Australian Shepherd Health & Genetics Institute

Australian Shepherd Health & Genetics Institute

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Are congenital diseases and defects hereditary?

Some are and some aren’t.  “Congenital” eans present at birth.  Any birth defect would be congenital.  For example, dogs born with the heart defect patent ductus arteriosus have an inherited congenital defect.  Dogs born with problems caused by a toxic exposure to the dam during pregnancy would have non-inherited congenital defects.

 How many genes to dogs have?

Scientists once thought that dogs and other mammals had tens of thousands of genes, possibly as many as 100,000.   However, once they learned to decode the DNA they realized the number was much lower.  In the late 1990s they reduced the estimate to around 40,000.  The more detailed their genome map became, the more they reduced the estimate.  Currently dogs are thought to have around 20,000 genes.

Why were early estimates of gene numbers so far off?

Huge numbers of genes were once though necessary to provide the instructions for all the parts and operations necessary to make a mammal function.  Now we know that genes aren’t specialists:  They multi-task.  One gene might be responsible for different things at different times of life or in different body tissues.  Recent study of the epigenome – the system that regulates the genes – has shown that various mechanisms turn genes on and off like a switch or up and down like a rheostat depending on life stage, what type of cell the gene is in, or in reaction to environmental factors.

What are linked genes?

Genes that lie close to each other on the same chromosome become linked when the all chromosomes except the sex chromosomes swap sections prior to dividing in half in the process that leads to the production of sperm and eggs.  Some chromosome areas are more prone to separation during recombination so certain genes almost always remain linked.  There is less linkage of dog genes because they have been divided up into more chromosome packages.   Dogs have 78 chromosomes (39 pair) compared to 46 (23 pair) for humans.  Dogs, humans and most other mammals have about the same number of genes.  The current best estimate is around 20,000.

What is complex inheritance?

Formerly the term “polygenic” (many genes) was used.  However, we now know that it is more complicated than that.  Complex inheritance can involve multiple genes, DNA sequences that influence gene function and in some cases environmental factors.

Is there a way to know how much a parent contributes to a complex trait like hip dysplasia?

Not precisely.  Observation and record-keeping may help you determine which crosses are the best bet for getting or avoiding a complex trait.  Hip dysplasia is the classic example; each parent must contribute something to the mix of genes that produces good hips or bad, but the contribution might not be equal.  Even with the best hip phenotype, some dogs will consistently produce sound hips while others with equally good hips will be more likely to produce HD.  Dogs which produce multiple offspring with HD (or any other complex trait), especially with multiple and unrelated mates, carry more “load” for HD – they have more of the gene versions and other elements necessary to produce HD even though they themselves have a superior hips.

If you want to know the potential of a dog that has not been bred, has only a few offspring, or whose offspring are too young to know if they exhibit the trait, you need to do pedigree research.  The more connections the dog has to the trait, whether wanted or unwanted, the more likely the dog is to produce the trait.  The IDASH pedigree analysis program is a method owners can use to determine risk for producing diseases.

What is the distinction between “incomplete dominant” and “dominant with incomplete penetrance”?

If a gene has incomplete dominance, the heterozygote (individual with one dominant and one recessive gene) has a phenotype (what you see or can measure) between the dominant and recessive types.  Pelger-Huet Anomaly is an incomplete dominant.  Dogs with two of the defective alleles will die before birth.  Dogs with only one copy will be healthy, but there will be minor abnormalities in some of their blood cells that indicate they carry the gene.  Dogs with two of the normal versions of this gene do not have these blood cell abnormalities.

Incomplete penetrance means that sometimes you see the trait and sometimes you don’t.  The dog could have one or two copies of the responsible allele but never have the disease.  The gene HSF4, which contributes to most hereditary cataracts in Aussies, is incompletely penetrant.  Dogs that do not have the dominant mutation are unlikely to have cataracts, those with one or two copies might, but might not.  As genetic mechanisms are becoming better understood these genes are now usually called risk factor genes.  Having a particular version makes you more likely to have the disease.  How likely it is will depend on the level of penetrance of that gene version, with high penetrance making disease more likely and low penetrance less so.  When a risk factor gene has been identified and studied, penetrance is expressed as being x (number) or (number)-fold. (Examples:  17-fold or x17)

Aren’t genes several generations back too diluted to worry about?

The idea that genes get “diluted” across the generations is inaccurate.  Genes don’t dilute, but every ancestor on average passes half of whatever genes it had to its offspring – one from each gene pair.  Statistically, a dog will have half of a parent’s genes, a quarter of a grandparent’s, an eighth of a great-grandparent’s and so on.  In actuality it may have more or less than that number but in most cases whatever came down will hover around those amounts.   A dog five generations back statistically only passes on 3.13% of its genes.  Chances are low that your dog has any particular one of that ancestor’s genes; however he almost certainly has some; 3.13% of 20 thousand is just over 626 genes.  Keep in mind that if a dog appears in a pedigree multiple times, then it is more likely that its genes will be passed on.

