Pets problem of genetic disease. When we humans began to domesticate other animals, we altered the natural process up until then, one that had always favored those best equipped to find food, reproduce, and survive on their own. It became a new kind of symbiosis, in which we called the shots. No doubt we made many good choices, often picking the strongest and the healthiest. But often we just liked a certain unusual look or something that served us, but not necessarily the animals: horses with thick, beefy legs for pulling heavy loads; toy versions of dogs for lap companions; no-tail novelty cats and the like.

Pets problem of genetic disease

Dogs have probably been more shaped by breeding than any other animal, ever since wolves first associated with us some 20,000 to 30,000 years ago (estimates vary). Over thousands of generations, we developed dogs of every size and purpose. With a social structure and instinct similar to our own, they have served us well as hunters, herders, sled dogs, watchdogs, guide dogs, religious symbols, personal companions, and even as a source of food.

Most of the traits we chose served us more than them. Yet, in some ways it was a coevolutionary process. As we took over a lot of their natural habitat, the ancestors of domestic animals adapted to a new habitat: the crops and abodes of humans. That was a version of “survival of the fittest.” Those most fit to live with us were those who best survived. If we were not providing that niche, my guess is that their genetic lines would revert to their original traits pretty quickly.

In any case, cats were much less bred by us. Independent sorts, they were the last animal to share human homes. They were lured by the many mice in the granaries of ancient Egypt and became a religious symbol of that culture, their regal mannerisms no doubt suggesting that role. Their natural talent as mousers made cats welcome in households all over the world. Since they were not easily trained, the cat had few other duties except companionship.

Just page through a picture book on breeds and it’s clear that dogs show much more variety in size, shape, and hair texture than cats. The difference in appearance between certain modern breeds and their wild ancestors is much more striking in dogs. This greater interference with natural selection explains why a study of birth defects in cats, cows, dogs, and horses showed that dogs had the most congenital malformations at birth and cats the fewest. That is despite the fact that cats are generally more sensitive to chemicals and other agents known to cause birth defects.

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Neotony: Selecting for Immaturity

How does selective breeding lead to defects and malfunctions? One of the most significant ways we created the many kinds of dogs we have today from the gene pool of wolves was simply that we often preferred the little cute ones that never fully developed. This was made possible due to a process called “neoteny” or “juvenilization,” selecting for primitive or undeveloped characteristics that occurred early in the species’ existence or those common to immature puppies or kittens: short legs and muzzles, silky hair, floppy ears, and the tendency to bark (adult wolves rarely bark).

It’s humbling to realize that many of the features we find appealing in purebred animals are actually the products of arrested development—either physical or psychological. In the process we often inadvertently created defects or losses of function that went along with “cuteness” or other desired traits.

For instance, breeding dogs for a short muzzle (upper jaw) has spelled trouble for breeds like bulldogs, boxers, and terriers. That’s because separate genes determine related features, such as the teeth and soft palate (which separates the mouth and throat), which are still normal size. This makes the teeth so crowded that they are forced to grow in crooked and sideways. The soft palate hangs so far back into the animal’s throat that it threatens constant suffocation and causes breathing problems.

Dogs with short legs (dachshunds and basset hounds) tend to have deformed spines. Tailless cats (the Manx breed) can have severe malformation of the urinary tract and genitals. The bulldog, Chihuahua, and others bred to have a small pelvis often require cesarean deliveries. Giant breeds such as Saint Bernards and Great Danes are known for their bone problems and their short lives. Generally, the largest and smallest breeds tend to suffer the most from genetic weaknesses.

Inbreeding adds to the problem. To fix a given characteristic into a breed (so that it will breed true, that is, reappear consistently), selected brothers and sisters must be mated, or a parent crossed with its offspring. Such intensive inbreeding might ensure the desired trait, but it might also perpetuate basic weaknesses in the line, such as poor resistance to disease, low stamina, low intelligence, birth defects, and inherited diseases that include hemophilia or deafness. Breeding to meet market demand can also lead to disaster.

For example, the recently imported Siamese cat became so popular that breeders mated siblings as well as parents and offspring to meet the demand. The kittens born of these matings so weakened the breed that it almost died out entirely. Sobered by this experience, breeders began to make wiser selections. Many breeds of dogs—such as the collie, the cocker spaniel, the beagle, and the German shepherd—have also suffered as a result of surges in popularity.

The Ethics of Breeding

The problem of genetic disease is particularly sad. Animals undergo much unnecessary suffering because they’re often bred for financial gain or for some trait considered “cute,” unusual (such as squat faces, long faces, curly or silky hair, hairlessness, wrinkly skin, floppy ears, or missing tails), or useful (such as short legs for access to dens in hunting or massive size for fighting or guarding). The question of whether the animal will have a comfortable, well-adapted, potentially healthy body rarely comes up.

We also don’t seem troubled by the high rate of defective pets. People rightly get alarmed at a human birth defect rate of 1 in 1,000, but many pet breeders simply accept statistics that predict 10 to 25 percent of their litters may be born defective. A related ethical issue is that breeding also produces an excess of puppies or kittens who either don’t have the desired trait or are clearly defective. They must in some way be disposed of. One small thing that helps is not to get stuck on having a purebred animal.

Instead one can adopt a mixed-breed animal, one that needs a home, and this gives a chance at life for the millions of mixed-breed animals that would make equally good pets that go begging. Up to 75 percent of the dogs and cats born each year face death by accidents, starvation, or euthanasia because they can’t find permanent homes.

Selecting a mongrel from the local animal shelter, however, won’t necessarily reduce the risk of acquiring a pet with congenital problems. Often an adopter chooses an animal that especially arouses pity, perhaps one with strangely colored eyes, or drooping ears and eyes that look sad, or a short, pushed-in, childlike face.