Genetics and Inheritance

Airedale Terrier Puppies

This section explains genetics, chromosomes, DNA, dominant and recessive genes, prepotency, and how to selectively breed for certain characteristics.

Genetic Material and Cells

Cell showing Nucleus and Mitochondria

Each living body is made up of masses of cells. In mammals these cells take various forms e.g. bone cells, muscle cells, blood cells etc. A cell consists of a body like substance of tissue and a central "heart" called a nucleus. It is the nucleus that contains most of the genetic material consisting of nucleoproteins or DNA commonly called genes. However, a few genes are also contained within each mitochondrion which are situated within the substance outside the nucleus of the cell.

Chromosomes, Genes and DNA

Golden Retriever pups

Chromosomes are chains of genes. The number of chromosomes contained within the nucleus of every living body is constant for that particular animal for example, man has 46 chromosomes, the dog has 78, and Drosophila the fruit fly, which genetic students study with because of their simplicity of having only 8 chromosomes. Each species of animal with a constant number of chromosomes, will usually only mate with that species with the same number of chromosomes. If you get a mating between two species of differing numbers of chromosomes the progeny is generally sterile e.g., a horse and donkey mated together gives a mule.

Each chromosome is made up of a chain of nucleoproteins or DNA which are responsible for a different characteristic in that particular animal. Each individual nucleoprotein is called a gene and the study of the characteristics it produces is called genetics.

Genes and Reproduction

Airedale Terriers

During normal growth, chains of genes or chromosomes divide equally along their length forming two new cells each genetically the same as the parent cell. However, when the cells divide to form the reproductive cells i.e. the sperm cells of the male or the eggs of the female the process of division gets far more complicated.

When fertilization occurs the chromosomes arrange themselves into pairs so that each gene from the original parent is in line or opposite its pair from the other original parent. There is then a complete random rearrangement of genes from one chromosome to the other and then the division takes place. The reproductive cells so formed hence contain only half the number of chromosomes of the parent and a random selection of genes derived from each of those original parents. However, there is always one gene still present for each characteristic. When fusion of two reproductive bodies from male and female animals (i.e. fertilization) takes place we once again get the original or double number of chromosomes typically constant for that animal carrying half the characteristics from each parent animal which produced those reproductive bodies.

Recessive and Dominant Genes

Dogue de Bordeaux

The simplest individual gene^[1] is that of eye colour. This is known to be controlled by a "single recessive gene". Dark eyes are dominant over light. So a light eyed dog mated to a dark eyed bitch will produce all dark eyes which carry one light eyed gene and one dark eyed gene. So if in turn these are mated to a light eyed animal this mating will theoretically produce half dark-eyed progeny and half light-eyed progeny. So it is important to mate again to a dark eyed dog coming from a family where at least both immediate parents have dark eyes if you are going to eliminate this ugly fault.

Prepotent

When a dog seems to have great effectiveness in transmitting the required characteristics to his offspring, he is said to be 'prepotent'.

Multiple Genes

Most other characteristics within the breed cannot be answered by a simple gene system. Most veterinary practitioners believe entropion (eyelids that roll inwards) is a simple recessive gene also, but in my personal observation this could not be so. The same also applies to coat texture. When two or more genes come into play for a particular characteristic the problem becomes one more for the statistician than the breeder, unless he has a number of dogs with which to experiment. As a dog has 78 chromosomes each one containing a whole chain of these genes, the inherent possibilities of each dog is enormous. This is where recognized genetic screening programmes and pedigrees come into their own.

References and Further Reading

^[1]   Jane Harvey, "Breeding a Better Airedale" published in The Airedale Terrier Year Book 1975 (National Airedale Terrier Association published 1986 UK) Pages 60-63.

"Breeding a Better Airedale" was republished with the appropriate breed name in the title in newsletters of various Club publications including the Airedale Terrier Club of Victoria, the Queensland Airedalers, the Terrier Club of Canberra, "Standfast" the newsletter of the Sporting Terrier Club of Victoria, the Gundog Club of Victoria, the Golden Retriever Club of Victoria, the Golden Retriever Club of South Australia, the English Springer Spaniel Club of Victoria, and "Bassett Ear" newsletter of the Basset Hound Club of Victoria.

The above material, complete with moving computer graphics are explained further in the DVD 'How to Make a Showdog' by Jane Harvey