How It Works: Dominant and Recessive Alleles

How It Works: Dominant and Recessive Alleles

Every organism inherits two copies of each gene — one from each parent. These two copies are called alleles. If both alleles are the same (e.g., BB or bb), the organism is homozygous for that gene. If the alleles are different (e.g., Bb), the organism is heterozygous.

When a dominant allele is present, it is always expressed in the phenotype — even if there is only one copy. A recessive allele is only expressed in the phenotype when two copies are present (homozygous recessive, e.g., bb). In a heterozygous individual (Bb), the dominant allele "masks" the recessive allele at the molecular level — the dominant allele produces a functional protein that determines the phenotype, while the recessive allele either produces a non-functional protein or none at all.

Punnett squares are used to predict the probability of offspring genotypes. Parents contribute one allele to each gamete (egg or sperm) through meiosis, and the Punnett square maps out all possible combinations when gametes fuse at fertilisation.