The Institute of Canine Biology
  • HOME
  • Blog
  • Courses
    • COI BootCamp (FREE!)
    • Basic Population Genetics (FREE)
    • The Science of Canine Husbandry
    • Managing Genetics For the Future >
      • Syllabus - Managing Genetics for the Future
    • The Biology of Dogs (Open Reg )
    • DNA For Dog Breeders >
      • Syllabus - DNA for Dog Breeders
      • Open Reg - DNA For Dog Breeders
    • Understanding Hip & Elbow Dysplasia >
      • Open Reg - Understanding Hip & Elbow Dysplasia
    • Genetics of Behavior & Performance >
      • Syllabus - Genetics Behavior & Performance
      • Open Reg - Genetics of Behavior & Performance (Open Reg)
    • Strategies for Preservation Breeding >
      • Open Reg - Strategies for Preservation Breeding
    • Group Discounts
    • MORE FREE COURSES >
      • Quickie Genetics (Free!)
      • Heredity & Genetics (Free!)
      • Useful Genetics (Free!)
      • Basic Genetics Videos
  • Breed Preservation
    • Breed Status
    • Breeding for the future >
      • BFF Breed Groups
    • The "Elevator Pitch"
    • What's in the Gene Pool?
    • The Pox of Popular Sires
    • What population genetics can tell us about a breed
    • What population genetics can tell you...Tollers & Heelers
    • How to use kinship data
    • Using EBVs to breed better dogs >
      • How population size affects inbreeding
      • EBV Examples
    • How to read a dendrogram
    • Global Pedigree Project >
      • The Database
    • Finding the genes without DNA
    • How to read a heat map
  • Health Data
    • Bloat (Purdue Study)
    • Body Condition Score >
      • % Dysplastic vs BCS
    • Breed Comparions
    • Cancer
    • Cardiac
    • Cataracts
    • Caesareans
    • Deafness
    • Degenerative Myelopathy
    • Elbow Dysplasia
    • Epilepsy
    • Genetic Diversity
    • Genetic Diversity (MyDogDNA)
    • Hip Dysplasia >
      • Hip Dysplasia (Hou et al 2013)
    • Inbreeding Effects
    • Inbreeding (Gubbels)
    • Inbreeding (Dreger)
    • Lifespan
    • Litter size
    • Metabolic
    • mtDNA
    • Orthopedic
    • Mode of Inheritance
    • Patella Luxation
    • Thyroid
    • Portosystemic shunt
    • Purebred vs Mixed (UC Davis)
    • Purebred vs Mixed Breed (Bonnett)
    • Spay & Neuter Effects
    • Calboli et al 2008
    • Hodgman (1963)
    • Scott & Fuller (1965)
    • Stockard: Purebred crosses
    • Summers (2011)
  • Projects
    • How To Interpret Breed Analyses
    • Afghan Hound
    • More details about the Toller study
    • Belgian Tervuren >
      • Belgian Terv p2
      • Belgians- why population size matters
    • Bernese Mountain Dog
    • Boxer
    • Brussels Griffon
    • Bullmastiff
    • Canaan Dog >
      • Canaan analyses
    • Cesky Terrier >
      • Cesky genetic history
    • Chinook
    • Curly-coated Retriever
    • Doberman
    • Entelbucher Mountain Dog
    • Flatcoat Retriever
    • French Bulldog
    • German Shorthair
    • Golden Retriever >
      • Golden Retriever Pedigree Charts
    • Irish Water Spaniel >
      • IWS (6 Nov 17)
    • Labrador Retriever
    • Manchester Terrier
    • Mongolian Bankhar >
      • Research Updates
      • Bankhar 1
    • Norwegian Lundehund
    • Plummer Terrier
    • Otterhound
    • Portuguese Water Dog >
      • Portuguese Water Dog (pt 2)
    • Ridgeback
    • Schipperke
    • Standard Poodle >
      • The Problem With Poodles
      • 3poodle pedigree charts
      • 3Poodle Wycliff dogs
      • Poodle Genetics
    • Tibetan Spaniel
    • Tibetan Mastiff
    • West Highland White Terrier
    • Whippet
    • Wirehaired Pointing Griffons
    • UK KC Graphs >
      • UK KC Breed Status
      • UK Groups
      • KC Gundogs
      • KC Hounds
      • KC Terriers >
        • Terriers (select breeds)
      • KC Pastoral
      • KC Toys
      • KC Working
      • KC Utility
      • Australian KC
    • Breed outcrossing programs
  • Resources
    • Genetics Databases
    • Stud Books >
      • American Kennel Club stud books
      • Field Dog stud books
      • The Kennel Club (UK)
    • Learn
    • Videos about dog genetics
    • The Amazing Things Dogs Do! (videos) >
      • Livestock Management
      • Livestock guarding
      • Transportation, exploration, racing
      • Conservation & wildlife management
      • Detection Dogs
      • Medicine & Research
      • Entertainment
      • AKC/CHF Podcasts
    • Read & Watch
    • Bookshelf
  • Preventing Uterine Inertia

