Genes and Alleles
A gene is a segment of DNA that codes for a trait. Each gene has two copies — one from each parent — called alleles. Alleles can be dominant (uppercase: B) or recessive (lowercase: b).
If you have even one dominant allele, the dominant trait shows. Recessive traits only show when you have two recessive alleles (bb).
Genotype and Phenotype
Genotype = the allele combination (BB, Bb, or bb). Phenotype = the visible trait that is expressed.
BB and Bb have the same phenotype (dominant trait shows), but different genotypes. A Punnett square predicts probability ratios — not guarantees.