A "Different" thread on GENETICS........

[tadkerson]

The problem with breeding for vigor or disease resistance is coming up with a way to measure the outcomes (dependent variable) of the breeding program. The average person does not run a controlled experiment to determine if there actually is a difference in one breeding program or another breeding program. So you really do not know if what you say is happening is happening.

In the case of Marek's, you will never know if a bird is resistant because the resistant birds do not show signs of being infected. They do not get sick. If you make sure every bird in a test group is exposed to mereks and all the birds contract the disease but one- you can be somewhat sure that the one bird was resistant. You would want to expose the bird to the virus again just to make sure. Then breed the bird and test the offspring to see if the resistance can be inherited.

I am not trying to rain on your parade.

Tim

 

[Krys109uk]

In the case of Marek's, you will never know if a bird is resistant because the resistant birds do not show signs of being infected. They do not get sick. If you make sure every bird in a test group is expose to mereks and all the birds contract the disease but one- you can be somewhat sure that the one bird was resistant. You would want to expose the bird to the virus again just to make sure. Then breed the bird and test the offspring to see if the resistance can be inherited.

Absolutely. I am not trying to prove anything. I am just recounting what I've been doing & what I've experienced.
Also there are lots of other factors which could affect the incidence of what appears to be Marek's. But I did read one can breed for immunity to Marek's, so, hopefully, I am going in that general direction. And if not I'm sure it won't make any difference. I'm not claiming anything.....just hoping.

 

[Junkmanme]

Interesting input.

I can see the subjective nature of trying to breed for disease resistance as expressed in Tim Adkerson's example of Marek's disease. I agree that it is unlikely that more than a small few of us "Backyard Chicken enthusiasts" would go to the trouble and expense of conducting a verifiable experiment to actually support the conclusion that disease-resistance had been achieved.

Along a similar line, my late wife spend considerable effort over a couple of decades in trying to get "rabies vaccination" approved for wolf-dogs (wolf-hybrids) with no success because people would not believe that it required extensive (and expensive) testing in order to get the approval from the government. Therefore, it is not approved YET. (People expected that approval could be accomplished "politically"...which is NOT the case.) Many folks get rabies vaccination for these "pets" by claiming that they are "German Shepherd/Huskie", etc.

NEVERTHELESS, I think by close observation a "Backyard Chicken Enthusiast" can make some reasonble, accurate, though "scientifically unproven" conclusions in their Breeding Programs simply by "selective breeding", much as was done in prior centuries.

But I WONDER if there have been "identifiable genes" that have been SHOWN to foster certain immunities and/or general health and vigor?

I seem to recall reading somewhere on these forums, for instance, that some particular European breed seems to be naturally immune to Marek's.

I was thinking that this would be something somewhat akin to the "naked-neck" genetics...?????

Certainly, I haven't the educational background to comment much, but I've surely got the questions! (Ha-Ha!)

-Junkmanme-

 

[Junkmanme]

Here is an ONLINE article that I "stumbled upon" that relates somewhat to the subject of this thread:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1569506/

Although the information in this article is LARGELY "beyond" MY education and knowledge, I'm sure that there are those on this particular Forum to whom this information would be interesting and more understandable.

I gathered from this article that chickens are very important in the study of how genetics affects disease.

-Junkmanme-

 

[patandchickens]

Quote:
The problem is that you cannot identify a gene (or alleles thereof) except by how they express themselves in the observable phenotype of the chicken. With something like color or weight gain or #eggs/year, this is easy to observe and there is not much to confuse the situation.

Whereas as Tim points out, there is a LOT to confuse the situation when the trait of interest is disease resistance, because there is such variation (among chickens, and as time goes by) in how and whether a chicken is *exposed* to a particular disease. So unless you are going to go deliberately infecting them as part of a study, you are stuck with the fact that chickens that don't fall sick with <whatever disease> are likely to be a MIXTURE of those who are genetically resistant AND those who happened not to be exposed.

It is my impression that, when disease resistance *has* been deliberately assessed by researchers, it is the sort of 'landrace', always-been-around-but-never-selected-for-foofy-show-characteristics types of chickens that tend to be most resistant to disease, and breeds/lines that have been heavily selected for *appearance* (sometimes from a small initial gene pool) that are more apt to be susceptible. Except that there are lots of exceptions

It seems to me the most practical thing for us backyarders is to simply not breed from birds that exhibit questionable health or come from a sire/dam of questionable health. Unfortunately this IS only practical IF you have a large enough flock to have a choice. Someone with, like, one roo and 8 hens is just not going to have a lot of scope to select for disease resistance.

Also, the better biosecurity you practice, the less useful this approach is, since if your chickens are seldom challenged by any diseases, you don't get to observe which are more or less resistant.

