Genetics Primer Anita Oberbauer, PhD, University of California, Davis Jerold Bell, DVM, Tufts University

The goal of dog breeders is to retain desirable traits while minimizing those traits that are less desirable or even deleterious. Effective breeding requires an understanding of parental contribution and the genetic transmission of traits. This talk will discuss how traits are passed down through generations, evaluation of pedigrees and assessing mode of inheritance for specific traits, making selection decisions, and various breeding approaches. Further, with the advent of tools that allow genetic testing, breeders are faced with even more information that needs to be interpreted in selecting sires and dams. How do these tests work, how they can be used, and how are they developed will also be covered along with what recourse breeders have if such tests are not yet available.

Biographical Profile

Dr. Anita M. Oberbauer is Professor and Vice-Chair in the Department of Animal Science at the University of California, Davis. She earned a B.S. at the University of California, Davis in zoology, a Ph.D. degree from Cornell University in animal physiology minoring in statistics and pathology, and then conducted postdoctoral research in growth factors and gene regulation at Loma Linda University and in Biological Chemistry at University of California, Los Angeles. She has received the Jim Corbin Award in Companion Animal Biology offered by the American Society of Animal Science and distinguished teaching awards from UC Davis and the Western Section of the American Society of Animal Science. She serves on the Board of Directors of the Orthopedic Foundation for Animals. Her research program emphasizes growth and development focusing on the relationship between skeletal size and overall body composition, as well characterizing the genetic basis for health disorders in dogs. She is the current health education chair of for the American Belgian Tervuren Club, and actively competes with her Belgian Tervuren which she shows in all AKC venues open to the herding group.

Dr. Oberbauer's research has been supported by the following grants:

1613: Development of a Genetic Marker for Idiopathic Epilepsy in the Belgian Tervuren and Belgian Sheepdog ;'~~ I657T: Identification and Inheritance of DNA Markers for Addison's Disease ;;" 2015: Development of a Genetic Marker for Idiopathic Epilepsy in the Belgian Tervuren and Belgian Sheepdog 2104: Defining Inheritance of Idiopathic Epilepsy in the Mastift Poodle and Giant Schnauzer 2226: Characterizing the Inheritance of Addison's Disease and Linked DNA Markers

2227: Defining the Mode of Inheritance for Primary Lens Luxation and Juvenile Cataracts in the Jack Russell Terrier

224: Characterizing Idiopathic Epilepsy in the Poodle, Giant Schnauzer, English Mastiff. An Assessment of Inheritance

225: Establishing a Genetic Linkage Between Addison's Disease and DNA Markers

2614: Refining the Genetic Linkage For Idiopathic Epilepsy in the Belgian Tervuren and Sheepdog

35I-A: Identifying the Mode of Inheritance for Primary Lens Luxation and Juvenile Cataracts in the Parson Russell Terrier

668-A: Inheritance of Addison's Disease in Nova Scotia Duck Tolling Retrievers 958: Idiopathic Epilepsy in the Poodle: Identifying Genomic Risk Factors

 

Genetics Primer – Anita Oberbauer, PhD, U of California, Davis – conference notes

                               

Dogs are the most genetically engineered species on the planet – differences in size, heads, etc.

 

How did we get there?  Maximize desirable traits while minimize undesirable traits while keeping our dogs healthy.

 

Determine if the trait is heritable

 

Breed to maximize benefit. – breed the best to the best philosophy.  The outcome of that approach relies upon the likelihood and probability.

 

DNA is made of repeated nucleotide bases – which are the template of genes.  Genes are regions of DNA that specify/code for a protein product.  They are the bases of the chromosomes.

 

Dogs have over 2 million bases, which is over 20,000 genes, 38 pairs of chromosomes, plus one pair of sex chromosomes.

 

For all genes on autosomes, there are two copies of the gene.  The different copies of the same gene are referred to as alleles.  Alleles are different.  Each autosome is paired: diploid.  A given gene is at a constant location (or locus) on its chromosome.  So diseases are always on the same location of a chromosome within a species.

 

Different alleles.  Differences in the underlying DNA caused by mutations or changes in the DNA.  Mutations are either positive (we like them – even if its not healthy for the dog), negative or neutral.

