Genetics Question!

[becks]

In one of my breeds, they can get Hereditary Cataract - but it isn't the normal recessive gene it is described as 'dominant with incomplete mode of inheritance'. Can someone explain to me how this is supposed to work?

[dancinbcs]

In one of my breeds, they can get Hereditary Cataract - but it isn't the normal recessive gene it is described as 'dominant with incomplete mode of inheritance'. Can someone explain to me how this is supposed to work?

This means that the dogs do not "carry" the gene as a recessive trait. They only need one copy of the gene from one parent to be affected but the degree they are affected will vary from barely noticable on examination to completely blind. Unaffected progeny (those that do not have a copy of the gene) from an affected dog can never produce the problem.

With recessive problems a dog has to inherit one faulty gene from each parent and therfore have two copies of the faulty gene to be affected. Dogs with one copy are unaffected carriers.

With dominant problems only one copy of the gene is required for the animal to be affected.

Using coat pattern as an example. A merle dog can vary from a tiny patch of merle to all over colour pattern but it must have one merle parent. Non-merle offspring of a merle can never produce the pattern unless they ate mated to a merle. Then the pattern will come from the other parent.

Edited October 10, 2011 by dancinbcs

[becks]

OK, so for the incomplete mode of inheritance just means that the dog can be affected in a range from barely to completely. Thanks, that was the bit I wasn't understanding! I was thinking you either inherit something or you don't, but the incomplete bit just refers to how it is expressed in the affected dog.

[zenchel]

In one of my breeds, they can get Hereditary Cataract - but it isn't the normal recessive gene it is described as 'dominant with incomplete mode of inheritance'. Can someone explain to me how this is supposed to work?

This means that the dogs do not "carry" the gene as a recessive trait. They only need one copy of the gene from one parent to be affected but the degree they are affected will vary from barely noticable on examination to completely blind. Unaffected progeny (those that do not have a copy of the gene) from an affected dog can never produce the problem.

This is not a correct definition of what I'm assuming the OP is enquiring about - "dominant with incomplete penetrance". As stated above a dominant gene will always express itself, so it can be inherited from one only parent. However the rider "incomplete penetrance" means that occasionally the dominant gene doesn't express itself, so any offspring of a dog that has say a cataract which is dominant but with incomplete penetrance, may not show clinical signs of the cataract, but genotypically is still affected, therefore can pass it on to the next generation. This mode of inheritance is really quite tricky to deal with, as if you have a clinically clear offspring of an affected parent, you would assume that the offspring is amongst the statistical 50% that have not inherited the dominant gene, however it may turn out to be one of those with "incomplete penetrance" and may go on to produce it in the next generation. It sort of behaves like a recessive, but not.....

I think what dancingbcs is referring to is "variable expression", which is as explained a varying degree of severity of expression of a particular disease, but not related to its mode of inheritance.

Sylvia

[becks]

Thanks Sylvia - no wonder it is so hard to track within the breed, especially as dogs in the breed have been testing 'clear' until 9 or 10 years old and then showing as affected at that late age, while others are affected on their first adult eye test!

[zenchel]

Thanks Sylvia - no wonder it is so hard to track within the breed, especially as dogs in the breed have been testing 'clear' until 9 or 10 years old and then showing as affected at that late age, while others are affected on their first adult eye test!

Yep, exactly the same in Labradors. It's a stinker

[becks]

Oh! Is it a dominant gene in labs? Do you have a test for it or just eye checks?

[zenchel]

Oh! Is it a dominant gene in labs? Do you have a test for it or just eye checks?

No DNA test, but Optigen are working on one.

Optigen Cataract Research

But as you say, they appear at widely varying ages - 12 months up to ten years - in Labs also. Is it the same disease? They rarely progress to anything worse than a star cataract in Labs, although thinking about it I wonder if the ones that appear early are the ones that do progress? I've had dogs diagnosed at five years and they never get any worse. I've never had one any younger than that, so don't know whether they progess.

Cheers,

Sylvia

[becks]

Posterior Polar Cataract is the name given to the type Giants get.

[Kia]

Posterior Polar Cataract is the name given to the type Giants get.

Australian Shepherds can be affected by these cataracts as well ....usually both eyes. We do have a DNA test available. While we are able to test for HSF4 cataracts there are still others that can show up so regular eye checks are important. There are aussies who carry a copy of the HSF4 gene who never cliniclly show signes.

[becks]

is it dominant in aussies or a recessive trait?

[Kia]

is it dominant in aussies or a recessive trait?

The HSF4 gene is dominant...so only needs 1 copy. It also has "incomplete penetrance" so a dog can be DNA'd as affected but never show cataracts.

[becks]

Thanks!

[sandgrubber]

Maybe this is heresy . . . and not to contest the validity of asking these questions. But I worry about the larger picture.

1. Hereditary cataracts, apart from PRA seem to be difficult to diagnose because its manifestation is often (usually??) mild. The dog may end out with poorer vision, but seldom goes blind. Sometimes there is no detectible problem. Dogs function well with partial vision impairment, and there's no indication that cataracts are painful.
2. Non-genetic testing is expensive, and not definitive. It requires regular examination by a specialist . . . for those living in rural areas this means long drives.
3. Genetic tests may be or may become available . . . in which case many of us will need to decide whether to apply them.
4. Something like 1/3 dogs dies of cancer. In humans, it's well known that there are genetic propensities to various cancers. To my knowledge, there are no tests available.
5. I haven't seen a lot of statistics about the incidence of hereditary contract. I'm under the impression that it's one percent of less in the breed I'm most familiar with.

I am all for health testing . . . but it's going to be the undoing of pedigree dog breeding if we end up with a long list of diseases for which we are expected to test, and mixed breed breeders have no testing obligations. If we are obliged to do all the tests the geneticists develop, and prioritization follows the lines of what the geneticists are able to come up with, rather than what ailments are most serious for dogs, it's going to get ugly.

How important are these tests? Are there higher priorities?

Edited October 19, 2011 by sandgrubber

[angelsun]

Testing is important but yes, there are higher priorities depending on the health issue that breeders are trying to 'manage'. We have cataracts in our breed, but with the exception of a few dogs with PRA that have lost their sight, we have no had any dogs go blind from punctates. However, we have a great deal of people that are willing to throw out dogs, with one or two small punctates that have remained stable for years, simply because they have them and show no cases of it being passed along to offspring or further generations. As a result, we have lost some good pedigree/balanced and structurally complete, dogs to the mix. Mediocre dogs of both physical and mental traits are being used simply because they 'eye test clear'.

I believe that our eye issues are not life threatening, nor debilitating and although I test yearly, and will base my breeding on those results, will not eliminate a dog with a few punctates, in favour of a lessor dog with clear eyes. I do my best to not double up on a problem, but I would hope that applies to ANY problem, whether it's eyes or crappy fronts or bad temperaments!

We can't lose sight of the bigger picture based on some single sided genetic issues that may not affect the breed as a whole, however we can not ignore them totally either. Too many people that are relatively new to breeding, seem to feel that things need to be either fixed in one generation or that they only, can 'save' their chosen breed.

[Kia]

It isn't about removing good dogs from the breeding pool but trying to minimise and eventually breed out the problems.

Yes on going testing is expensive both financially and time wise. I do the DNA testing so I can make informed decisions.