How do dominant and recessive alleles affect an animal's visible traits?

In animal breeding, an animal's visible traits—known as its phenotype—are determined by the interaction of its genotype and environmental factors. When a trait is controlled by a single gene with a dominant/recessive relationship, the expression of that trait depends on the specific alleles inherited from the parents.123

• Dominant Alleles: A dominant allele exerts greater influence and will be expressed in the phenotype even if only one copy is present. In a heterozygous pairing (where the two alleles are different), the dominant allele masks or overrides the effect of the recessive allele.4561
• Recessive Alleles: A recessive allele is only expressed in the physical appearance if the animal inherits two copies of it—one from each parent. If an animal is heterozygous, the recessive trait remains hidden, though it can still be passed to offspring.41

Because dominant alleles mask recessive ones, breeders often find it difficult to distinguish between homozygous dominant and heterozygous individuals, which complicates the selection process. In some cases, alleles may also be codominant, meaning neither allele masks the other, resulting in a combined or distinct expression of both.16