What Genetics Actually Is (And Why Mendel Got Lucky)
Genetics is the study of how traits pass from parents to offspring. Simple enough. But what makes it fascinating — and what makes it show up in every NEET paper — is that inheritance follows mathematical rules, not random chance.
Gregor Mendel discovered these rules by spending eight years counting pea plants. Not exciting, but the man was systematic. He chose the garden pea (Pisum sativum) because it has clearly distinguishable traits, a short life cycle, and — crucially — he could control which plants mated with which. His results gave us the foundation of classical genetics.
Here’s the core idea: traits are controlled by discrete units we now call genes. Each organism carries two copies of each gene (one from each parent). These copies, called alleles, can be identical or different. When they’re different, one allele often masks the other — that’s dominance. The masked one is recessive. This one observation explains why two dark-haired parents can have a light-haired child, and it’s what Mendel worked out a century before anyone knew what DNA was.
For NEET, genetics typically carries 5-6 questions. For CBSE Class 10, it’s a guaranteed 3-mark question minimum. Class 12 goes deeper — incomplete dominance, codominance, linkage — but all of it builds on what we cover here.
Key Terms and Definitions
Gene — A segment of DNA that codes for a specific trait. Located at a fixed position (locus) on a chromosome.
Allele — Different versions of the same gene. For height in peas: T (tall) and t (dwarf) are alleles of the height gene.
Dominant allele — The allele that expresses its trait even when only one copy is present. Written in uppercase (T).
Recessive allele — Expresses its trait only when both copies are present (homozygous recessive). Written in lowercase (t).
Genotype — The actual genetic makeup: TT, Tt, or tt.
Phenotype — What you can observe: tall or dwarf. Two plants with TT and Tt look identical (both tall) but have different genotypes.
Homozygous — Both alleles are the same: TT (homozygous dominant) or tt (homozygous recessive).
Heterozygous — Alleles are different: Tt. Also called a hybrid.
F1 generation — First filial generation — the direct offspring of a cross between two pure-breeding parents.
F2 generation — Offspring obtained by crossing two F1 individuals.
Test cross — Crossing an organism of unknown genotype with a homozygous recessive (tt). If any offspring show the recessive trait, the unknown parent was Tt, not TT.
In NEET MCQs, “hybrid” always means heterozygous. Don’t confuse it with the everyday usage of “hybrid” as a mix of two species — that’s something else entirely.
Mendel’s Laws
Law of Dominance
When two plants that are pure-breeding for contrasting traits are crossed, all F1 offspring show only the dominant trait. The recessive trait disappears completely in F1 — but it hasn’t been lost. It reappears in F2.
Law of Segregation (First Law)
During gamete formation, the two alleles of a gene separate from each other so that each gamete carries only one allele for each gene.
This is the most fundamental law. It explains why gametes are haploid. Each parent has two alleles; each gamete gets one; the offspring gets one from each parent, restoring the pair.
Why this matters: A Tt plant produces two types of gametes — T and t — in equal proportions (50:50). This is why F2 ratios work out to whole numbers.
Law of Independent Assortment (Second Law)
Alleles of different genes assort independently into gametes — meaning the inheritance of one trait does not affect the inheritance of another (when genes are on different chromosomes).
This holds true only for genes on different chromosomes (or far apart on the same chromosome). Linked genes don’t follow this law — that’s Class 12 territory.
The Punnett Square — Your Best Friend in Genetics
A Punnett square is a grid that shows all possible combinations of gametes from two parents. It predicts the probability of each genotype in the offspring.
How to Draw One
- Write the gametes of Parent 1 along the top
- Write the gametes of Parent 2 along the left side
- Fill each cell by combining the gamete from the top column with the gamete from the left row
- Count the genotype and phenotype ratios
Monohybrid Cross
A cross involving one pair of contrasting traits.
