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Health and Genetics

GENETICS

Genetics is the scientific study of genes, heredity and individual variation.  In dogs, it also covers the genetic diversity of different breeds.

Genetics in its simplest terms is what a dog inherits from its ancestors through its biological parents.

All that a dog develops into is NOT however totally dependent on what it inherits biologically.  Much of how that genetic potential is ultimately expressed in the dog is a result of the environmental conditions in which the dog is raised and nurtured.  You may recognize this as the nature versus nurture discussion.


Having said that, there are specific genetic boundaries or absolutes that cannot be changed by environment or nurture – things like gender, coat color, and inherited diseases.

It’s important to understand some basics in the discussion of genetics as it relates to Labrador Retrievers.

What is a Phenotype?

What is a Genotype?

What is a Method of Transmission?

What is Simple or One-Gene-Pair inheritance?

What is Polygenetic Inheritance?

We’ll try to explain these phenomena in simple terms to help you understand why they are important to Labradors.

What is a Phenotype?  Phenotype is what a dog looks like.  Tall or short, coat color, single or double-coated, light or dark eyes – these are phenotypes.  You can also think of them as the outward appearance of the dog.

o Phenotype is important because there are some characteristics of Labradors that we can ONLY gauge or know about by examining their phenotype.

o Hip Dysplasia, Elbow Dysplasia, Cardiac Murmurs and Tricuspid Valve Dysplasia and some Eye Conditions are examples of characteristics that we believe are inherited but for which we have

No specific genetic test

Or they are something that we believe is polygenetic [more than one gene is probably responsible for this trait]

o When we have no specific genetic test or a trait is polygenetic we must rely on precise evaluation of the dog’s phenotype from which we infer information about the genetic information it is likely to be carrying

o We use phenotypic evaluation in Labradors for

Hip and Elbow dysplasia

Cardiac Auscultation and Doppler Evaluation for Tricuspid Valve Dysplasia, Heart Murmurs, and other cardiac anomalies

AVCO eye examination for conditions other than PRA/RD-OSD including some conditions that may be noted as “breeder option” http://www.offa.org/eye_genetics.html

o Using phenotypic evaluation results in a wider range of results for these traits in offspring – it is less rigorous or less precise than if we had specific genetic tests for these traits.  

Example:  two Labrador parents both rated OFA Excellent do not always produce OFA Excellent Offspring, and they can produce Dysplastic offspring.  

Example:  two Labrador parents both rated OFA Normal for Elbows do not always produce OFA Normal Elbows in their offspring.  

OFA provides some good discussion and statistics for these phenomenon on their web site:  http://www.offa.org/hd_grades.html


What is a Genotype? Genotype is the genetic information coded at a specific location on the canine DNA that has been identified through extensive research as THE location for a given individual trait or disease.  Progress in genetic sequencing and demand for genetic identification of common traits and diseases has led to many such locations being identified and DNA testing becoming increasingly available at fairly reasonable costs.

o Genotype is important because it allows breeders and owners to accurately identify what traits and diseases an individual dog OR proposed breeding will display.

o Genotype testing allows responsible breeders to eliminate the occurrence of debilitating diseases like PRA blindness, RD-OSD blindness, Exercise Induced Collapse, Central Nuclear Myopathy, Hereditary Nasal Parakeratosis, Degenerative Myelopathy, Narcolepsy, and skin conditions associated with Dilution.

o Genotype testing is usually done either with a sample of blood drawn by your veterinarian and shipped to a testing laboratory OR with a sample of cheek cells swabbed by the owner and mailed to the testing laboratory.  Results typically are reported within two weeks of the laboratory receiving the sample.

o Almost all genotype testing is for traits or diseases that are inherited through a Simple Dominant/Recessive means. [see below]

o Genetic testing represents a huge step forward in responsibly breeding healthy Labradors and you should look for breeders who maximize testing.

o Open data base listing of genetic testing results is a clear indicator of a breeder’s dedication to the betterment of the Labrador breed.  All Blackhawk LRC members are encouraged and urged to list ALL of their health screenings publicly on the OFA data base, even those with Carrier or At-Risk or Affected status.  http://www.offa.org/index.html will take you to the search engine


What is a method of transmission? Method of transmission explains just how the genes that are passed from parents to offspring are expressed in the offspring.  

o The expression of traits and diseases in Labradors may occur in one of several ways.  

Simple dominant/recessive

Polygenetic

Epistatic over-ride

o There may be different means of expression, but the method of transmission IS ALWAYS THE SAME for each specific trait or disease.  Example:  coat color is always transmitted the same way from generation to generation.  The disease gene for Exercise Induced Collapse is always transmitted the same way from generation to generation.

o The method of transmission is critical to understand so that you will NOT produce puppies affected by debilitating disease.

Simple dominant/recessive transmission.

o This is the easiest to understand.

o When a disease or trait is transmitted by simple dominant/recessive the trait is most often on the RECESSIVE gene allele. [However some diseases ARE on the Dominant gene allele, but those are more rare in dogs and discussion of them belongs in an advanced theoretical course].

o Think of the Dominant Gene Allele as Dominant and the Recessive Gene Allele as recessive.  

o Every dog has a PAIR of gene alleles for a specific trait/disease – and ONE allele comes from each parent.

o A dog parent that has ONLY Dominant gene alleles will only pass ONE of those dominant genes to each of its offspring.

o A dog parent that has ONLY recessive gene alleles will only pass ONE of those recessive genes to each of its offspring.

o It only takes ONE Dominant gene for an offspring to be normal.

o It takes TWO recessive genes for an offspring to be affected/have the disease/trait you are trying to avoid.

o That part is easy to understand.  The tricky part is when you have a prospective dog parent who carries ONE Dominant and ONE recessive gene.  The beauty of genetic testing is that you can identify that prospective dog parent AND ONLY choose a mate for it that has TWO Dominant gene alleles, and that makes sure ALL OF THE OFFSPRING will NOT have a chance to receive TWO recessive genes and thus have the disease/trait you want to avoid.

Polygenetic transmission.

o Polygenetic transmission means that MORE than one pair of genes is responsible for the trait/disease you are interested in.

o Polygenetic traits appear to skip generations.  

o The current thinking is that unless there is a genetic test for a disease or trait, then we can cautiously consider that it MIGHT BE polygenetic.  Elbow dysplasia is an example of a disease/trait/occurrence that many breeders and researchers believe is polygenetic.

o Until something is positively identified in the gene sequence and proven to be polygenetic, the best rule of thumb to eliminate it is this:

Do not breed affected dogs.

Do not breed clear dogs with affected siblings to other clear dogs with affected siblings.

Do not breed clear dogs with affected parents or affected grandparents.

Do not breed clear dogs to untested dogs even if you want to introduce and older bloodline via frozen semen UNLESS a substantial record of offspring data exists in a public data base.

Epistatic over-ride.

o Coat color inheritance in Labradors is an example of epistatic over-ride.

o If a dog has 2 copies of the Recessive Epistatic gene – ee – then the Epistatic color [yellow] will over-ride the Black and Chocolate color genes.