Question
State Mendel’s three laws of inheritance and explain each using his pea plant experiments. What are the phenotypic and genotypic ratios in a monohybrid and dihybrid cross?
(NEET and CBSE 12 — one of the most consistently tested genetics topics)
Solution — Step by Step
When two organisms with contrasting traits (homozygous) are crossed, the F1 generation shows only one of the two traits — the dominant trait.
Mendel crossed tall () and dwarf () pea plants. All F1 plants were tall (). The tall trait is dominant over dwarf.
During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele.
When F1 () plants were self-pollinated:
- F2 genotypic ratio:
- F2 phenotypic ratio: 3 tall : 1 dwarf
The reappearance of the dwarf trait in F2 proved that alleles don’t blend — they segregate intact.
Alleles of different genes assort independently during gamete formation (when genes are on different chromosomes).
Mendel’s dihybrid cross: (round-yellow x wrinkled-green)
F1: all (round-yellow)
F2 ratio: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green
graph TD
A["P: TT x tt"] --> B["F1: All Tt - Tall"]
B --> C["F1 x F1: Tt x Tt"]
C --> D["TT - Tall"]
C --> E["Tt - Tall"]
C --> F["Tt - Tall"]
C --> G["tt - Dwarf"]
H["Phenotypic Ratio"] --> I["3 Tall : 1 Dwarf"]
J["Genotypic Ratio"] --> K["1 TT : 2 Tt : 1 tt"]Why This Works
Mendel’s genius was in choosing pea plants with clearly contrasting traits and working with large sample sizes (he counted thousands of offspring). The 3:1 ratio isn’t exact in a small sample — it is a statistical prediction that becomes more accurate with larger numbers.
The molecular basis we now understand: each “factor” (allele) is a DNA sequence on a chromosome. During meiosis, homologous chromosomes separate (segregation), and non-homologous chromosomes assort independently. Mendel figured out the rules without knowing about DNA.
Alternative Method
For NEET, use the branch diagram method (forked-line method) for dihybrid crosses instead of the 4x4 Punnett square. Consider each gene separately, then multiply the probabilities.
For : probability of tall = 3/4, dwarf = 1/4
For : probability of yellow = 3/4, green = 1/4
Round-yellow = . This is much faster than drawing a 16-box Punnett square.
Common Mistake
Students often confuse the Law of Dominance with the Law of Segregation. The Law of Dominance explains why the F1 looks like one parent. The Law of Segregation explains why the recessive trait reappears in F2.
Another frequent error: applying the Law of Independent Assortment to linked genes. This law holds only when genes are on different chromosomes. Genes on the same chromosome (linked genes) do not assort independently — they tend to be inherited together, which is why Mendel’s ratios break down for linked traits.