Last Updated: April 2026
Genetics and Inheritance is one of the highest-scoring chapters in NEET Biology 2027, contributing 5-8 questions per year from NCERT Class 12 Biology Unit 3. Mendel’s laws, chromosomal disorders, sex determination, and linkage are virtually guaranteed in every NEET paper. This complete guide covers every concept with NEET-style explanations, diagrams in text form, and 40 practice MCQs with detailed solutions.
NEET Genetics Weightage and Important Topics
| Topic | NEET Questions/Year (Avg.) | Difficulty |
|---|---|---|
| Mendel’s Laws + Monohybrid Cross | 2-3 | Easy-Medium |
| Dihybrid Cross + Incomplete Dominance | 1-2 | Medium |
| Chromosomal Disorders | 1-2 | Easy |
| Sex Determination | 1 | Easy |
| Linkage and Crossing Over | 1 | Medium |
| Mutations | 1 | Medium |
| Pedigree Analysis | 1 | Medium-Hard |
Chapter 1: Mendel’s Laws of Inheritance
Gregor Johann Mendel’s Experiments
Mendel worked on Pisum sativum (garden pea) from 1856-1863 and selected 7 pairs of contrasting traits. His success factors:
- Large sample size (eliminating chance variation)
- Use of true-breeding (pure) varieties
- Study of one trait at a time (monohybrid) before studying two (dihybrid)
- Mathematical analysis of results
Mendel’s 7 Contrasting Characters in Pea
| Character | Dominant Trait | Recessive Trait |
|---|---|---|
| Seed shape | Round (R) | Wrinkled (r) |
| Seed colour (cotyledon) | Yellow (Y) | Green (y) |
| Flower colour | Violet/Purple (V) | White (v) |
| Pod shape | Inflated/Full (I) | Constricted (i) |
| Pod colour | Green (G) | Yellow (g) |
| Flower position | Axial (A) | Terminal (a) |
| Plant height | Tall (T) | Dwarf (t) |
Law 1: Law of Dominance
In a cross between two pure-breeding organisms differing in one character, the F1 generation shows only the dominant trait. The recessive trait is suppressed but NOT lost.
Monohybrid Cross Example: TT (tall) × tt (dwarf)
- F1 generation: All Tt (tall) — dominance of T over t
- F1 × F1: Tt × Tt → 1TT : 2Tt : 1tt (genotypic ratio) = 3 tall : 1 dwarf (phenotypic ratio)
Law 2: Law of Segregation (Purity of Gametes)
The two alleles of a gene pair segregate during gamete formation. Each gamete receives only one allele of each pair. This is the most fundamental Mendelian law and is supported by meiosis.
Law 3: Law of Independent Assortment
Genes for different traits (located on different chromosomes) assort independently during gamete formation. Basis of dihybrid cross.
Dihybrid Cross (RRYY × rryy):
- F1: All RrYy (Round-Yellow — both dominant)
- F2 phenotypic ratio: 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green
- F2 genotypic ratio: 1:2:1:2:4:2:1:2:1 (9 types)
Chapter 2: Extensions of Mendelian Genetics
Incomplete Dominance
F1 shows intermediate phenotype. Example: Red snapdragon (RR) × White (rr) → Pink F1 (Rr). F2 ratio: 1 Red : 2 Pink : 1 White (phenotypic = genotypic ratio = 1:2:1).
Co-dominance
Both alleles are equally expressed. Classic example: ABO blood group. IA and IB alleles are co-dominant; i is recessive. Blood group AB shows both A and B antigens on RBCs.
ABO Blood Group — NEET Must-Know Table
| Blood Group | Genotype | Antigen on RBC | Antibody in Plasma |
|---|---|---|---|
| A | IAIA or IAi | A | Anti-B |
| B | IBIB or IBi | B | Anti-A |
| AB | IAIB | A and B | None |
| O | ii | None | Anti-A and Anti-B |
Pleiotropy
A single gene affecting multiple phenotypic traits. Example: Sickle cell anaemia (HbS gene) causes not only anaemia but also organ damage. Phenylketonuria (PKU) — mutation in phenylalanine hydroxylase gene causes mental retardation, skin/hair hypopigmentation.
Chapter 3: Chromosomal Theory of Inheritance
Sutton and Boveri (1902) proposed chromosomal theory — genes are located on chromosomes. Morgan confirmed this through Drosophila experiments.
Sex Determination
| Organism | Female | Male | Mechanism |
|---|---|---|---|
| Humans, Drosophila | XX | XY | Male determines sex through Y chromosome |
| Grasshopper (XO type) | XX | XO | Females have 2 sex chromosomes; males have 1 |
| Birds (ZW type) | ZW | ZZ | Female determines sex (opposite to humans) |
| Honey bee | Diploid (2n) | Haploid (n) | Haplodiploidy — unfertilised eggs become males |
Chapter 4: Chromosomal Disorders
These are caused by non-disjunction during meiosis — failure of chromosomes to separate properly.
