Question
Explain Mendel’s Law of Segregation using a monohybrid cross. Take the example of tall (T) and dwarf (t) pea plants. Show the F₁ and F₂ generation results using a Punnett square (checker board). What ratio do we get in F₂?
Solution — Step by Step
Mendel crossed a pure-breeding tall plant (TT) with a pure-breeding dwarf plant (tt). “Pure-breeding” means both alleles are identical — homozygous. These are the P (parental) generation.
Each parent contributes one allele to the offspring. TT gives only T gametes; tt gives only t gametes. Every F₁ plant gets one T and one t.
All F₁ plants are tall because T is dominant over t. This is the Law of Dominance at work — but the t allele hasn’t disappeared. It’s hiding.
Now we self-pollinate the F₁ (Tt × Tt). Each Tt plant produces two types of gametes: T and t, in equal proportions. Fill the checker board:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
F₂ genotypes: 1 TT : 2 Tt : 1 tt
TT and Tt both appear tall (T is dominant). Only tt is dwarf.
- Tall (TT + Tt) = 3 boxes
- Dwarf (tt) = 1 box
This 3:1 ratio in F₂ is the signature result of every monohybrid cross.
The Punnett square shows WHY the law works. During gamete formation (meiosis), the two alleles of a gene segregate from each other — each gamete gets only one allele. They come back together at fertilisation, randomly.
This is Mendel’s Law of Segregation (also called his First Law).
Why This Works
The key insight is that alleles exist in pairs in somatic cells but travel solo into gametes. Meiosis physically separates homologous chromosomes, and since each chromosome carries one allele, each gamete is guaranteed to carry only one copy.
When two Tt plants cross, each parent independently sends either T or t into its gamete. The four possible combinations (TT, Tt, Tt, tt) occur with equal probability — this is why the Punnett square works as a probability tool, not just a diagram.
The t allele wasn’t lost in F₁ — it was simply masked by T. The 3:1 ratio in F₂ is direct proof that the recessive allele survived intact inside the heterozygous Tt plants.
NEET shortcut: Whenever you see a 3:1 phenotype ratio, immediately recognise it as a monohybrid cross with complete dominance. The genotype ratio underneath is always 1:2:1. These two ratios are tested together almost every year.
Alternative Method — Using Probability
Instead of drawing the full checker board, use the multiplication rule. For Tt × Tt:
- Probability of T gamete from each parent = 1/2
- Probability of tt offspring = 1/2 × 1/2 = 1/4
- Probability of TT = 1/2 × 1/2 = 1/4
- Probability of Tt = 2 × (1/2 × 1/2) = 2/4 = 1/2
Phenotype ratio: P(tall) = 3/4, P(dwarf) = 1/4 → 3:1. Same answer, faster calculation. This method scales much better when you move to dihybrid crosses.
Common Mistake
Don’t say “the recessive allele disappears in F₁.” Many students write that the dwarf trait is “lost” in F₁. It is not — it is only unexpressed. The F₂ generation bringing back dwarf plants (tt) is Mendel’s proof that the allele was preserved unchanged inside the F₁ Tt plants. NCERT explicitly tests this concept: the term is “blending inheritance vs particulate inheritance” — Mendel’s model is particulate, alleles stay distinct.
Quick Summary
| Generation | Cross | Result |
|---|---|---|
| P | TT × tt | All Tt (tall) |
| F₁ | Tt × Tt (self) | 1TT : 2Tt : 1tt |
| F₂ phenotype | — | 3 tall : 1 dwarf |
The 3:1 ratio and the reappearance of the recessive trait in F₂ are the two observations that Mendel used to build the Law of Segregation. Every monohybrid cross question in NEET and CBSE boards comes back to these two numbers.