The science of genetics is an ever evolving field that has experienced significant growth over more recent years. It can be quite difficult to assess the breeding potential of individual animals but with the development of new tools, breeders can improve their breeding decisions by selecting better genetically matched animals. In this article, Dr Angela M Hughes explains the basics of genetics and how breeders can use new diagnostic tools to protect the genetic health of the offspring and the entire breed.
The field of genetics has progressed far beyond most people’s recollections of Darwin’s Galapagos finches and Mendel’s wrinkled peas. In fact, genetics has experienced significant exponential growth in the last few years alone.
With many new tools, scientists have learned a tremendous amount about how traits and genetic diseases occur and are inherited. Breeders are now able to use these tools to improve their breeding decisions, resulting in puppies with fewer medical concerns and improving the overall health of their breed.
The basic blueprint for life, the genetic material, is deoxyribonucleic acid (DNA). DNA is composed of a double strand of nucleotide bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
These bases align to form complementary base pairings such that A always pairs with T and G always pairs with C. These strands can be completely separated and replicated in preparation for cellular division.
Alternatively, these strands can be partially separated, transcribed into ribonucleic acid (RNA), and translated into functional proteins. DNA is contained in the nucleus of most cells in the body and is arranged into chromosomes. In dogs, these chromosomes consist of 38 autosomes and a pair of sex chromosomes, X and Y. Each offspring inherits one set of autosomes and a single sex chromosome (X or Y) from each parent. Within each of these chromosomes, one can think of DNA as a series of letters (bases) that make up words and sentences. These "sentences" can be equated to genes that are the instructions for cells to create different proteins. It is estimated that dogs have approximately 20,000 genes.
Dogs are one of the most diverse species on the planet. So how can a single species range from a two pound Chihuahua to a 200 pound Great Dane, and yet have essentially the same DNA instructions in each? The answer is in the alleles.
Alleles are sequence alterations at the level of the DNA that are passed onto future generations. Some alleles are translated into differences in the proteins, potentially resulting in a structural or disease difference between individuals. The dogs within a breed tend to share a lot of the same alleles, therefore they tend to look and act similarly.
Genetics in Breeding: Optimal Selection
Many breeders face difficult decisions when choosing potential breeding pairs. They have many things to consider in terms of health, physical and behavioral traits in their quest to produce the best possible puppies.
While they can consider what a dog looks like and acts like, and even what their relatives are like, they may still have some concerns about potential health issues that could arise based on the breed and family history. There are several ways that breeders can examine potential breeding dogs for genetic diseases and traits of concern. These include direct mutation tests for known mutations, phenotype tests like taking hip radiographs and performing eye exams to infer a dog’s genes with regard to these diseases, and a new genetic tool to evaluate dogs for their genetic diversity.
A number of recent studies have demonstrated a link between inbreeding, or lack of diversity, with a significant decrease in litter size and a significant increase in the number and percentage of stillborn puppies.1-3
Other studies have also shown an association between increased inbreeding coefficients and genetic predisposition for hip dysplasia in German Shepherds,4
higher prevalence for primary cataracts in English Cocker Spaniels,5
and decreased hunting ability.6
As a result many researchers are recommending that dog breeds be treated like an endangered species and it is important to consider genetic diversity as a factor in dog breeding programs to at minimum maintain, and if possible increase, the genetic diversity within each breed.
While many of these studies focus on the inbreeding coefficient, which is a measure of the relatedness of the ancestors for the offspring in a litter, this statistic is still a relatively rough measure and an average for the litter.
Within that litter, the puppies can vary quite widely in terms of their genetic diversity within themselves or as compared to the breed as a whole (e.g. one or more puppies could carry a particularly rare haplotype).
Thus, a new genetic test called Optimal Selection™ was developed as a tool to look at the DNA of breeding dogs and provide a better understanding of the specific chromosomal patterns, or haplotypes, that each dog carries. This information can be used in breeding decisions and work to maximize the genetic diversity, or heterozygosity, of the puppies.
Studies of Dandie Dinmont Terrier breeders utilizing Optimal Selection have shown an increase in the number of puppies per litter and an increase in their genetic diversity while maintaining the traits that breeders are selecting for and producing AKC championship dogs. They have also noted increased post-thaw semen motility in dogs with greater levels of genetic diversity.
Future of Genetics in Breeding
Veterinary genetics is a very quickly evolving field. Many new tools like mutation testing and Optimal Selection can help breeders unravel the mysteries held within the DNA of their dogs.
While the genetics of a breeding animal should never be the only means of determining a desirable mating, the genetic status and diversity of the individuals should be included as a factor in order to protect the genetic health of the offspring and the entire breed.
- Van der Beek S., Nielen A.L., Schukken Y.H., Brascamp E.W. (1999) Evaluation of genetic, common-litter, and within litter effects on preweaning mortality in a birth cohort of puppies. Am. J. Vet. Res., 60, 1106–1110.
- Gresky C., Hamann H., Distl O. (2005) Einfluss von Inzuchtauf die Wurfgro¨ße und den Anteil tot geborener Welpen beim Dackel. Berl. Mu¨ nch. Tiera¨ rztl. Wschr., 118:134–139. (Quoted in Voges and Distl, 2009)
- Calboli FC, Sampson J, Fretwell N, Balding DJ. (2008) Population structure and inbreeding from pedigree analysis of purebred dogs. Genetics. 179:593-601.
- Janutta V., Hamann H., Distl O. (2008) Genetic and phenotypic trends in canine hip dysplasia in the German population of German shepherd dogs. Berl. Munch. Tierarztl. Wschr., 121:102–109.
- Engelhardt A., Stock K.F., Hamann H., Brahm R., Grußendorf H., Rosenhagen C.U., Distl O. (2007) Analysis of systematic and genetic effects on the prevalence of primary cataract, persistent pupillary membrane and distichiasis in two color variants of English Cocker Spaniels in Germany. Berl. Munch. Tierarztl. Wschr., 120:490–498.
- Voges S, Distl O. (2009) Inbreeding trends and pedigree analysis of Bavarian mountain hounds, Hanoverian hounds and Tyrolean hounds. J Anim Breed Genet. 126:357-65.