Home » Pet Health » Genes and Cancer » Dominant and Recessive Allelese Explained


Genes contain information, like a blueprint, about the features of living things. There is a unique set of genes in every organism. Most human and animal features (for example, eye color) and illnesses (for example, cancer) result from genetic factors.

Genes have different versions called alleles, where every individual inherits one allele per gene from each parent. An allele can be dominant or recessive, and a dominant allele expresses its trait to a greater degree than the other allele of that gene. Dominant alleles can determine the characteristic even when the individual has only one dominant allele. On the other hand, two recessive alleles must be present to have the recessive trait expressed. In essence, the dominant allele masks or overrides the effects of a recessive allele.

For instance, the allele for brown eyes is dominant (represented by ‘B’) over blue eyes (represented by ‘b’), which is recessive. The inheritance of even one copy of an individual’s dominant brown eye allele (Bb) will lead to brown eyes, as you can see in the table below. Mom and dad have brown eyes (Bb), even though one of the alleles is dominant (B) and one is recessive (b). The offspring with brown eyes can be the same as mom and dad, or they can have both copies of the dominant allele for brown eyes (BB). For a recessive trait, like blue eyes, to be expressed, the individual must inherit two copies of the recessive allele. In the case of eye color, one blue eye allele (b) comes from each parent to give blue eyes (bb).

B (mom) b (mom)
(Brown eyes)
(Brown eyes)
(Brown eyes)
(Blue eyes)

Similarly, when it comes to diseases, a dominant allele will lead to the development of the disease even when an individual inherits a single dominant allele from either of the parents. Examples of dominant diseases include Huntington’s disease in humans and polycystic kidney disease in dogs.

As in eye color, some diseases result from two recessive alleles. Cystic fibrosis in humans is an example of a recessive disorder. Similarly, degenerative myelopathy in dogs, a condition that damages the spinal cord leading to paralysis, is a recessive disorder.

If an individual inherits only one recessive copy of a disease-causing gene from a parent, the individual is a carrier. These individuals do not show the symptoms of the disease, but they can pass on the mutation to their offspring. For example, if you breed two dogs that are carriers for a recessive illness, like in the eye color example above, there is a 25 percent chance that a pup will get the disease. Therefore, it is essential to identify dogs and cats that are carriers before breeding. If you want to get your pet from a breeder, speak to them about the genetic line. Many reputable breeders will offer certification of a disease-free genetic heritage, providing you with the peace of mind you need.

Sometimes it’s hard to know your pet’s risks, especially if you don’t know your pet’s origin. Therefore, speaking to your vet and discussing potential genetic testing and risks your pet may be facing is important.

The Pet Cancer Foundation’s Website Editorial team is comprised of veterinarians, veterinary oncologists, and veterinary technicians, as well as scientific writers and editors who have attained their PhD’s in the life sciences, along with general editors and research assistants. All content found in this section goes through an extensive process with multiple review stages, to ensure this extended resource provides pet families with the most up-to-date information publicly available.

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Last Updated: October 20, 2022

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The following sources were referenced to write the content on this page: 

Gargani, M, Valentini, A & Pariset, L 2011, ‘A novel point mutation within the EDA gene causes an exon dropping in mature RNA in Holstein Friesian cattle breed affected by X-linked anhidrotic ectodermal dysplasia’, BMC Vet Res, vol. 7, no. 1, pp. 35-42.

Gharahkhani, P, O’Leary, CA, Kyaw-Tanner, M, Sturm, RA & Duffy, DL 2011, ‘A non-synonymous mutation in the canine Pkd1 gene is associated with autosomal dominant polycystic kidney disease in Bull Terriers’, PLoS One, vol. 6, no. 7, pp. e22455-e22455.

Holder, AL, Price, JA, Adams, JP, Volk, HA & Catchpole, B 2014, ‘A retrospective study of the prevalence of the canine degenerative myelopathy associated superoxide dismutase 1 mutation (SOD1:c.118G > A) in a referral population of German Shepherd dogs from the UK’, Canine Genet Epidemiol, vol. 1, pp. 10-16.

Mellersh C 2012, ‘DNA testing and domestic dogs’, Mamm Genome, vol. 23, no 1-2, pp. 109-23.

Ratjen, F & Döring, G 2003, ‘Cystic fibrosis’, Lancet, vol. 361, no. 9358, pp. 681-689.

Roos, RA 2010, ‘Huntington’s disease: a clinical review’, Orphanet J Rare Dis, vol. 5, pp. 40-48.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.