3. Alleles, Genotype & Phenotype

Most genes come in different versions, called alleles — for example, an allele for a tall plant and an allele for a short plant. Since you inherit one allele from each parent, you carry two alleles for each gene.

Some key vocabulary, using T (tall) and t (short):

• A dominant allele (capital, T) shows up even if only one copy is present.
• A recessive allele (lowercase, t) shows only when both copies are recessive.
• Genotype is the pair of alleles you have (TT, Tt, or tt); phenotype is the visible trait (tall or short).
• Homozygous means two of the same allele (TT or tt); heterozygous means two different (Tt).

So TT and Tt both look tall, because one dominant T is enough. Only tt is short.

⚠️ Common misconception: 'dominant' does not mean 'stronger,' 'better,' or 'more common.' It only means one copy is enough for the trait to appear.

💡 Tip: if an organism shows a recessive trait, its genotype must be two recessive alleles (tt) — there's no other way.