Why do mixed breed dogs have so many mutations?

8/23/2019

 
By Carol Beuchat PhD
As we accumulate data from DNA testing, some interesting patterns are starting to emerge.

​A recent study has accumulated data for about 100,000 dogs, including about 83,000 mixed breed dogs and 18,000 purebred dogs representing 330 breeds (Donner et al 2018). For testing, the authors used a SNP array that tests for 152 known mutations, or "variants," at the same time. These tests cannot identify all mutations, only the ones that have been identified and localized at a particular SNP. So the data represent only a fraction of the actual number of mutations in the entire gene pool.
They organized their results in a Venn diagram, which identifies the number of mutations found in mixed breeds, purebreds, both, and not found in this population of dogs.

Picture

The data reveal that 34 mutations were only seen in mixed breed dogs, 13 were only seen in purebreds, and 25 were not found in this sample population. Of the mutations identified, 80 were found in both mixed and purebred dogs. If we add up the numbers for mixed and purebreds, we find that a total of 114 mutations (34 + 80) were found in mixed breed dogs, and 93 occurred in purebred dogs.
These results are very clear. More mutations were found in mixed breed dogs than in the purebreds. 

You might look at these data and conclude that this is proof that purebred dogs are healthier than mixed breeds. Would you be correct? 
Let's remember a few things. Recessive mutations are usually not expressed (i.e., they have no effect) unless the dog is homozygous for the mutation; i.e., to be affected, the dog must inherit two copies of the defective allele, one from each parent. A dog with only one copy of the mutation would be called a "carrier" and, for most recessive mutations, carriers suffer no ill effects.

One consequence of this behavior of recessive mutations is that a dog can be a carrier of many mutations but suffer no ill effects from any of them. In fact, because of this, recessive mutations can be passed from parent to offspring generation after generation without affecting the health of the dog.

Dogs that inherit two copies of a mutation (called homozygous, affected, or "at risk") are likely to display some negative effects like blindness, exercise collapse, a nervous disorder, or some other disturbance to a body function. If the consequences are severe enough, natural selection or the breeder will remove the dog from the population of breeding animals and those mutations are not passed on to offspring.

So we have two situations: in one, a dog carries a single copy of the mutation and suffers no ill effects (heterozygous), and in the other the dog carries two copies and the mutation is expressed as a genetic disorder.
Now look back at our Venn diagram. There are more mutations found in the population of mixed breed dogs. But what we need to know is whether they are homozygous and affect the dog, or heterozygous and are harmless.

This pair of graphs show the proportions of mixed breed and purebred dogs in which the mutations were heterozygous (left). The graph on the right shows the proportion of dogs in which the mutations were homozygous (right). Mutations in the heterozygous state on the left were found in both mixed and purebred dogs, but as we would expect from the Venn diagram above, more of the mixed breed dogs were carrying mutations (yellow arrow). The graph on the right shows what we really need to see: both mixed and purebred dogs had homozygous mutations, but they were much higher in purebred than mixed breed dogs.
​
Picture

​Here is what the study's authors had to say:
"The distribution of the number of disease variants carried in the heterozygous state differed significantly between mixed breed dogs and the combined purebred sample, with a higher ratio of mixed breed dogs being carriers of the common analyzed disease risk alleles....

However, when we compared the groups for the number of common recessive disease 
variants carried in the homozygous state, an opposite pattern emerged... ​Purebred dogs were 2.7 times more likely than mixed breed dogs to be genetically affected for at least one of the common recessive disorders (3.9% vs. 1.4% of dogs, respectively)."

