We all know that our dogs can be afflicted with many of the same diseases and conditions that we humans suffer from: the flu, allergies, GI upsets, and—at least for me—the possibility of stumbling over or slipping on an object and ending up with something like a torn ligament. Then there are the diseases with genetic components like certain cancers, epilepsy, kidney disease, and diabetes, just to name a few.
But there are some conditions—especially genetic ones—that are unique to humans. One of them is Down Syndrome. So, the answer to the question in the title is: No, dogs cannot have Down Syndrome.
Why Doesn’t Down Syndrome Affect Dogs?
The answer lies in genetics, which is how traits are passed down from one generation to the next. These traits are found in DNA, chromosomes, and the genome. DNA is the molecule in the cell nucleus that contains genetic instructions for all living organisms. Chromosomes, which come in pairs (one from the father and one from the mother) are comprised of DNA and a protein. All of an organism’s genes make up the genome, which is unique to each organism.
Genetically, we have many similarities with our dogs—in fact, we share almost 85 percent of our DNA with dogs—but there are also some very important differences. The main difference is that dogs have 39 sets of chromosomes (78 in total), while humans have 23 sets, for a total of 46. Down Syndrome happens when there is a full or partial copy of chromosome 21, bringing the total to 47. Because dogs have a total of 78 chromosomes, there is no equivalent to the human 21st chromosome in a dog’s genetic makeup.
Other Canine Genetic Abnormalities that Mimic Down Syndrome
While dogs can’t have Down Syndrome, they can possess chromosomal abnormalities that lead to conditions similar to that disorder. These include developmental disabilities, difficulty with coordination, as well as distinct physical features such as a flattened face, shorter limbs, and smaller stature. One example is canine congenital hypothyroidism, which is caused by low or absent levels of thyroid hormone at birth, resulting in slow growth, a large and protruding tongue, short limbs, and cognitive delay.
There are other conditions in dogs that could be confused with Down Syndrome. These include pituitary dwarfism, which comes with increased risk of kidney disease, dental issues, and hearing loss; congenital hydrocephalus, a buildup of fluid in the brain that causes brain dysfunction; deficiency of growth hormone; and portosystemic shunt, which results in stunted growth, poor muscle development, and abnormal behaviors such as disorientation and circling, staring into space, and seizures.
Genetic Similarities Between Humans and Dogs
Sometimes, the effect of a shared gene is identical between humans and dogs, as is the case with EPAS1. Sparked by low-oxygen conditions, this gene stimulates the body’s responses to hypoxia—low oxygen levels in body tissues—at high altitudes and is shared by people living in the 16,000-foot-elevation Tibetan Plateau and the dogs that live there. Similarly, the EPS8L2 gene is involved in childhood deafness, as well as hearing loss in Rhodesian Ridgeback dogs.
Other times, the shared gene’s influence is similar, but not exactly the same. One example is a defect in the SCN3A gene, which affects human speech development. In dogs, however, genetic changes near the canine equivalent of the SCN3A gene are associated with frequent howling. Howling may not be the same as speech, but perhaps they’re two sides of a similar coin!
Some Common Canine Genetic Disorders
More than 900 inherited disorders have been identified in dogs. Here are some of the most prevalent:
Allergies
Both skin and internal allergies are very common in dogs, as they are in humans. One especially prevalent inherited allergy is atopic dermatitis (a chronic skin condition that causes intense itching), commonly caused by allergies to substances in the environment such as pollens. Some dogs are also genetically predisposed to developing food allergies; approximately 10 percent of dogs develop itchy or chronic skin problems due to food allergies. The most common dog food allergens are proteins such as chicken, beef, dairy, lamb, and eggs, along with wheat and soy.
Hip Dysplasia
Hip dysplasia, most commonly seen in large dogs, is a deformity of the hip that causes abnormal looseness of the hip joint, leading to degenerative joint disease or osteoarthritis. Although hip dysplasia is most often genetic, it can also be affected by factors such as diet, environment, exercise, growth rate, muscle mass, and hormones.
Diabetes Mellitus
Diabetes is a disease in which the cells of the pancreas either stop producing insulin or can no longer produce adequate amounts of it. The most common form of canine diabetes is similar to human type 1 diabetes, requiring lifelong insulin therapy. Most often seen in older dogs, the risk of developing the disease varies between different dog breeds; research suggests that genetic factors contribute to an animal’s susceptibility to developing diabetes.
Mitral Valve Disease
Mitral valve disease (MVD), the most common genetic cardiovascular disease in dogs, represents 75 percent of all canine cardiovascular disorders. MVD is a chronic, degenerative heart condition that affects the valve between the left atrium and left ventricle of a dog’s heart. In studies, up to 30 percent of dogs with MVD will progress to heart failure and death as a consequence of the disease.
Hereditary Cancers
While not all cancers are hereditary, some dogs carry specific genes that predispose them to developing certain types of cancer. The most frequently diagnosed inherited canine cancers are lymphoma, hemangiosarcoma, mast cell tumor, and osteosarcoma. It is generally believed that dogs develop cancer about twice as frequently as humans.
In the end, if you’ve been worried that your dog may have Down Syndrome, you can rest assured that he doesn’t. However, if your pup displays any signs similar to human Down Syndrome, contact your veterinarian for a thorough evaluation.
