Dominant and Recessive Alleles Determine Traits
- Recall that genes exist in different forms called alleles, and these alleles determine the traits expressed in an organism:
- Dominant Alleles: Mask the effect of recessive alleles. Represented by a capital letter (e.g., T for tall plants).
- Recessive Alleles: Expressed only when both alleles are recessive. Represented by a lowercase letter (e.g., t for dwarf plants).
Dominant alleles don't "destroy" recessive ones, they simply mask their expression.
Understanding Homozygous and Heterozygous Genotypes
- Homozygous-Dominant (TT): Two dominant alleles result in the dominant trait being expressed.
- Heterozygous (Tt): One dominant allele is enough to mask the recessive allele, producing the dominant phenotype.
- Homozygous-Recessive (tt): Two recessive alleles are required for the recessive trait to appear.
Gregor Mendel studied the inheritance of traits like plant height:
- Crossing tall plants (TT) with dwarf plants (tt) produced F₁ offspring that were all tall (Tt).
- When the F₁ plants self-pollinated, the F₂ generation showed a 3:1 ratio of tall to dwarf plants.
Why Recessive Traits Are Hidden in Heterozygotes
- Recessive alleles require both copies to be recessive (tt) to express the trait.
- In heterozygotes, the dominant allele ensures the production of functional proteins, overshadowing the recessive one.
Recessive alleles are not “weaker” or less important, they are simply masked when a dominant allele is present.
