University of Alberta heritage breeds

ipf wrote:By selecting and breeding strongly for the first set, one can't help but diminish variation (to a greater or lesser degree) in the second.

I understand what you are saying, but why? If you have a large enough population and select for, say, a specific comb, and you also want to select for resistance to a certain disease, some with the comb will be resistant and some will not. If you further select the resistant birds from the population with the correct comb can't you have your cake and eat it too?

I realize this is a very simplified example, and things are far more complicated, genetically. Is it then this complexity the reason to have diversity for diversity's sake?

"If you have a large enough population" is key to this. Large enough (to geneticists) may mean in the thousands, or at least hundreds.

Backing up a bit.You all know that genes come in pairs on chromosomes, right? The location of a specific gene (call it G) on its chromosome pair is called its locus (plural=loci). So every individual has two Gs. There may be several or many different versions (called alleles) of G1 in a species; call them G1, G2, etc. but each individual has exactly two Gs. One individual may be of genotype(=genetic makeup) G1G1, another G1G5, another G4G3. Lots of possibilities. If, through selection and breeding, you've managed to get rid of all alleles except the one you want, all individuals in your population will be (say) G2G2, and the allele/trait is said to be "fixed" in the population. Thing is, that when you select, inbreed, backcross, sib mate, all those things, in an effort to fix alleles at one locus, you reduce diversity not only at the locus you're interested in, but at others as well. This may or may not seem intuitively obvious to you. There's a lot of math involved in proving this, but the math has been well done, well understood, and well accepted for decades.

If through inbreeding you accidentally lose valuable alleles at other loci, you risk damaging the adaptive capability of your population.

Some alleles are present at very low frequencies, e.g. one in 10,000, one in a million. This is due partly to chance, and partly to the selective advantage conferred by the mutuation that gave rise to that allele. Some alleles may have no selective advantage at present, but may provide genetic resistance immunity to some as-yet-unknown disease or environmental challenge in the future. Even alleles that are present at relatively high frequencies (say one in 50) are at very high risk of loss over even one or two generations in a breeding population of only 25. Thus the value of keeping diversity for diversity's sake.

An excellent explanation, thank you for typing it all out!

IPF, since we are on the subject could you touch on genetics where one trait is linked to another? I don't know if there are any instances in chickens- perhaps lethal gene combinations would fall under this, but in dogs often deafness and blindness is linked to color. Dalmatians and Shelties being good examples here. Select for the trait of a certain color and often deafness or blindness hitch a ride.

Technically, linkage is the term for genes that are located on the same chromosome, so when the gametes (egg and sperm cells) are formed, and one chromoosome of each pair goes into the gamete, those that are on the same chromosome tend to stay together. One example is the pea comb gene, P or p, and the blue egg colour gene, O or o.

This gets useful when you have a hen that is the offspring of a single combed, non- blue-egg breed (eg barred rock, leghorn, whatever) crossed with a pure ameraucana (or araucana). The female offspring will ALL have pea (ish) combs, and lay blue eggs. Their genotype will be PO/po - heterozygous at both loci. Their gametes will (almost all) be either PO or po (note that the P stays with the O, and the p with the o).

This gets REALLY useful and interesting when you hatch eggs from the offspring, when mated to a pp/oo (non-pea, non-blue) bird. A female offspring from this cross with a pea comb is almost guaranteed to lay blue/green eggs, and a female with a straight comb is almost guaranteed to lay non-blue/green eggs!

However, note the "almost". In the stage of gamete formation the chromosome pairs sometimes twist around each other and break and reattach, so that you could have a Po or pO gamete. This happens about 4% of the time, for this particular pair of genes - the % recombination depends how far apart on the chromosome the genes in question are - (the farther apart, the greater the likelihood that the break will lie between them - makes sense, right?)

I'm not sure that the connection between deafness and coat colour in dogs is technically "linkage" (as defined above) - I suspect it might be rather that the allele that causes a particular coat colour also has direct effects on hearing (or vision), particularly if homozygous. Many genes affect more than one trait. As i say, i'm not positive about this; dogs are not something I know much about. If I find out more, I'll re-post.

Thanks IPF. I appreciate how this info links to my other question.

This has been a great conversation so far.

I have been reading and re reading the info shared and posting in this thread.