The mongrel conundrum Print E-mail
News - Rubrieke
Thursday, 22 April 2010 17:04
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In this column veterinarian Dr Liesel van der Merwe provides practical assistance for common problems in companion animals. She is a specialist physician at the Onderstepoort animal teaching hospital and a senior lecturer in the section of small animal medicine. Send your questions to This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Dr Liesel van der Merwe

I want to discuss the pros and cons of selecting pure-bred versus random-bred dogs and cats. Everyone has a set of chromosomes, each containing two halves, one from each parent. Each chromosome is made up of genes which link up in pairs to create a double helix. Groups of genes carry all the body’s information.

The “dominant” gene is the one that is expressed, such as for brown eyes. A “recessive” gene produces a particular trait - for instance, for blue eyes - only if its effects are not over-ridden by those of a dominant gene. Both genes have to be of the recessive type for a recessive characteristic to be expressed.

Genes are normally transmitted unchanged from one generation to the next, but sometimes a mutation occurs: the structure of the gene is changed.

There are more than 300 dog breeds in the world. All dogs, no matter what breed, belong to the species Canis familiaris. To create specific dog breeds, individual dogs with desirable physical characteristics or traits were bred with each other.

This resulted in a narrower genetic pool within a species and a huge variation between breeds. With purebred dogs genetic variations are well documented:

Dachshunds are predisposed to disc prolapse and spinal cord damage; Labrador retrievers are predisposed to elbow and hip dysplasia; huskies are predisposed to epilepsy; fox terriers are predisposed to severe skin allergies; boerboels may develop compression of the spinal cord due to neck vertebra malformation as well as dilated heart disease causing failure.

This is because current breeding practices within the pedigreed dog community result in the reduction of genetic diversity and the increasing physical expression of recessive genetic traits.

A litter of puppies from an unplanned mating may have more than one father, thus the determination of parentage in many dogs is virtually impossible. Predicting the dog’s appearance and personality is more of a lucky dip. However, mongrels do have some advantages to their purebred cousins.

As a group, dogs of varied ancestry will be healthier than purebred dogs because the gene pool is expanded and more diverse, with a decreased likelihood of two recessive genes being present to express as a physical condition.

It is not a guarantee that they will not have genetic problems or diseases, but the chances are less. Purebred and mongrel dogs have equal susceptibility to non-genetic diseases such as biliary fever, distemper and other infections.

Most breed societies try to limit the continued breeding with dogs affected by recessive conditions. With the increase in understanding of genetics, an increasing number of conditions can be determined by testing the DNA of a puppy or kitten.

Prior to genetic testing an animal would have to exhibit signs of the disease or condition before he/she could be identified as having the gene. Many conditions only manifest later in life and the dog could have already produced offspring.

Carriers of certain traits or conditions could only be identified if their offspring showed symptoms. The number of diseases or conditions which can be tested for is increasing yearly.

The message is that if you want to buy a purebred dog, make sure you understand which conditions are commonly associated with the breed. There are cost implications as well as implications to your lifestyle with many of these conditions.

We read up the stats on a car we buy and replace every three years, but don’t think to check up on a pet which may live for 10 years.

Also speak to the breeders and ensure that they are taking the steps established by their specific breed standards to limit the continued transmission of these defects, for example the German Shepherd Dogs and Labrador retrievers need to be certified free of hip dysplasia. Persian cats can be tested to see if they carry polycystic kidney disease (PKD).

Responsible pet ownership is informed pet ownership.


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