Question
Mendel crossed a tall pea plant (Tt) with a short pea plant (tt). What are the expected genotypes and phenotypes of the offspring? Draw the Punnett square and state the ratios.
This is the classic testcross — a cornerstone of genetics that shows up in NEET and CBSE Class 10 boards almost every year.
Solution — Step by Step
The tall parent is Tt (heterozygous — one dominant allele, one recessive). The short parent is tt (homozygous recessive — both alleles recessive). We’re told this, so no guesswork needed.
A parent can only pass one allele per gamete (this is Mendel’s Law of Segregation).
- Tt parent produces gametes: T and t (in equal proportion)
- tt parent produces gametes: t and t (only one type)
Set up a 2×2 grid. Parent 1 gametes go across the top, Parent 2 gametes go down the side:
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
Two boxes give Tt, two boxes give tt.
Genotype ratio: 1 Tt : 1 tt
Half the offspring are heterozygous tall, half are homozygous short. Each with 50% probability.
Since T (tall) is dominant over t (short):
- Tt → Tall (one dominant allele is enough)
- tt → Short (no dominant allele)
Phenotype ratio: 1 Tall : 1 Short
The final answer: 50% tall (Tt) and 50% short (tt) offspring.
Why This Works
Mendel’s Law of Segregation says alleles separate during gamete formation — each gamete carries only one allele for each trait. When gametes fuse randomly at fertilisation, we get the combinations shown in the Punnett square.
The reason we cross with tt (not TT or another Tt) is that tt contributes only t gametes. This means whatever shows up in the offspring comes entirely from the other parent. That’s why geneticists use testcrosses — it reveals the hidden genotype of the unknown parent.
If the offspring were all tall, you’d know the other parent was TT. If half are short, the other parent must have been Tt. The tt parent acts like a genetic “revealer.”
Alternative Method — Forked Line / Branch Method
For quick calculations without drawing the full grid, use the probability method:
- P(T from Tt parent) = 1/2, P(t from Tt parent) = 1/2
- P(t from tt parent) = 1 (certainty)
Same result: 1/2 Tt : 1/2 tt, which is ratio 1:1.
This method is faster for MCQs in NEET where drawing a full Punnett square wastes precious seconds.
In NEET, any time you see a 1:1 phenotype ratio, think testcross (Tt × tt). A 3:1 ratio means both parents were Tt × Tt. These two ratios are the most tested in genetics MCQs.
Common Mistake
Students often write the genotype ratio as 2 Tt : 2 tt instead of simplifying to 1:1. Both are technically correct, but NCERT and NEET answer keys always expect the simplified ratio. Always divide by the GCF. Also, many students confuse genotype ratio with phenotype ratio here — since T is dominant, all Tt plants look tall, so phenotype ratio stays 1:1 (same as genotype ratio in this specific cross, but that won’t always be the case).