Advantages and drawbacks of mixed-breed dogs
Dog crossbreeds (or hybrids – the first and subsequent generations of crosses from two purebred dogs) are not breed and are not considered purebred. Though hybrids from the same purebred dogs can have same qualities that you would expect from that cross between purebreds but with greater genetic variability.
However, hybrids are not true breeds; they only can be reproduced by returning to the original purebred parents. In theory, the offspring should have consistent reproducible and predictable characteristics. But actual outcome is quite unpredictable. It is equiprobable to get the puppies with unique and inimitable appearance combining share and share alike traits of either parents or puppies inherited greater share of only one parent in the same litter. When crossing two genetically depleted samples the offspring may inherit the worst traits of both parents. In not so rare cases the signs of both breeds have regressed in the progeny giving the puppies more similar to mongrels or mutts, with mild characteristics of the breed. It is frequently observed in dogs taken from the puppy mills.
Mixed-breed dogs are more diverse genetically due to disordered nature of parents crossing. Progeny of such breeding is less inclined to certain genetic disorders, as the probability for both parents to have equally harmful recessive alleles is reduced. However, certain harmful recessive disorders have been developed in many seemingly unconnected breeds because a breed mixing does not guarantee absolute genetic health.
When two poor specimens are bred, there is a possibility the hereditary diseases or abnormalities of both parents will progress in progeny. Industry of hybrid dogs itself, how hard to aware it, is a fraudulent business at each stage of its "production". Right from puppy mills where a random dog crossing prevails there is no guarantee that the offspring was produced as a cross between two pure breeds. Cruelty and insanitary conditions of dogs’ reproduction multiply eventually tortured, ill and confined animals on most farms. At the final production point - network of pet shops, skilled dealers are able to create a "purebred" pedigree to any dog using the online commercial dog’s registers. So anyone may have a high chance to buy a mutt dog with poor health for a big money given for the supposed cross of two pure breeds.
However, for many reasons selection of hybrid plant varieties and animal breeds has been developing rapidly bringing new species with genetic improvements.
Some studies have shown that mixed breed dogs have a significant advantage in health. A German study found that "mongrels require less veterinary care". Studies conducted in Sweden, found that "mongrel dogs are less prone to diseases than the average purebred congeners" based on mortality data, "mixed breeds were totally in the category of low-risk". Data from Denmark showed that mixed breeds have a longer average life compared to purebred. A British study produced similar results, but only for a few species (such as Jack Russell Terrier, Miniature Poodles and Whippets) that lived longer than other mixed breeds.
A landmark study of the breed effect on the life expectancy of pets has been done, using data on mortality from more than 23,535 pet dogs. Data were obtained from the veterinary referral hospitals in North America. The average age at death was determined for pure and mixed breeds with different body weight. Thus, the average age at death in the study was "8.5 years for all mixed breeds, and 6.7 years for all purebred dogs."
In a study in 2013, adjusted for different body weight, was found that mixed breeds live 1.2 years longer than purebred dogs.
However, a study in 2014 carried out by the Royal Veterinary College, involving nearly 150,000 dogs came to conclusion that mixed dogs were as hardy and tenacious as mutts.
Studies have shown that the mixing of breeds has a number of desirable reproductive characteristics.
Scientists John Paul Scott and John L. Fuller (authors of the book “Genetics and the social behavior of dogs”) found that mixed-breed dogs were the best of mothers compared to purebred females producing more milk and better caring for the offspring. These advantages have led to even lower mortality in the hybrids progeny.
If the hybrids are crossed together for a long period and their breeding is well documented they may finally be considered as a new dog breed by leading kennel clubs. The example of a recent hybrid, which became recognized by all major clubs, is the Cesky Terrier. New breeds of dogs should have a breeding club, able to document the pedigree of each individual member of the breed from its original representatives. Each kennel club has separate rules for documenting new animal breeds.
The American Canine Hybrid Club (ACHC) is a leading designer dogs’ registry. This club recognizes all hybrids derived from purebred ancestors preferably registered with any recognized registration service.