | Ch 5, 5.4 to 5.7 | Meselson and Stahl; 61 sense codons, 3 stop codons |
| Lac operon | Ch 5, 5.8.1 | Genes i, z, y and a; lactose as inducer |
| Human genome project, DNA fingerprinting | Ch 5, 5.9 and 5.10 | 1990 to 2003; VNTR probe, Alec Jeffreys |
Two gaps between the syllabus and the book are worth knowing. First, the syllabus names only thalassemia as a Mendelian disorder, but NCERT explains colour blindness, haemophilia, sickle-cell anaemia, phenylketonuria and thalassemia, and it shows sickle-cell anaemia as its example of an autosomal recessive pedigree. Second, pedigree analysis (section 4.8.1) and mutation (section 4.7) are taught in Chapter 4, while the syllabus places "mutation and recombination" under evolution. The safe reading for genetics for NEET: learn the whole NCERT text of both chapters. Nothing in either chapter is marked as outside the syllabus, and our page on the NEET 2027 deleted syllabus confirms these chapters came through the rationalisation intact.
Which ratios and crosses must you know for genetics for NEET?
Most Chapter 4 questions are ratio questions in disguise. Get the three core results automatic and the rest of genetics for NEET becomes arithmetic.
| Cross or pattern | NCERT example | Phenotype ratio in F2 | Genotype ratio in F2 |
|---|---|---|---|
| Monohybrid, complete dominance | Tall (TT) by dwarf (tt) pea | 3 : 1 | 1 TT : 2 Tt : 1 tt |
| Incomplete dominance | Red (RR) by white (rr) snapdragon | 1 red : 2 pink : 1 white | 1 : 2 : 1 (same as phenotype) |
| Dihybrid, independent assortment | Round yellow by wrinkled green pea | 9 : 3 : 3 : 1 | NCERT asks you to work it out (it is not 9:3:3:1) |
| Co-dominance and multiple alleles | ABO blood groups | 4 blood types | 6 genotypes from 3 alleles |
| Linked genes | Morgan's Drosophila crosses | Far from 9:3:3:1 | 1.3% recombination (white, yellow) vs 37.2% (white, miniature wing) |
The trap in the first row: phenotype and genotype ratios differ under dominance but match under incomplete dominance. Questions love that contrast. The linkage row matters too. Morgan's student Alfred Sturtevant used recombination frequency as a measure of distance between genes, which is why tightly linked genes show low numbers.
How does sex determination work in each NCERT example?
NCERT gives four mechanisms, and the syllabus names three of them (humans, birds, honey bee). For genetics for NEET, learn all four. The grasshopper is the fourth, and it comes up as the classic XO example.
| System | NCERT example | Female | Male | Who decides the sex of the offspring |
|---|---|---|---|---|
| XO type | Grasshopper | XX | XO (one X only) | Sperm (male heterogamety) |
| XY type | Humans, Drosophila | XX | XY | Sperm (male heterogamety) |
| ZW type | Birds | ZW | ZZ | Egg (female heterogamety) |
| Haplodiploid | Honey bee | Diploid, 32 chromosomes | Haploid, 16 chromosomes | Fertilisation: fertilised egg is female, unfertilised egg is a drone |
Two NCERT lines are easy marks. In humans, 50% of sperm carry X and 50% carry Y, so every pregnancy has a 50% chance of either sex, and it is the sperm, not the mother, that decides. In honey bees, males make sperm by mitosis, have no father and so cannot have sons, but do have a grandfather and can have grandsons.
Which genetic disorders does NCERT cover?
This table is where the syllabus and the book differ most, so learn genetics for NEET from the book, not from the syllabus list. NCERT also names cystic fibrosis in its list of common Mendelian disorders, without explaining it.
| Disorder | Type | How it is inherited | Cause, in NCERT's words |
|---|---|---|---|
| Colour blindness | Mendelian | Sex-linked recessive | Defect in red or green cone; about 8% of males and 0.4% of females |
| Haemophilia | Mendelian | Sex-linked recessive | One protein of the clotting cascade affected; carrier female to sons |
| Sickle-cell anaemia | Mendelian | Autosomal recessive | Glutamic acid replaced by valine at position 6 of the beta globin chain (GAG to GUG) |
| Phenylketonuria | Mendelian | Autosomal recessive | Enzyme that converts phenylalanine to tyrosine is missing |
| Thalassemia | Mendelian | Autosomal recessive | Too few globin chains; alpha form on chromosome 16, beta form on chromosome 11 |
| Down's syndrome | Chromosomal | Trisomy | Extra copy of chromosome 21; first described by Langdon Down in 1866 |
| Klinefelter's syndrome | Chromosomal | Extra X | 47, XXY; masculine development with gynaecomastia, sterile |
| Turner's syndrome | Chromosomal | Missing X | 45 with XO; females with rudimentary ovaries, sterile |
The comparison NCERT spells out, and examiners like: thalassemia is a quantitative problem (too few globin molecules), while sickle-cell anaemia is a qualitative one (a globin that works incorrectly). Also note that phenylketonuria appears twice in the chapter, once as a Mendelian disorder and once as the example of pleiotropy.
