With the recent completion of the canine and feline genome sequences, the unique traits of breeds, many hereditary disorders and genetic predispositions to disease have been characterized from the clinical signs to the gene defect. Moreover, with DNA tests it is now possible to determine the ancestry of mixed breed dogs. The recent advances in canine clinical genetics will be covered and illustrated with clinical case examples.

The topic of clinical genetics in small animals has often been overlooked by veterinarians, presumably because hereditary disorders were thought to occur rarely in clinical practice, to offer relatively little for a clinician to do, or to represent an area where clinicians defer to breeders or academic institutions. However, many of the characteristic breed traits and common and rare genetic diseases seen in veterinary practice and disease predispositions, now recognized to have a heritable basis, have taken on an increasingly important role in veterinary medicine as many infectious diseases, nutritional deficiencies, and intoxications have been controlled. Today, many hereditary diseases are well characterized from clinical signs to the gene defect, precise diagnostic tools have been developed to detect affecteds but also carriers, specific treatments can be offered for a few, and genetic counseling with breeder clients can improve the health of small animals in future generations.

Because of the increased awareness of breeders, pet owners, and veterinarians of genetic defects and the improved diagnostic abilities in clinical practice, the number of reported hereditary diseases in small animals is rapidly growing. At present, >900 and >200 hereditary diseases in dogs and cats, respectively, have been adequately documented, and every year over a dozen new defects are being reported. For the small animal practitioner, it can be a daunting, nearly impossible task to remember all these diseases and be aware of the many novel tests and their appropriate management and control.

Many of the characteristic breed traits and common and rare diseases seen in veterinary practice have a heritable basis. Recent exciting advances in our current knowledge of the completed dog and feline genome sequences offer the opportunity to clinicians to use these emerging tools in clinical practice and have a positive impact on the health of dogs as well as cats and in particular the diagnosis, management, and control of hereditary diseases. Genetic screening tests are available for >100 diseases. Identification of complex trait markers, such as those responsible for temperament and trainability, will likely prove extremely valuable to guide- and service-dog organizations, but also the common pet dog and cats. For the first time, the investigation and identification of polygenic diseases is a realistic proposition. Lastly, collaborative comparative veterinary-human studies will serve to accelerate the rate of discovery and the extent to which both human and veterinary medicine accrues benefits from gene-based research.

Genetic diseases are caused by chromosomal alterations or gene mutations. Disease-causing mutations are heritable changes in the sequence of genomic DNA that alter the expression, structure, and function of the coded protein. The genotype refers to the animal's genetic makeup, reflected by its DNA sequence, whereas the phenotype relates to the clinical manifestation of specific gene(s) and environment, or both. The molecular genetic defect is now known for ~60 hereditary disorders in dogs and ~20 in cats. These molecular genetic changes include point mutations, deletions, and insertions in the DNA sequence that result in a missense or nonsense sequence with an altered codon sequence. For approximately half of the disorders suspected to be of a genetic nature the mode of inheritance remains, however, unknown. The canine genome (7.5x) has been sequenced during the past years, which has and will continue to greatly facilitate the characterization of molecular bases of simple and complex hereditary diseases in dogs and cats. The high quality canine DNA genome sequence of a Boxer makes up a total of ~20,000 genes. Similarly a high density (7x) feline genome sequence has discovered ~19,000 genes. Based upon the vast variety of polymorphic markers (microsatellites and Single Nucleotide Polymorphisms [SNPs]) spread throughout the genome new genes can be discovered and associated with disease traits. Moreover, certain SNP panels can be used to group breeds of dogs and cats. Recently this technology has been applied to develop a mixed breed test (veterinary-based Mars Veterinary Wisdom Panel Mx mixed breed analysis based upon >150 AKC breeds or MetaMorphix Canine Heritage Breed Test based upon >100 breeds). Knowing the breed contributions of a mixed breed puppy may predict the size, temperament and other traits as well as possibly some hereditary disease predilections.

The dog has 76 autosomes (38 pairs) and 2 sex chromosomes (78XX or 78XY), and the cat's karyotype is 38XX or 38XY. The pattern of inheritance depends mainly on two factors: 1) whether the mutation is located on an autosome (autosomal) or on the X-chromosome (X-linked), and 2) whether the phenotype, the observable expression of a genotype as a disease trait, is dominant, i.e., expressed when only one chromosome of a pair carries the mutation, or recessive, i.e., expressed when both chromosomes of a pair carry the mutation. Thus, it is the phenotype rather than the mutant gene or protein that is dominant or recessive. Whereas in humans most diseases are dominantly inherited, recessive traits are favored by the common inbreeding practices in small animals. In addition, complex genetic traits where more than one gene alteration (polygenic) and environmental factors play are role in the expression and severity of a disease. Many susceptibilities to disease, such as inflammatory, immune-mediated, and neoplastic diseases as well as drug reactions, are considered to be transmitted by a complex trait.

Preliminary Observations of the Breed Composition of Mixed Breed Dogs

P. Markwell, BSc, BvetMed, MRCVS; N. Fretwell, BSc, PhD
Mars Veterinary, a Division of Mars Inc., Rockville, MD, USA

Mixed breed dogs comprise a significant proportion of the dog population in many countries, e.g., ~50% in the USA. Genetic tests designed to determine the breeds present in mixed breed dogs have recently become available commercially. These tests can provide insight for both veterinarians and pet owners into the genetic background of mixed breed dogs. The most comprehensive of the current tests cover >150 breeds (WISDOM Panel^TM MX, Mars Veterinary, Rockville, MD, USA). The aim of this study was to use this test to collect data on the breed composition of mixed breed dogs in the USA.

Blood samples were collected in veterinary hospitals from >20,000 dogs. DNA was extracted and typed at >300 different single nucleotide polymorphisms (SNPs) across the genome using selective hybridization and PCR amplification, followed by a discriminatory single base-pair primer extension reaction. The SNP variants were detected by mass spectrometry. A Bayesian generative model was then used to infer the family tree of a dog from comparison of detected genotypes with 157 breed signatures developed previously from more than 8700 pure bred dogs. Inference was performed on 11 different family tree models, and the best-fit model selected using the deviance information criterion. The number of breeds detected per dog averaged 2.5 with German shepherd dog the most commonly detected breed. In addition to collecting data on the breed composition of mixed breed dogs, work was also undertaken to determine performance metrics for the test. Large populations of well-defined complex mixed breed dogs are not readily available, thus the study focused on first cross (F1) hybrids.

The WISDOM Panel test was run on blood samples collected under veterinary supervision from more than 180 F1 hybrids bred from parents registered to a recognized Kennel Club; >25 breeds were represented in the sample set. Two metrics were calculated, positive predictive value (PPV) and sensitivity of breed detection (S). PPV (true positive breed calls (TP) / (TP + false positive calls) was 90%. S (TP/ (TP + false negative calls), was 97%. These metrics did not include breed calls that were made at the lowest level of certainty.

Breeds most commonly detected in mixed breed dogs from US by WISDOM Panel^TM MX.

This study provides a preliminary assessment of the mixed breed dog population of the USA, and indicates that the genetic test used for this study has a high PPV and sensitivity. The data suggest that this test could be of value in both veterinary practice and research to determine the breeds present within mixed breed dogs.