Question
Explain Mendel’s Law of Segregation using a monohybrid cross between a tall (TT) and dwarf (tt) pea plant. Show the F1 and F2 generations using a Punnett square.
(NCERT Class 12, fundamental NEET question)
Solution — Step by Step
Parents: Tall plant (TT) x Dwarf plant (tt)
T = dominant allele (tall), t = recessive allele (dwarf)
The tall parent produces only T gametes. The dwarf parent produces only t gametes.
All F1 offspring receive one T from the tall parent and one t from the dwarf parent.
F1 genotype: All Tt (heterozygous)
F1 phenotype: All Tall (because T is dominant over t)
This is the principle of dominance — in a heterozygote, only the dominant trait is expressed.
Cross: Tt x Tt
Each F1 parent produces two types of gametes: T and t (in equal proportion).
Punnett square:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
F2 Genotypic ratio: 1 TT : 2 Tt : 1 tt
F2 Phenotypic ratio: 3 Tall : 1 Dwarf = 3:1
Mendel’s Law of Segregation (First Law) states: During gamete formation, the two alleles of a gene separate (segregate) from each other so that each gamete carries only one allele.
In the Tt parent, during meiosis, the T and t alleles separate into different gametes. Half the gametes carry T, half carry t. This random segregation produces the 3:1 ratio in F2.
Why This Works
The law of segregation has a physical basis in meiosis. Homologous chromosomes (one carrying T, the other carrying t) separate during Anaphase I of meiosis. Each daughter cell receives only one chromosome of the homologous pair, and therefore only one allele.
The 3:1 phenotypic ratio emerges because there are three genotypes that produce the tall phenotype (TT + 2Tt) and only one that produces the dwarf phenotype (tt). The ratio 1:2:1 in genotypes translates to 3:1 in phenotypes due to dominance.
For NEET, know the test cross: crossing F1 (Tt) with homozygous recessive (tt) gives a 1:1 ratio (Tt : tt). This test cross was Mendel’s tool to confirm that F1 plants were heterozygous, not homozygous dominant.
Common Mistake
Students sometimes write the F2 ratio as 3:1 for genotypes. The 3:1 ratio is the phenotypic ratio. The genotypic ratio is 1:2:1 (1 TT : 2 Tt : 1 tt). Confusing phenotypic and genotypic ratios costs marks in NEET.
Another error: assuming the law of segregation means alleles always separate in a 1:1 ratio among offspring. The 1:1 separation happens in gametes of a heterozygous individual. The offspring ratio depends on which gametes combine — the 3:1 ratio appears only in F2 of a monohybrid cross between two heterozygotes.