Classic example: Tall (TT) × Dwarf (tt)
P generation: TT × tt
Gametes: T only × t only
F1: All Tt (all tall)
F1 × F1: Tt × Tt
Gametes from each parent: T and t
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
F2 genotype ratio: 1 TT : 2 Tt : 1 tt
F2 phenotype ratio: 3 tall : 1 dwarf
CBSE 10 marking scheme: For a monohybrid cross question (3 marks), you need: (1) correct P generation cross with gametes, (2) correct Punnett square, (3) correct F2 ratio. Missing the gamete step costs 1 mark — write it explicitly.
Dihybrid Cross
A cross involving two pairs of contrasting traits. Mendel crossed Round Yellow (RRYY) × Wrinkled Green (rryy).
- Round (R) is dominant over Wrinkled (r)
- Yellow (Y) is dominant over Green (y)
F1: All RrYy — Round Yellow (all dominant phenotype)
F1 × F1: RrYy × RrYy
Each parent produces 4 types of gametes: RY, Ry, rY, ry (in equal proportions, 25% each)
The F2 Punnett square is 4×4 = 16 cells.
F2 phenotype ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
This 9:3:3:1 ratio is the signature of a dihybrid cross. Memorise it — NEET asks for it directly.
9 : 3 : 3 : 1
(Both dominant) : (Dominant 1, recessive 2) : (Recessive 1, dominant 2) : (Both recessive)
Shortcut — Multiply monohybrid ratios:
Rr × Rr → 3 Round : 1 Wrinkled
Yy × Yy → 3 Yellow : 1 Green
Combined: (3:1) × (3:1) = 9:3:3:1 ✓
This multiplication trick works only if the genes are on different chromosomes (independent assortment).
Solved Examples
Example 1 — CBSE Class 10 Level
Q: In peas, round seed (R) is dominant over wrinkled (r). A round-seeded plant (Rr) is crossed with a wrinkled-seeded plant (rr). What fraction of offspring will be wrinkled?
Solution:
Parent 1 (Rr) produces gametes: R and r
Parent 2 (rr) produces gametes: r and r only
| R | r | |
|---|---|---|
| r | Rr | rr |
| r | Rr | rr |
Offspring: 2 Rr : 2 rr → 1 Round : 1 Wrinkled
Fraction wrinkled = 1/2 or 50%
This is a test cross. The 1:1 ratio tells us the round parent was heterozygous.
Example 2 — JEE Main / NEET Level
Q: Two heterozygous plants for two independently assorting genes (AaBb × AaBb) are crossed. What is the probability of getting an offspring with genotype AAbb?
Solution:
For gene A: Aa × Aa → P(AA) = 1/4
For gene B: Bb × Bb → P(bb) = 1/4
Since genes assort independently: P(AAbb) = 1/4 × 1/4 = 1/16
Example 3 — NEET Application Level
Q: A man with blood group A (genotype I^A i) marries a woman with blood group B (genotype I^B i). List all possible blood groups in their children.
Solution:
Parent 1 gametes: I^A and i
Parent 2 gametes: I^B and i
| I^A | i | |
|---|---|---|
| I^B | I^A I^B | I^B i |
| i | I^A i | ii |
Genotypes: I^A I^B, I^B i, I^A i, ii
Phenotypes (blood groups): AB, B, A, O — all four groups are possible!
This appeared in NEET 2022. Key insight: when both parents are heterozygous carriers of recessive allele i, blood group O is possible.
Exam-Specific Tips
CBSE Class 10: Genetics is in Chapter 8 (Heredity and Evolution). Expect one 5-mark question with a Punnett square or one 3-mark + one 2-mark combination. Show all work — partial marks are given generously if your cross is set up correctly even if the final ratio is wrong.
CBSE Class 12: Chapter 5 (Principles of Inheritance). Goes beyond Mendel — incomplete dominance, codominance, multiple alleles (ABO blood groups), and sex-linked inheritance (colour blindness, haemophilia). The 5-mark genetics question in boards almost always tests Punnett square + ratio + explanation.
NEET weightage: Genetics + Molecular Basis of Inheritance together contribute 5-8 questions per year. From Genetics specifically: Mendelian crosses, deviations from Mendelism (incomplete dominance, epistasis), and sex determination are highest-yield. PYQ trend: NEET 2019, 2021, 2022, 2023 all had at least one dihybrid or trihybrid ratio question.
