Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

Is it better to choose pure-bred or random-bred dogs and cats? 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, but has also resulted in a huge variation between breeds, if you consider the relative sizes of a Yorkshire terrier and a Great Dane.

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, such as 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.

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.

The list is endless. 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. Mongrels, crossbreeds, Heinz 57’s, pavement specials . . . whatever you may want to call them; they do have some advantages to their purebred cousins.

The theory of hybrid vigour or ‘basterkrag’ is that as a group, dogs of varied ancestry will be healthier than purebred dogs. This is because the gene pool is actually expanded and is more diverse, with a resultant decreased likelihood of two recessive genes, one from each parent, being present to express as a physical condition or disease.

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 statistics 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).