Molecular Basis of Inheritance: the numbers and experiments to know
Chapter 5 is the half of genetics for NEET that rewards exact recall. Every figure below is printed in the NCERT chapter, and almost all of them have turned up as options in some form.
| Fact | Value in NCERT Chapter 5 |
|---|---|
| Bacteriophage phi X 174 | 5386 nucleotides |
| Bacteriophage lambda | 48502 base pairs |
| E. coli | 4.6 × 10^6 base pairs |
| Human DNA, haploid content | 3.3 × 10^9 base pairs |
| Distance between two base pairs | 0.34 nm |
| Length of DNA in a typical mammalian cell | About 2.2 metres (6.6 × 10^9 bp × 0.34 nm) |
| DNA per nucleosome | 200 base pairs, around a histone octamer |
| Codons | 64 in total, 61 code for amino acids, 3 stop codons (UAA, UAG, UGA) |
| Start codon | AUG, which also codes for methionine |
| Human Genome Project | Launched 1990, completed 2003 |
| Human genome | 3164.7 million bp; about 30,000 genes; under 2% codes for proteins; 99.9% of bases identical in all humans |
| Most and fewest genes | Chromosome 1 (2968) and Y (231) |
| SNPs identified | About 1.4 million locations |
Notice the two genome sizes: 3.3 × 10^9 bp for haploid human DNA in section 5.1 and 3164.7 million bp in section 5.9. Both are NCERT's own figures, from different contexts, so read the question for which one it means.
The experiments come with names and years, and the order matters because each answered the question the previous one left open.
- Griffith, 1928. Heat-killed virulent S strain Streptococcus pneumoniae transformed live R strain bacteria, so mice died. Something was transferred, but he could not say what.
- Avery, MacLeod and McCarty, 1933 to 1944. Only DNA from S cells transformed R cells, and DNase stopped it. DNA was the transforming principle.
- Hershey and Chase, 1952. Using radioactive phosphorus for DNA and radioactive sulphur for protein in bacteriophages, they showed it is DNA that enters the bacterium.
- Watson and Crick, 1953. The double helix model, built on X-ray diffraction data and Chargaff's base-pairing rule.
- Meselson and Stahl, 1958. Heavy nitrogen (15N) in E. coli DNA proved replication is semi-conservative. Taylor and colleagues showed the same in Vicia faba in 1958.
- Jacob and Monod. The lac operon: regulatory gene i makes the repressor, structural genes z, y and a make beta-galactosidase, permease and transacetylase, and lactose acts as the inducer.
How should you revise genetics for NEET, step by step?
The order below works whether you are in Class 11 starting early, in Class 12 meeting the chapters for the first time, or a dropper on a second pass.
- Read both NCERT chapters once without notes. Just follow the story from Mendel to the genome. Our guide on [how to study NCERT for NEET](/blogs/how-to-study-ncert-for-neet-2027/) explains how to read a chapter for detail on the second pass.
- Solve crosses by hand. Draw the Punnett square for the monohybrid, incomplete dominance and dihybrid crosses until the ratios in the table above come out without looking. Then do the ABO combinations.
- Learn the figures as figures. The pedigree symbols, the replication fork, the transcription unit and the lac operon diagram are all questioned. Our list of [NEET Biology diagrams](/blogs/neet-biology-diagrams/) counts 17 numbered figures in Chapter 4 and 16 in Chapter 5.
- Put every number on a card. The Chapter 5 table and the disorders table are made for [NEET flashcards](/blogs/neet-flashcards/), one fact per card, reviewed on a spaced schedule.
- Practise mixed questions under time. Our [NEET Biology MCQ](/blogs/neet-biology-mcq/) guide walks through all 90 Biology questions, and [NCERT Exemplar for NEET](/blogs/ncert-exemplar-for-neet-2027/) has more conceptual genetics problems.
