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The Beginners Guide to Genetics
Breeding dogs can be a bit hit and miss so a rudimentary understanding of genetics can help. The information that follows is basic and much simplified. Unfortunately the inheritance of characteristics is not always straightforward but there are some aspects of genes that it is useful to understand.
Inheriting the genes
Genes are located on chromosomes. Each species of animal and plant will have is own characteristic number of chromosomes. Humans have 46 chromosomes which occur in pairs. Dogs have 78 chromosomes. The chromosomes vary in length, with the longer chromosomes containing a larger number of genes.
However, there will be an identical number and order of genes on each pair of chromosomes.The pairs are referred to as homologous chromosomes because they have the same origin. A dog inherits 39 chromosomes from its father and 39 from its mother. These are not just any 39 chromosomes, but one from each of the pairs of chromosomes.
In order that the chromosome number does not double from generation to generation, each parent only passes on 39, not 78 chromosomes. The halving of the chromosome number takes place in the sex cells. The gametes, the eggs and sperm contain 39 single chromosomes. When a sperm fuses with an egg cell, the original number of 78 is restored.
Genotype and Phenotype
There are two terms to remember when reading the information below : genotype and phenotype. The genotype refers to the genetic composition of an individual. The phenotype refers to an individual's appearance as a result of having a certain genotype.
Dominant and Recessive
Most genes occur in two forms, or alleles, known as dominant or recessive. A dominant allele will always show itself, whereas a recessive allele can be masked by the presence of a dominant allele. Consider the inheritance of eye colour. Most people fall into one of two categories - brown eyes or blue eyes (forget about the many variants of blue). The allele for brown eye colour is dominant to the allele for blue eye colour. An individual inherits one allele for eye colour from each of its parents.
If it inherits a dominat allele from each parent, the individual will have brown eyes. However, brown eyes will also result if just one dominant brown eye allele is inherited. In order to have blue eyes, an individual must inherit a blue eye allele from each parent. This can be shown graphically, using symbols to represent eye colour.
Let B represent brown eye allele, b represent blue eye allele.
If the parents were brown eyed, but each carried a blue eye allele their genotype would be represented as follows
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If the alleles are the same, for example BB or bb, the individual is described as being homozygous. If the alleles are different, for example Bb, the individual is heterozygous. This example assumes that there is a single gene controlling a single characteristic. Single genes probably control eye colour, hair colour and length in dogs, for example long hair in Weimaraners.
Many breeds of dog suffer from a number of genetic disorders which are also controlled by single genes. This means that it is possible to track the inheritance of the gene and eventually breed out the disorder. Tests are available for a number of genetic disorders and this makes the process even easier. However, it is more usual for characteristics to be controlled by groups of genes, rather than single genes and this makes the inheritance far more complex.Genetics and breedingDog breeders are often faced with the problem of choosing a suitable stud dog for their bitch.
This is where is simple understanding for genetics can help. If the bitch has been line-bred, then there is a good chance that the animal is homozygous for most of their genes, i.e. the alleles for each of the genes are the same. This means that there will be less variation among the gametes. For example, an dog that is BB for black coat will only produce gametes that contain B alleles. By choosing a stud dog that is from a related line it is possible to reduce the variation in the puppies.
This is because the alleles carried by the stud will be similar to those of the bitch. It is this type of mating that will produce offspring that most resemble the parents. There is a risk - it is possible that there are some recessive genes lurking in the line and by crossing two related parents there is greater chance of the offspring inheriting a recessive gene from each parent and showing the recessive trait in their phenotype.
Outbreeding involves crossing the bitch with an unrelated dog. This is where it is impossible to determine the outcome of the cross. If both parents have been line breed, they will probably be homozygous. This cross results in a litter of puppies, each of which is heterozygous for most of their genes. These puppies may look 'of type' but they may have lost some of the more desirable characteristics of their parents. Sometimes, of course, the new combination of genes produces a great dog.
One cross of this type that comes to mind is that of Flimmoric Fanclub and Gunalt Joy, both line-bred dogs. The problem with this type of cross crops up in the next generation. Because the out bred dog is heterozygous for most of its genes, its gametes will be very variable. The best approach is probably to cross the out bred dog back to a related line to regain some homozgyous characteristics.
Gene pools and population genetics
The term 'gene pool' refers to the sum of all the genes in a population of a particular species or in this case, a breed of dog. A large gene pool leads to a stable population. The influx of new genes from outside does not have too large effect on the gene pool as a whole. There are sufficient heterozygous individuals to ensure that the recessive genes do not disappear from the population.
If the gene pool becomes smaller, it is more susceptible to change. New genes, especially dominant ones, can spread rapidly through the population and the number of individuals carrying recessive genes (which is always small) decreases. The genotype of the individuals becomes more alike and the whole population is more vulnerable to threats such as disease.
A good example is a field of wheat of the same variety. Because all the wheat plants are genetically similar, they are all vulnerable to the same diseases. This means that a single disease could wipe out the whole population. During the 1970s the gene pool of Weimaraners in the UK was small. There were relatively few individuals and most of them were related. The import of weimaraners during the 1980s and early 1990s increased the gene pool. During this time, the number of weimaraners also increased rapidly.
It is likely that the gene pool was at its most diverse during the early1990s. Since then the variation seen in the dogs has decreased. Why? A small number of stud dogs have been used to service a large number of bitches - the result is a generation of weimaraners that are probably more alike in appearance and genotype than at any other time.