Last Updated: April 2026
Genetics and Inheritance is one of the highest-weightage chapters in NEET Biology. On average, 9–10 marks come directly from this chapter every year — making it non-negotiable for any serious NEET 2027 aspirant. This comprehensive guide covers Mendel’s experiments, all three laws, the Chromosomal Theory, sex determination, linkage, and crossing over — with NCERT alignment and MCQ practice built in.
Why Genetics Matters in NEET 2027
NEET Biology is divided into Botany and Zoology, each carrying 45 questions (180 marks). Genetics appears across both sections and is one of the most consistent high-yield chapters. According to historical NEET papers (2018–2024), Genetics + Inheritance contributes approximately 9–11 questions per year. A student who masters this chapter effectively secures 36–44 marks.
Chapter-wise Weightage Table
| Topic | Avg. Questions (NEET) | Marks |
|---|---|---|
| Mendel’s Laws and Monohybrid Cross | 2–3 | 8–12 |
| Dihybrid Cross and Incomplete Dominance | 1–2 | 4–8 |
| Chromosomal Theory of Inheritance | 1 | 4 |
| Sex Determination (XX/XY, ZW/ZZ) | 1 | 4 |
| Linkage and Crossing Over | 1 | 4 |
| Mutation and Genetic Disorders | 1–2 | 4–8 |
| Total | 9–10 | 36–40 |
Mendel’s Experiments with Pea Plants
Gregor Johann Mendel (1822–1884) performed his landmark experiments using the garden pea plant, Pisum sativum, over a period of seven years (1856–1863). He chose pea plants because they:
- Have a short life cycle and produce many offspring
- Can self-fertilize naturally, ensuring pure lines
- Show clearly contrasting traits (7 pairs studied)
- Are easy to cross-pollinate artificially
The 7 Contrasting Traits Mendel Studied
| Trait | Dominant | Recessive |
|---|---|---|
| Seed Shape | Round | Wrinkled |
| Seed Colour | Yellow | Green |
| Pod Shape | Inflated | Constricted |
| Pod Colour | Green | Yellow |
| Flower Colour | Violet | White |
| Flower Position | Axial | Terminal |
| Plant Height | Tall | Dwarf |
Mendel’s Three Laws of Inheritance
Law 1: Law of Dominance
When two homozygous parents differing in one pair of contrasting traits are crossed, the trait that appears in the F1 generation is called the dominant trait, and the one that is suppressed is called the recessive trait. In a monohybrid cross:
- F1 ratio: All dominant phenotype (e.g., all Tall)
- F2 ratio: 3 Dominant : 1 Recessive
- F2 genotypic ratio: 1 TT : 2 Tt : 1 tt
Law 2: Law of Segregation (Purity of Gametes)
The two alleles of a gene separate (segregate) from each other during gamete formation, so each gamete receives only one allele. This is also called the Purity of Gametes principle. Key points:
- Alleles do not blend — they remain distinct
- Each gamete carries only one allele per gene
- Alleles reunite randomly at fertilization
Law 3: Law of Independent Assortment
When two pairs of contrasting traits are considered simultaneously (dihybrid cross), each pair of alleles segregates independently of the other. This applies only when genes are located on non-homologous (different) chromosomes. Key outcomes:
- F1 dihybrid: All dominant phenotype (e.g., Round Yellow)
- F2 phenotypic ratio: 9:3:3:1
- F2 genotypic ratio: 1:2:1:2:4:2:1:2:1 (9 genotypes)
Test Cross
A test cross involves crossing the unknown individual (possibly dominant) with a homozygous recessive parent (tt or rryy). It reveals the genotype of the organism being tested. If offspring show 1:1 ratio, the organism is heterozygous; if all offspring are dominant, it is homozygous dominant.
Chromosomal Theory of Inheritance
Proposed independently by Walter Sutton (grasshoppers) and Theodor Boveri (sea urchins) around 1902, this theory states:
- Genes are carried on chromosomes
- Chromosomes occur in pairs (homologous chromosomes)
- One chromosome of each pair comes from each parent
- Chromosomes — and therefore genes — segregate independently during meiosis
This theory directly explained the mechanism behind Mendel’s Laws. Morgan later confirmed the theory through experiments on Drosophila melanogaster.