In a nutshell, here's what they found:

a) The collective population of mixed breed dogs contains more recessive mutations than that of purebreds, but those mutations are more likely to be heterozygous and therefore harmless.

b) On the other hand, there are fewer mutations found in purebred dogs, but they are far more likely to be homozygous, and therefore expressed as a genetic disorder, than in mixed breed dogs.

The fact that mixed breed dogs have so many mutations does not make them more prone to illness. In fact, they are less likely to be affected by a genetic disorder than purebred dogs, because the mutations are more often found in the harmless, heterozygous state. The mutation load in purebred dogs is smaller than in mixed breeds, but it is far more dangerous, because the mutations are much more often homozygous.

It is this simple property of recessive mutations, of only being expressed when homozygous, that results in the higher rate of genetic disorders in purebred than mixed breed dogs. 

This comparison of the health of mixed and purebred dogs is oft debated, and breeders usually argue that purebred dogs are just as healthy as mixed breed dogs. For problems caused by non-genetic disorders (broken bones, allergies, and the like), this could be true. But, for genetic disorders, the purebred dogs fare far worse than mixed breed dogs. It's a simple matter of the genetics of recessive mutations.

Why are there more mutations hanging around in the huge gene pool of mixed breed dogs? It's because recessive mutations are harmless if they are heterozygous, so they are not eliminated from the population by selection. The frequency of a specific mutations in the population is usually very low (< 1%, with a few notable exceptions, like the mutation associated with degenerative myelopathy, DM). The chances of any two random mixed breed dogs having the same mutations is low. Although a mixed breed dog can be affected by a genetic disorder caused by a recessive mutation if it inherits two copies, the chance of this happening is very low.

Breeding purebred dogs in a closed population means that all of the dogs in a breed are related and therefore share many genes in common. Some of these shared genes will be mutations. The more closely related the sire and dam, the more likely the puppies are to be afflicted with a genetic disorder caused by a recessive mutation.

This is why the best strategy for reducing the incidence of genetic disorders in purebred dogs is not endless DNA testing. To control the risk from all mutations, the ones you know about as well as the ones you don't, the best strategy is to reduce the relatedness of the parents. Reducing genetic disease in dogs boils down to reducing the risk of a puppy inheriting two copies of the same recessive mutation. The way to accomplish this is to reduce the relatedness of the parents. 
Mixed breed dogs have more mutations than purebreds. But they are less likely to be affected by genetic disorders because they are more likely to be heterozygous; i.e. have only one copy of the mutation.

REFERENCES

Donner J, H Anderson, S Davison, AM Hughes, and others. 2018. Frequency and distribution of 152 genetic disease variants in over 100,000 mixed breed and purebred dogs. PLOS Genetics 14(4):31007361. https://doi.org/10.1371/journal.pgen.1007361

You can learn more about the genetics of dogs in ICB's online courses.
Check out ICB's course "COI Bootcamp"

*** Visit our Facebook pages ***

ICB Institute of Canine Biology
...the latest canine news and research

ICB Breeding for the Future
...the science of animal breeding

Comments are closed.

    Archives

    January 2025
    November 2022
    July 2022
    May 2022
    April 2022
    March 2022
    February 2022
    November 2021
    October 2021
    December 2020
    January 2020
    August 2019
    July 2019
    June 2019
    May 2019
    April 2019
    March 2019
    February 2019
    January 2019
    December 2018
    November 2018
    September 2018
    August 2018
    July 2018
    June 2018
    May 2018
    October 2017
    August 2017
    May 2017
    April 2017
    March 2017
    February 2017
    January 2017
    December 2016
    November 2016
    September 2016
    August 2016
    July 2016
    June 2016
    April 2016
    March 2016
    February 2016
    January 2016
    December 2015
    November 2015
    October 2015
    September 2015
    August 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    January 2015
    December 2014
    November 2014
    October 2014
    September 2014
    August 2014
    July 2014
    June 2014
    May 2014
    February 2014
    December 2013
    October 2013
    September 2013
    July 2013
    March 2013
    July 2012
    April 2012

    Categories

    All
    Behavior
    Border-collie
    Herding

Blog

News


About Us

Contact Us








Copyright © 2012-2017 Institute of Canine Biology
Picture
Picture