Common Mistakes to Avoid
Mistake 1 — Confusing genotype ratio with phenotype ratio. F2 monohybrid cross gives genotype ratio 1:2:1 (TT:Tt:tt) but phenotype ratio 3:1 (tall:dwarf). These are different things. NEET MCQs deliberately offer both as options.
Mistake 2 — Writing gametes wrong for dihybrid cross. For AaBb, students often write AB, Ab, aB, ab correctly but miss that each gamete appears with equal frequency (25%). When calculating probabilities, each gamete has probability 1/4, not 1/2.
Mistake 3 — Applying independent assortment to linked genes. The second law works only when genes are on different chromosomes. If genes are linked (same chromosome), the ratio deviates from 9:3:3:1. Class 12 students mix this up under pressure.
Mistake 4 — Test cross vs. back cross. A test cross is always with homozygous recessive (aa). A back cross is crossing F1 with either parent. These are not the same. NEET 2023 had a direct question on this distinction.
Mistake 5 — Saying recessive traits “skip a generation.” They don’t skip — they’re hidden when a heterozygous carrier mates with another carrier or homozygous dominant. Saying “skipped” in your CBSE answer will cost you marks.
Practice Questions
Q1. In a monohybrid cross between two tall plants, 1/4 of the offspring are dwarf. What are the genotypes of the parent plants?
Both parents are Tt (heterozygous). Only a Tt × Tt cross produces the 3:1 ratio where 1/4 offspring are tt (dwarf). If either parent were TT, no dwarf offspring would appear.
Q2. A black-coated rabbit (BB) is crossed with a white-coated rabbit (bb). What will be the coat colour of F1 rabbits? If F1 rabbits are interbred, what ratio of coat colours appears in F2?
F1: All Bb — all black (B is dominant over b).
F2 from Bb × Bb:
- Genotype ratio: 1 BB : 2 Bb : 1 bb
- Phenotype ratio: 3 black : 1 white
Q3. What is the probability that a child of two parents, both with genotype Aa, will be homozygous?
Aa × Aa → 1 AA : 2 Aa : 1 aa
Homozygous offspring = AA + aa = 1 + 1 = 2 out of 4
Probability = 1/2 (50%)
Q4. In a dihybrid cross AaBb × AaBb, how many offspring out of 16 will show both dominant phenotypes?
Both dominant phenotype = A_B_ (at least one A and one B allele)
From the 9:3:3:1 ratio, 9 out of 16 show both dominant phenotypes.
Answer: 9/16
Q5. A plant with round yellow seeds (RrYy) is crossed with a plant with wrinkled green seeds (rryy). What phenotypic ratio do you expect?
This is a dihybrid test cross (heterozygous × double recessive).
RrYy × rryy:
Gametes from RrYy: RY, Ry, rY, ry (equal proportions)
Gametes from rryy: ry only
Offspring:
- RrYy — Round Yellow (1/4)
- Rryy — Round Green (1/4)
- rrYy — Wrinkled Yellow (1/4)
- rryy — Wrinkled Green (1/4)
Phenotypic ratio: 1 : 1 : 1 : 1
This is how Mendel confirmed independent assortment — the 1:1:1:1 test cross ratio proves the four gamete types are produced in equal numbers.
Q6. A colour-blind man marries a woman with normal vision whose father was colour blind. What percentage of their sons will be colour blind? (Colour blindness is X-linked recessive.)
Man: X^c Y (colour blind)
Woman: Her father was colour blind, so he gave her X^c. Her own vision is normal, so she must be a carrier: X^N X^c
Cross: X^c Y × X^N X^c
Gametes: X^c, Y × X^N, X^c
Sons get Y from father and either X^N or X^c from mother.
Sons: X^N Y (normal) or X^c Y (colour blind) — in 1:1 ratio
50% of sons will be colour blind.