If a dog has only 50% of each parent’s genes, when you breed full siblings together can you re-creating the original dog? 

Since there are only two individuals (each appearing twice) on the grandparent line it is true that the grandpups will, statistically, have half of the common grandsire’s genes and half of the common granddam’s.  However, during the creation of sperm and eggs the chromosome pairs split; only one from each pair goes into each germ (reproductive) cell.  This will have happened twice between grandparent and grandpup therefore, while most offspring of a full-sib cross will have close to the statistically predicted portion of each grandparents’ genes, the specific collection of gene copies each pup gets could vary considerably and they are very unlikely to even come close to exactly duplicating the genome of either grandparent.

How much do the grandparents contribute to a litter?

Statistically, each grandparent will contribute 25% of its genes to its grand puppies, great grandparents would provide 13.5%, and so forth as you go back in generations.  However, these are just statistical norms; the actual number will be around the percentage in most cases but can vary.

Are genetic traits passed to 50% of the offspring?

While half of a dogs’ genes come from each parent, bur the pup gets one copy of each gene from each parent.  What particular combination of gene versions a puppy inherits will determine whether it does or does not have the trait.  Statistically, if you breed two Black that carries red to a red, you will have 50% black pups and 50% red, but in any given litter you can have all possible combinations from all of one color to all of the other.  Single litters are too small to reliably have a perfect statistical result.

Couldn’t we use frozen semen to get back to the type of Aussie we had before big hair and big bone became the rage?  And couldn’t we also get back to a place where health issues weren’t as bad as they are now?

Frozen semen is a mixed blessing.  It has great potential for positive benefit, but the dogs whose semen will most frequently be used or stored are those who have already been bred extensively. Wide use of popular sires reduces genetic diversity within the breed and has the potential to spread widely whatever undesirable genes those dogs have, with the possibility of making a formerly infrequent genetic issue common.

On the plus side, long-term semen storage can also be a way of bringing the breed back to an older type or for preserving bloodlines that have been neglected – IF breeders are willing to use semen from dogs that have become “out of fashion.”

When it comes to using semen from long-ago dogs, the technology is relatively recent and significant early Aussies lived and died before semen storage was possible.  On a practical level, keeping semen from males that excelled at real work or performance events could be beneficial in the future because it is their brains and physical abilities that are important and these will remain desirable years down the line.  For conformation dogs which are subject to fads and fashion, the top dogs of decades past may be so out-of-date nobody would be interested 20 years hence.

From a health standpoint, it wasn’t that the diseases we are dealing with today weren’t hapening, they just weren’t as frequent as some of them are now.  You might be able to bring in blood that didn’t have something that has since become very common, like epilepsy or hemangiosarcoma.  But you have to keep in mind that every dog carries bad genes for something and overemphasis on the “new”  blood from a particular long-ago dog or line could lead to a new frequent problem.

Before storing frozen semen or using it consider the big picture rather than a single breeding.  Look at the potential for both good and ill from every angle, rather than just health or appearance.

 Is there any legitimate reason why someone would crossbreed?

Yes.  It’s done all the time in commercial livestock.

In purebred dogs it is generally shunned, not that it doesn’t happen.  For example, the color merle has appeared in several breeds that had no previous history of merle.  Odds are that some creative individual either introduced the gene and then fixed breed type by backcrossing or there was an “oops” litter that someone decided to capitalize on.  Merle isn’t a gene that mutates from the non-merle wild type readily.

Legitimate reasons for crossbreeding include:

  1. Increasing the diversity of a rare breed’s gene pool.
  2. Reducing the frequency of genes for a serious or lethal disease in a breed population where many individuals carry those genes.
  3. To produce dogs more suitable for a particular type of work or performance event.
  4. Developing an entirely new breed.

Many breeds have a history of crossbreeding in their past; the Doberman was created in this manner.  More recently crosses have been done to address important issues (Dalmatian and Pointer to get away from bladder stones, Boxer and Corgi to introduce a bobbed tail).  There is nothing wrong with crossbreeding if it is open and for a well-defined purpose.  (Yes, Labradoodles have a purpose.)  Crossbreeding has been used, legitimately, to produce dogs better for a purpose, like guide dogs or agility.

However, fraudulent crossbreeding within an established breed through use of falsified documentation is ethically wrong and against the rules of every legitimate registry.