Sex Determination
XX/XY System (Humans and Most Mammals)
- Females: XX (produce X-type eggs only)
- Males: XY (produce X or Y sperm)
- Sex is determined by the father (which sperm fertilizes the egg)
- Probability: 50% XX (female), 50% XY (male)
ZW/ZZ System (Birds, Some Reptiles and Fish)
- Males: ZZ
- Females: ZW
- Sex is determined by the mother (which egg is produced)
XO System (Grasshoppers)
- Females: XX (2 sex chromosomes)
- Males: XO (1 sex chromosome)
- No Y chromosome present in males
Linkage and Crossing Over
Linkage
Genes located on the same chromosome tend to be inherited together — this is called linkage. Linked genes do not show independent assortment. T.H. Morgan discovered linkage while working with Drosophila. He found that genes on the same chromosome are transmitted as a group called a linkage group.
Crossing Over
During Prophase I of meiosis, homologous chromosomes pair up and exchange segments at points called chiasmata. This process is called crossing over. It results in:
- Recombinant gametes with new combinations of alleles
- Increased genetic variation in offspring
- Recombination frequency = (recombinant offspring / total offspring) × 100%
The recombination frequency is used to construct genetic maps (linkage maps) — genes farther apart show higher recombination frequency.
Sex-Linked Inheritance
Genes located on sex chromosomes are called sex-linked genes. Most sex-linked traits are X-linked. Examples in humans:
- Haemophilia A — X-linked recessive (factor VIII deficiency)
- Colour Blindness — X-linked recessive
- Duchenne Muscular Dystrophy — X-linked recessive
Males (XY) are more frequently affected by X-linked recessive traits because they have only one X chromosome. Females (XX) can be carriers without showing the trait.
Important Formulas and Ratios for NEET
| Cross Type | F2 Phenotypic Ratio | F2 Genotypic Ratio |
|---|---|---|
| Monohybrid (complete dominance) | 3:1 | 1:2:1 |
| Monohybrid (incomplete dominance) | 1:2:1 | 1:2:1 |
| Dihybrid (independent assortment) | 9:3:3:1 | 1:2:1:2:4:2:1:2:1 |
| Test Cross (heterozygous) | 1:1 | 1:1 |
Practice MCQs — NEET 2027
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Frequently Asked Questions (FAQs)
Q1: What is the difference between genotype and phenotype?
Genotype refers to the genetic makeup of an organism (e.g., Tt, TT, tt). Phenotype refers to the physical expression of that genotype (e.g., Tall or Dwarf). The same phenotype can have different genotypes (e.g., both TT and Tt are Tall).
Q2: Why did Mendel choose 7 traits for his experiments?
Mendel chose 7 pairs of traits because the pea plant (Pisum sativum) has 7 chromosomes. Each of the 7 traits happened to be located on a different chromosome, which allowed them to assort independently — perfectly demonstrating the Law of Independent Assortment. This was largely fortuitous but gave clear, unambiguous results.
Q3: What is incomplete dominance and how is it different from co-dominance?
In incomplete dominance, neither allele is fully dominant and the F1 shows an intermediate phenotype (e.g., red × white = pink in snapdragons). In co-dominance, both alleles are expressed simultaneously in the F1 (e.g., ABO blood group — IAIB shows both A and B antigens). The key difference: incomplete dominance gives blending, co-dominance gives both traits simultaneously.
Q4: How many marks can I score from Genetics in NEET 2027?
Based on 2018–2024 NEET paper analysis, Genetics typically contributes 9–11 questions worth 36–44 marks. Scoring 8/10 correct in this chapter alone can mean a significant leap in your overall rank. Mastering Mendel’s Laws, dihybrid ratios, sex determination, and linkage guarantees high returns.
Start Your NEET 2027 Preparation Today
Genetics is the gateway to scoring 360+ in Biology. Combine your NCERT reading with strategic MCQ practice, revision of ratios, and pedigree analysis to maximize your marks. Explore our complete Biology course at NEET Gurukul Courses for chapter-wise video lessons, mock tests, and expert doubt resolution.
Tip: Revise the 9:3:3:1 ratio, test cross, and ZW/ZZ system — these appear almost every year in NEET!