Genetics

Allele

A variant form of a gene occupying a specific position (locus) on a chromosome. Dogs inherit two alleles for each gene — one from each parent — and the combination determines traits such as coat color, size, and disease susceptibility.

An allele is one of two or more variant forms of a gene that can exist at a particular locus (position) on a chromosome. Because dogs are diploid organisms — meaning they carry two copies of each chromosome — every dog has two alleles for each gene: one inherited from the sire and one from the dam. The specific combination of alleles a dog carries is its genotype; the observable trait that results is its phenotype.

Dominant, Recessive, and Co-dominant Alleles

Alleles interact in several ways:

  • Dominant alleles produce their associated trait even when only one copy is present. A dog with one dominant and one recessive allele (heterozygous) expresses the dominant phenotype.
  • Recessive alleles require two copies (homozygous recessive) to produce the associated trait. This is why a dog can be a carrier of a disease-causing allele without showing symptoms.
  • Co-dominant or incompletely dominant alleles produce an intermediate or blended phenotype when both are present. Merle coat patterning is a well-known example: one merle allele produces the merle pattern, while two copies (double merle) can cause deafness and eye defects.

Alleles and Disease Risk

Many hereditary conditions in dogs follow predictable inheritance patterns based on allele combinations:

  • Progressive retinal atrophy (PRA) is typically autosomal recessive — two copies of the mutant allele are needed for clinical disease.
  • Von Willebrand disease involves alleles affecting clotting factor production; Type 1 is autosomal recessive in most breeds.
  • Degenerative myelopathy (SOD1 mutation) requires two copies of the risk allele for clinical disease, though penetrance is incomplete.

Why Alleles Matter for Longevity

Understanding a dog’s allele profile through DNA panel testing allows owners and breeders to:

  • Identify carriers of recessive disease alleles before breeding
  • Assess risk for late-onset conditions and begin early screening
  • Make informed decisions about inbreeding coefficients and genetic diversity
  • Monitor for conditions linked to specific allele combinations (e.g., the MDR1 drug sensitivity allele common in herding breeds)

Genome-wide association studies (GWAS) in dogs have identified alleles associated with lifespan variation between and within breeds, connecting specific genetic variants to aging rate, cancer susceptibility, and metabolic efficiency.