JMHO,

Pat, currently facing the decision of whether to keep the only sussex cockerel I managed to hatch out of my sussex pen this year, because although he is *very much* the body type I want, he has had somewhat loose stools for the past several months and I just dunno. Sigh.

 

[lauralou]

Maybe this isn't what you are talking about, but we recently culled a weak chick. It was half the size of the others, and just plain pitiful, in my opinion. It cried all the time. It was really hard to convince my husband to do the deed.

I've also heard that you should never help a chick out of it's shell. That if it isn't strong enough to hatch, well, you get the picture. In theory, I think I agree with that. In practice, I've helped a chick out of it's shell. I had a lot of eggs under a broody hen, and on hatch day I noticed that one egg had been crushed. The entire shell was crackled up with the membrane drying out. I knew that the chick had little chance of zipping that egg. So I waited and waited, and finally extracted the chick. I'm really glad that I did. I legbanded her, and now I see that she is a she, and is as healthy and active as any of the others. I only got 3 pullets out of that hatch of nine, so each pullet is very valuable to me!

 

[Pathfinders]

Quote:
We call chicks like that "puny", and at this point any chick that fails to thrive gets culled (doesn't happen often, maybe one out of 200 or so.) I know it's hard, but it's best. When my younger daughter was about 10 we had a chick like that. We had helped her out of her egg, and she was just tiny and runty. My daughter named her Little Pip. She lived for about a year, but had to be penned separately from the other birds, as she was just too small to be in with them. It was a sad life, and taught my daughter not to fall in love with the ones who should really be left to die (although she still is a jam tart when it comes to the small and malformed...) <sigh>

 

[McSpin]

Quote:
Immunity to disease is definitely inherited. Immune system genetics is quite complex and very difficult to explain in detail. In simple terms, the DNA recombines into many (millions) of different random combinations, each one capable of producing an antibody that is designed to combat a specific pathogen. When the genes recombine, they do so geometrically. So a small increase in diversity creates a geometric increase in disease resistance. This is why out-crosses to unrelated stock or to stock known to carry the needed antibodies is so important. When line-breeding for a long time, the immune system looses diversity through a few different mechanisms. Genes are lost through cross-overs and homozygosity increases.

One of the big problems with backyard chicken keepers is that they do not introduce new, unrelated stock with enough frequency. Show people's birds are especially affected. They tend to get the best stock (whoever has the best show birds at the time) and when they introduce new birds, there is a high probability that that stock also descended from the same winning line.

This also helps explain why hybrids tend to have so many great qualities. Gene diversity is a good thing when talking about immunity and general health of an animal. The only thing it's not good for is winning shows on a consistent basis

 

[Ridgerunner]

I think you should be careful not to confuse hybrid vigor with selective breeding. Here is a definition of hybrid vigor I found.

Increased vigor or other superior qualities arising from the crossbreeding of genetically different plants or animals. Also called heterosis.

And one for selective breeding. True for plants as well as animals.

The intentional mating of two animals in an attempt to produce offspring with desirable characteristics or for the elimination of a trait

Of course, whether a trait is superior or not is often in the eye of the beholder, but the way I understand it, heterosis has been accepted by a lot of the scientific community. Genetic crosses often give a more vigorous offspring. I think an example is a lot of the hybrid vegetables we grow. Specific crosses produce specific traits that most gardeners find superior. You cannot safely save seeds from these hybrids because you never know what you will get.

Not all crosses are going to give you superior traits. For example, a dairy farmer will often breed his heifers (cows not yet having produced a calf) to a black angus bull since the black angus bull tends to father calves that are small at birth. For a cow having her first calf, it is good for the calf to be small at birth. There is less likelihood that the cow will have birthing difficulties. In this case, small calves are considered a superior trait. In subsequent years, this is not as important a trait since the cow is more mature and can more safely birth a larger calf. However, if you breed a dairy bull to a black angus heifer, the calf is likely to be large at birth, endangering the cow.

Going back to the vegetables, if you do plant the seeds from a hybrid, you are very likely to get a lot of plants that produce poor results. However, you might find some that produce very good results. If you save the seeds from those plants and use them, you will again get mixed results, but maybe more are acceptable. If you go through this process several generations, you may come up with a plant that consistently produces good results. But you have lost that genetic diversity and may have also enhanced some undesirable qualities.

I think sex links are a good example of hybrid vigor. They tend to lay very well. But the next generation cannot be sexed at hatch and may or may not be as good layers. That's where selective breeding comes in. Out of the second and following generations, you reproduce only from the ones with the traits you want. But you also concentrate any bad traits that they may have.

If you have the desire and facilities for a major breeding program, you can try to get pretty sophisticated, but I expect for most of us the best we can do is select healthy breeding stock that exhibits the traits we want and eliminate those that exhibit traits we don't want and we are doing the best we can. And I'm one of those that Pat described. I do not have enough different birds to avoid including some bad traits, so I will at some point have to introduce new stock.