A series of articles by Mike Tempest
Welcome to this the first in a series of articles that will attempt to explain the complicated subject of Genetics in a reader-friendly way. In this series of articles, I shall build up from simple to more complex situations. We shall deal with how characteristics are transmitted from parent to offspring; define chromosomes, genes and alleles (yes, we have to use some scientific terms); describe how genes and alleles relate and react; consider simple forms of inheritance and work our way through to complex forms illustrating these with examples on the way; discuss breeding methods and the use of pedigrees and finally see what we can do to assess whether we are making any progress in breeding better dogs.
The dog that we see
First, we need to consider whether the dog that we see is all down to its breeding. What a dog looks like is known as its phenotype, which we can conveniently abbreviate to P. There is a considerable amount of variation in the phenotype of dogs within a breed, but is all of this variation due to the dogs’ genes? The answer is no - what we see in the whole dog is not exclusively genetically produced. What we see is made up of a genetic component known as the genotype (G) and an environmental component (E) and we could express this in the simple formula P = G + E.
The tail example
One of the most obvious examples of an environmental influence on the appearance of a dog is that of tail docking. Except in naturally docked breeds, dogs have the genes for a ‘long’ tail, but man has traditionally exerted his influence by cutting off puppy dogs’ tails, usually for the purpose of preventing damage in later life that may be caused by the work that certain breeds are used for. In these circumstances short tails are not the result of a dog’s genetic make-up, but are the consequences of a direct environmental influence – man. One of the consequences of this interference is that knowledge of the natural appearance and carriage of tails in different breeds has been lost over decades of docking, and in the wake of a ban on docking, it will prove difficult to achieve a description of the ideal tail carriage in breed standards.
Some breeds and strains within breeds are however naturally docked due to the dog having inherited the genes for a short tail, and some individual dogs in a breed can have a natural dock whilst others will have long tails. One well-known breeder, Dr Bruce Cattanach, has attempted to produce naturally docked Boxers by incorporating the docked genes through crossing Boxers with Pembroke Corgis from a strain with a long history of natural docks. Once a naturally docked cross results, this is then mated back to Boxers to get a three-quarter bred Boxer with a natural dock. Continual mating back to Boxers will eventually after several generations produce dogs that are essentially Boxers with naturally docked tails.
Some less obvious and more sophisticated examples are concerned with bone and coat. Let us assume that a dog inherits the genes for good bone growth. If it is not fed enough calcium it may never realise its genetic potential for bone. Similarly, in coated breeds if a dog inherits the genes for a good coat but is not fed the necessary nutrients it will not realise the potential of its inheritance for coat. The appearance of a dog can never exceed what it acquires in its genes, but it is very easy to restrict the appearance below the genetic potential.
Other examples of environmental influences are the coat of a black dog going a rusty, red appearance due to continual exposure to sunlight, and the appearance of a faded black nose or ‘winter nose’ due to hormonal changes associated with the breeding cycle, reproduction or even stress.
Relative importance of G and E
The relative importance of the genetic and environmental components in determining the physical appearance of the dog will be different for the different individual characteristics that make up the whole dog. Some individual characteristics may be entirely due to genes with no environmental influence, such as the retina in the eye with the associated disorder of progressive retinal atrophy (PRA). Other characteristics may have a large environmental influence, such as the structure of the hip joint and the degree of hip dysplasia.
Those characteristics influenced 100% by the genes are usually those with a simple mode of inheritance, whilst complex characteristics are almost invariably influenced to some extent, and maybe a large extent, by the environment. However, they still have a genetic component and can therefore be influenced by selective breeding, although improvement will be slow.
The G component
This is the bit that we want to pay more attention to in subsequent articles, and it goes without saying that this can be a difficult concept to understand. The genetic component is made up of what is known as an additive component (A) and a non-additive component (NA) which we can represent as G = A + NA. The additive component can be taken to be the cumulative effect of genes added together, and the non-additive component is the effect of interactions between genes (known as epistasis with the symbol I) or an interaction between different alternative forms of a gene (known as dominance with the symbol D). So, we can now say G = A + D + I. The interactions produce an effect greater or lesser than would be expected from the simple sum of the effect of the separate genes. The additive component is usually of greatest interest to dog breeders because it is the chief cause of resemblance between relatives.
The first message
All that you see in a dog may not be what it seems. There is a well known saying that “half the pedigree goes in at the mouth” – in other words you may not see the genetic potential of a dog being fully displayed unless it is provided with the environment, in this case feeding, that will allow it to reach its potential, and the extent to which a dog reaches its genetic potential will depend on the environment to which it is exposed.
This article was originally published in Show Dogs Ireland and is reproduced here with the kind permission of its editor Caroline Reynolds and the author Mike Tempest#