Mendel's Laws > The Law of Dominance

The law of dominance states that, out of a pair of allelomorphic characters, one is dominant and the other recessive.

1. In a pair of contrasting traits, only one trait is expressed—this is the dominant trait.
2. The trait that remains unexpressed is called recessive.
3. The recessive trait can express itself only when both alleles are recessive (homozygous recessive).

Or

When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F₁ are dominant and those which do not appear in F₁ are recessive.

The Law of Dominance states:
"When parents with pure, contrasting traits are crossed together, only one form of the trait appears in the F₁ generation. The hybrid offspring exhibit only the dominant trait in their phenotype. The allele that is expressed in the hybrid condition is called the dominant allele, and the allele whose expression is suppressed is called the recessive allele."​

• Also known as Mendel's First Law of Inheritance​
• The law governs the expression of traits, not their transmission (that is governed by the Law of Segregation)​
• Applies specifically to conditions involving a heterozygous (dissimilar) pair of alleles

Step 1: Parental Selection:

Mendel selected pure-breeding plants - Tall (TT) and Dwarf (tt) - as parents. Both were true-breeding, meaning they consistently produced only their own type across generations.

Step 2: P-Generation Cross:

P: TT × tt (Tall) (Dwarf)

Gametes: T and t

Step 3: F₁ Observation

All F₁ offspring were Tall (Tt) - not a single dwarf appeared.

• Genotype: Tt (100%)
• Phenotype: All Tall → Tall "dominates" over Dwarf

Step 4: F₂ Generation (Self-cross F₁)

F₁ × F₁: Tt × Tt

Punnett Square: 

Genotype Ratio: 1 TT : 2 Tt : 1 tt

Phenotype Ratio: 3 Tall : 1 Dwarf

The dwarf trait reappeared in F₂, proving it was only masked in F₁ - not destroyed.

Step 5: Conclusion

• T allele → produces a functional protein → dominant trait expressed
• t allele → produces no/non-functional protein → recessive trait hidden
• In Tt, one working copy of T is enough to show the dominant phenotype
• Only in tt (no functional protein at all) does the recessive phenotype appear