Aneuploidy Disorders in Humans — NEET Quick Reference
| Disorder | Chromosomal Makeup | Key Features |
|---|---|---|
| Down’s Syndrome (Trisomy 21) | 47, +21 (2n+1) | 47 chromosomes, short stature, characteristic facial features, intellectual disability, palm crease |
| Turner’s Syndrome | 45, XO (2n-1) | 45 chromosomes, female, sterile, short stature, webbed neck, no secondary sexual characters |
| Klinefelter’s Syndrome | 47, XXY (2n+1) | 47 chromosomes, male, gynaecomastia, sterile, tall, small testes |
| Super Female (Trisomy X) | 47, XXX | Female, fertile, generally normal phenotype |
| Jacob’s Syndrome | 47, XYY | Male, tall, normal intelligence, sometimes aggressive behaviour (disputed) |
Mendel’s Postulates vs Modern Understanding
- Mendel’s “factors” = Genes (located on chromosomes)
- “Alternative forms” = Alleles
- Independent assortment only applies to genes on DIFFERENT chromosomes; genes on the same chromosome show linkage
Chapter 5: Mutations
A sudden heritable change in genetic material. Types:
- Point/Gene mutation: change in single nucleotide pair (e.g., sickle cell anaemia — single base change in β-globin gene)
- Chromosomal mutation: deletion, duplication, inversion, translocation
- Frameshift mutation: insertion or deletion of nucleotides — shifts reading frame
- Mutagens: agents causing mutation — UV radiation (thymine dimers), X-rays, chemical mutagens (colchicine, EMS)
Pedigree Analysis for NEET
NEET pedigree questions test your ability to determine inheritance pattern from family trees. Quick rules:
- If trait skips a generation → recessive
- If trait appears in every generation → dominant (usually)
- If more males affected → X-linked recessive (e.g., haemophilia, colour blindness)
- If only females affected → X-linked dominant possible
- Carrier females: phenotypically normal but have one X with mutant allele
40 Practice MCQs — Genetics and Inheritance
Practice 10 MCQs below. For full 40-question NEET-level practice set, attempt our mock test.
- Mendel’s Law of Independent Assortment applies to genes located on: (a) same chromosome (b) different chromosomes (c) mitochondria (d) plasmids — Answer: (b)
- Down’s Syndrome is caused by trisomy of chromosome number: (a) 18 (b) 21 (c) 13 (d) X — Answer: (b)
- In ABO blood group, IAIB genotype results in blood group: (a) A (b) B (c) AB (d) O — Answer: (c) — Co-dominance
- Turner’s Syndrome individual has how many chromosomes? (a) 44 (b) 45 (c) 46 (d) 47 — Answer: (b) 45 (XO)
- Sickle cell anaemia is an example of: (a) Co-dominance only (b) Incomplete dominance only (c) Both Co-dominance and Pleiotropy (d) Sex-linkage — Answer: (c)
- In birds, sex determination follows: (a) XY system (b) XO system (c) ZW system (d) Haplodiploidy — Answer: (c)
- F2 phenotypic ratio in a monohybrid cross is: (a) 1:2:1 (b) 3:1 (c) 9:3:3:1 (d) 1:1 — Answer: (b)
- Klinefelter’s syndrome individuals have: (a) 44+XX (b) 44+XO (c) 44+XXY (d) 44+XYY — Answer: (c)
- Linkage was first discovered by: (a) Mendel (b) Morgan (c) Sutton (d) Watson — Answer: (b) T.H. Morgan (in Drosophila)
- Colchicine causes mutation by: (a) causing base pair substitution (b) inhibiting spindle formation leading to polyploidy (c) UV-induced thymine dimers (d) frameshift insertion — Answer: (b)
Frequently Asked Questions (FAQ)
How many questions come from Genetics in NEET 2027?
Genetics and Principles of Inheritance (Chapter 5, NCERT Class 12 Biology) typically yields 5-8 questions in NEET. This makes it one of the highest-weightage chapters after Ecology, Plant Kingdom, and Human Physiology. Combined with Molecular Basis of Inheritance (Chapter 6), the genetics unit can contribute 10-14 questions — making it critical for NEET 2027 Biology.
What is the difference between genotype and phenotype?
Genotype is the genetic constitution of an organism — the actual alleles present (e.g., TT, Tt, tt). Phenotype is the observable physical expression of the genotype (e.g., tall or dwarf). Two organisms with different genotypes (TT and Tt) can have the same phenotype (both tall, since T is dominant). The phenotype depends on both genotype AND environment.
Why did Mendel choose Pisum sativum for his experiments?
Mendel chose garden pea for several reasons: (1) Easy to grow with short generation time; (2) Available in true-breeding varieties; (3) Has clearly distinguishable contrasting traits; (4) Flowers are bisexual allowing self-pollination (controlled breeding); (5) Cross-pollination is also easily achievable by hand; (6) Produces large numbers of seeds for statistical analysis.
Related Reading: NEET Biology Human Reproduction | NEET Cell Division Notes | NEET Ecosystem Chapter