Q7. If a wrinkled-seeded (rr) plant is crossed with a plant of unknown genotype for seed shape, and ALL offspring have round seeds, what is the genotype of the unknown plant?
If the unknown were Rr: offspring would be 1/2 Rr (round) and 1/2 rr (wrinkled). Some wrinkled offspring would appear.
Since ALL offspring are round with no wrinkled, the unknown parent must be RR (homozygous dominant).
RR × rr → all Rr (round)
Q8. In peas, yellow seed colour (Y) is dominant over green (y), and round shape (R) is dominant over wrinkled (r). Mendel obtained 315 Round Yellow, 108 Round Green, 101 Wrinkled Yellow, and 32 Wrinkled Green in F2. Show that this approximates a 9:3:3:1 ratio.
Total offspring = 315 + 108 + 101 + 32 = 556
Expected ratio 9:3:3:1 means: 9/16, 3/16, 3/16, 1/16 of 556
- Expected Round Yellow = 9/16 × 556 = 312.75 ≈ 315 ✓
- Expected Round Green = 3/16 × 556 = 104.25 ≈ 108 ✓
- Expected Wrinkled Yellow = 3/16 × 556 = 104.25 ≈ 101 ✓
- Expected Wrinkled Green = 1/16 × 556 = 34.75 ≈ 32 ✓
The observed numbers closely match expected — confirming the 9:3:3:1 dihybrid ratio.
FAQs
Why did Mendel use pea plants and not some other organism?
Peas have several advantages: they breed true (pure lines exist), they have short generation time, they produce large numbers of offspring, and they have easily observable contrasting traits. Critically, peas are naturally self-pollinating, which let Mendel maintain pure lines and control crosses precisely. He also lucked out — most of the 7 traits he studied happen to be on different chromosomes, so independent assortment held up beautifully.
What is the difference between F1 and F2 generation?
F1 (first filial) is the direct offspring of two pure-breeding parents. F2 is obtained by crossing two F1 individuals (or self-fertilising F1 in plants). The recessive trait that disappeared in F1 reappears in F2 in a predictable ratio — this was Mendel’s key observation.
Can dominant traits skip a generation?
Dominant traits cannot skip generations — if the allele is present, it expresses itself. What can appear to “skip” is a recessive trait: when two carriers (Tt) have a child with the dominant phenotype (Tt or TT), the recessive allele is hidden. It shows up again when two carriers marry.
What is incomplete dominance and how is it different from Mendel’s results?
In incomplete dominance, neither allele fully dominates. A cross between red snapdragon (RR) and white (rr) gives pink F1 (Rr). F2 gives 1 red : 2 pink : 1 white — the phenotype ratio equals the genotype ratio (1:2:1 instead of 3:1). Mendel’s 3:1 ratio holds only when dominance is complete.
How do you find the genotype of an organism showing a dominant phenotype?
Perform a test cross — cross it with a homozygous recessive organism. If any offspring show the recessive phenotype, the organism is heterozygous. If all offspring show the dominant phenotype (and the sample size is large enough), the organism is likely homozygous dominant.
What does “true breeding” mean?
A true-breeding (pure-breeding) organism produces offspring identical to itself generation after generation when self-fertilised. Tall true-breeding plants are TT — all offspring are tall. Mendel started his experiments with true-breeding lines to ensure his crosses were clean.
Why is the 9:3:3:1 ratio only for dihybrid crosses?
The 9:3:3:1 ratio assumes two independently assorting genes, each with complete dominance, from two heterozygous parents (AaBb × AaBb). Change any condition — add a third gene, link the genes, or introduce incomplete dominance — and the ratio changes. Trihybrid cross (AaBbCc × AaBbCc) gives a 27:9:9:9:3:3:3:1 ratio across 64 total combinations.
Is Mendel’s Law of Independent Assortment always valid?
No. It holds only for genes on different (non-homologous) chromosomes or genes very far apart on the same chromosome. Genes physically close together on the same chromosome are linked and tend to be inherited together, violating independent assortment. Morgan’s work on Drosophila demonstrated this — it’s covered in Class 12.