 

A single dog has two copies of a gene so has at most two different alleles.  But the dog population can have different mutations.

 

Homozygous – the alleles are identical.  Heterozygous – the alleles are different on both chromosomes.

 

Dominant – alleles are those genetic versions that mask the effects of another allele.  When referencing a dominant allele it is designated with a capital letter.

 

Recessive – alleles are those genetic versions that are identical at locus producing a particular characteristic, trait or disease.  When referencing a recessive allele it is designated with a lower case letter.

 

Incomplete dominance – alleles may not be fully dominant or masking of the recessive allele.  Each allele contributes to the phenotype.  Phenotype equals genotype.

 

Maternal effect – via the egg, contributes all the cellular content during the conception. 

 

Mendel’s laws state that independent assortment genes assort into gametes randomly and independent.  Except when genes are linked then genes physically close together on a chromosome.  During meiosis, alleles from one gene travel together with alleles of linked gene.

 

Observed traits may not be genetic.  Therefore cannot be selected for in breeding programs.  Examples include – black fur that sun bleaches red, cataracts due to eye damage. 

 

Genetic but not inherited but somatic.  The DNA that is altered are not in the gametes.  For example: developmental damage due to chemicals, xrays during development in the womb. 

 

Genetic and inherited traits are regulated by DNA within the egg and sperm.  Heritability between 0 and 1; <1 reflects an environmental contribution.  If heritable it is important to know how many genes are involved in the trait.  Complex traits are polygenetic – eye color, hip formation, speed of a dog.

 

Traits are also influenced by sex.  Sire determines the sex of the offspring.  In mammals, females are XX and males are XY.  Males have only a single copy of each sex chromosome.  An altered allele will always be expressed.  Sex linked traits are regulated by genes on sex chromosomes (i.e. hemophilia, calico traits in cats).  Sex limited traits – traits that can only be expressed in one sex (cryptoidism). 

 

Traits can be Polygenetic/Complex

 

Both parents contribute alleles to offspring.  Many genes contribute to the trait expression – some with small and some with small contributions.  No predictable rations unless you know what genes are involved.

 

Epistasis – what happens at one locus affects expression at another.  Sometimes called modifiers.  Associated with the penetrance of a gene: genotype specifies trait expression but that trait is not always seen.

 

Pleiotropy – the expression of one gene affects the phenotypic affects of multiple genes.

 

Inbreeding – goal is to create uniformity and increase the likelihood of desirable traits expressed.  With increased homozygosity, recessive traits are expressed.  Genes will become fixed for single alleles: predictable expression generation to generation – breed creation.  Inbreeding will uncover deleterious alleles present in the population.  Cons: effectively decreasing population size (gene pool).  Loss of genetic variation,, loss of alleles – especially true if coupled with popular sire syndrome.  Reduction in fitness traits – litter size, etc.

 

Linebreeding – Goals are very similar to inbreeding.  Accentuate the desirable traits, increase predictability of trait expression.  Increases homozygosity at loci.  Often tries to perpetuate the favored genetic combination of a strong ancestor.  Will to uncover recessive genes.

 

Phenotypic breeding.  To maintain desired traits.  Low to little consideration of pedigree.  Outcome is variable.  Parental phenotypes may be similar but due different alleles, it may not express the trait.

 

Outcross – breeding genetically dissimilar dogs.  Goal is to introduce genetic diversity.  Improve dogs b y compensating for deleterious recessive alleles with dominant alleles.  Introduces hybrid vigor.

 

Compensatory – Goal is to correct an obvious fault.  Not always successful.

 

The goal is to combine genetic knowledge with breeding strategies. 

 

The mode of inheritance is important to consider.  Reach and drive – recessive, temperament is dominant.  When evaluating the breeding, do not just look at the pedigree – LOOK at the dog.

 

Breeding requires having a vision, objective and knowledge of the mode of inheritance and the relative contribution of the sire and dam.  Prioritize traits.  Breed the best to the best – but better define the best.

 

Need a long term view, nothing happens in one generation.  Even with genetic tests, it still will take generations to maintain desirable traits.