- Finish with previous papers. Use [NEET previous year question papers](/blogs/neet-2027-previous-year-question-papers/) to see how genetics for NEET is actually asked, and mark every wrong answer by type: ratio, disorder, number or experiment.
For a full Biology plan that places genetics among the other nine units, see the NEET 2027 Biology preparation strategy. If you want the whole three-subject syllabus in one view, futureMBBS keeps a plain NEET syllabus overview, and its explainer on what the NCERT book is and why NEET relies on it helps if you are new to the board books. For how to begin the 2027 cycle as a whole, read its guide to starting NEET UG 2027 preparation.
Five traps in genetics for NEET questions
- Mixing phenotype and genotype ratios. The monohybrid F2 is 3:1 by phenotype and 1:2:1 by genotype. Answer the one that is asked.
- Assuming 9:3:3:1 for any two genes. Linked genes on the same chromosome break the ratio. That is the whole point of Morgan's work.
- Blaming the egg in birds. In humans the sperm decides sex. In birds it is the egg, because the female is ZW.
- Calling thalassemia and sickle-cell the same problem. One is too little globin, the other is the wrong globin.
- Guessing on a half-remembered number. A wrong answer costs one mark. Our page on NEET negative marking shows when leaving a question blank is the better choice.
Want to see whether ratios or DNA numbers are costing you marks? NEETsheet is our NEET UG prep app, with NCERT-aligned question sets and timed mocks that show exactly where Biology slips.
Sources
Every syllabus line, ratio, disorder and number above comes from these official documents, all opened and read on 13 October 2026.
- National Medical Commission, Undergraduate Medical Education Board, "Syllabus for NEET (UG) 2026", 22 December 2025, circulated by NTA (Biology Units 1 to 10, including Unit 7, Genetics and Evolution): https://nta.ac.in/Download/Notice/Notice_20260108180635.pdf
- National Testing Agency, Information Bulletin NEET (UG) 2026 (pattern of the test: Biology 90 questions and 360 marks of 720; marking of plus four and minus one; inter-se merit with Biology first): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/02/202602231394640855.pdf
- NCERT, Biology, Class 12, Chapter 4 "Principles of Inheritance and Variation" (Reprint 2026-27): https://ncert.nic.in/textbook/pdf/lebo104.pdf
- NCERT, Biology, Class 12, Chapter 5 "Molecular Basis of Inheritance" (Reprint 2026-27): https://ncert.nic.in/textbook/pdf/lebo105.pdf
- NCERT, Biology, Class 12, Chapter 6 "Evolution" (the third part of Unit 7, including the Hardy-Weinberg principle): https://ncert.nic.in/textbook/pdf/lebo106.pdf
- NCERT, online textbooks index, accessed 13 October 2026: https://ncert.nic.in/textbook.php
- National Testing Agency, NEET (UG) public notices, checked on 13 October 2026 for any NEET (UG) 2027 notice: https://neet.nta.nic.in/public-notices/
Frequently Asked Questions
Genetics for NEET covers two NCERT Class 12 Biology chapters: Chapter 4, Principles of Inheritance and Variation, and Chapter 5, Molecular Basis of Inheritance. Both sit in Unit 7, Genetics and Evolution, of the NEET (UG) 2026 syllabus. Evolution, the third part of that unit, is taught in Chapter 6.
NTA does not publish a chapter-wise question count, so no official number exists. What is fixed is that Biology has 90 questions worth 360 of the 720 marks, and Biology marks are the first tie-breaker. Any genetics-specific count you see online comes from unofficial paper analyses.
For content, yes. Every ratio, disorder, experiment and genome figure in this guide is printed in the current NCERT Class 12 Biology chapters, marked Reprint 2026-27. What NCERT alone will not give you is speed with crosses and pedigrees, which comes from solving timed mixed questions.
The syllabus names haemophilia and colour blindness as sex-linked, thalassemia as a Mendelian disorder, and Down's, Turner's and Klinefelter's syndromes as chromosomal disorders. NCERT also explains sickle-cell anaemia and phenylketonuria in the same section, so learn all eight from the chapter.
In incomplete dominance the F1 is in between the parents, as with pink snapdragons from red and white parents, and the F2 is 1 red to 2 pink to 1 white. In co-dominance the F1 shows both parental traits at once, as with the AB blood group, where both A and B sugars are made.
If you are revising genetics for NEET alongside the rest of Biology, NEETsheet is our app for practice questions and full-length mocks on the road to an MBBS seat: neetsheet.in.
