PART IV

OF PEAS AND PUPS

PART IV

INTRODUCTION

The conclusion of our last installment touched linkage in general and sex-linkage in particular. We hope to discuss "Mother-line" breeding in a future article....maybe this should be the place because it is surely, in part, sex-linked. From a purely scientific angle, it is difficult to attribute the entire success of this method of breeding to sex-linkage. With but two Chromosomes out of 78 being involved and even these carrying fewer genes than the AUTOSOMES (ALL THE CHROMOSOMES EXCEPT THE SEX CHROMOSOMES) their percentage of the whole is small. It is possible however that these few genes may have a far greater influence than their numbers indicate....It is not wise to criticize breeding methods which have proven successful int he hands of old experienced breeders even when there is no apparent scientific justification....We still subscribe to the prime importance of the 'Artistic Touch' in breeding. Of course, line breeding is involved in Motherline breeding too, as the name indicates...Studying the pedigree of a Mothering breeding, occasionally someone will deny that any line breeding has taken place. We have become so accustomed to thinking of breeding terms as being defined solely by pedigree...by a mechanical formula...we completely overlook the most important; The Artistic Portion...We foolishly deny that which we cannot see on paper. We must remember THAT WE ARE TRYING TO MATCH GENES...not names on a piece of paper. One who oversees the cohabitation of a canine pair Is Not, Thereby, A Breeder........

Likeness of pedigree usually means some likeness of genes but this is by no means infallible. The sex-linked characteristics may furnish, to the discerning eye, that little additional inkling needed to make selection more effective....It might well indicate to the experienced breeder wherein the dog and his pedigree differ genetically. So merely glancing at a scrap of paper and saying, "Such and such a dog is neither inbred or linebred", indicates no grasp of the complexities of the problem.......If we could see the ideal which the breeder sought (by seeing others of his strain), if we could see the dog itself.......if we could test the dog and if we could study the pedigree, we MIGHT be justified in making such a statement. Comments based solely on a scrap of paper are valueless...This is certainly not to condemn the pedigree does indicate a genetic similarity as well as kinship. It is only to emphasize that we cannot consider as genetic certainties that which is only probable...

Now back to the risque, sexy part. The male dog is Heterogametic (X-Y) (HETERO A PREFIX MEANING UNLIKE) and all of his sex-linked genes come from his dam...She is Homogametic (X-X) (HOMO-LIKE) and her sex-linked genes assort, cross over and play all the games that genes play, and chance alone decides the inheritance of the sone. What ever he does receive is going to show dominant or recessive, if we can see that well. 50% of her sex-linked genes are 100% of his. This makes the genetic relationship between mother and son closer than between mother and daughter. The father-daughter relationship is the reverse, 100% of him is 50% of her. The relationship is just as close but more difficult to "read". Genetically, the sex-linked relationship between father and son is zero.

CROSSING OVER

Let us consider a hypothetical problem based on this sex-linkage. We have a bitch which carries the defective recessives for hemophilia and color blindness on the same X Chromosome. We know this because there was one little male which had both defects, in the last litter. Merely for explanation, we will breed her again. This time one little colorblind male comes up without hemophilia. How can this be? Both defects were on the same X and here one shows up without the other. They were supposed to be linked together on the same chromosome. How is this possible?...The phenomenon which explains this apparent contradiction is called CROSSING OVER. It is an exchange of the corresponding parts between the Chromatids of Homologous Chromosomes. In the case above the recessive hemophilic gene merely changed places with its normal Allemorph. A diagram will make it clearer.

It is thought that Crossing Over takes place during the early phase of Meiosis when the Chromatids are tightly entwined and it may be that the stress of the twisting causes them to break and they then become attached to their neighbor. (fig#7 Meiosis) The process is not clearly understood, however, if linkage were always complete and the genes maintained their same alignment constantly, ALL OUR DOGS MIGHT STILL BE WOLVES! There would have been but limited variation and changes would have had to depend solely on mutations.....Under such circumstances, the Chromosomes would be the units of heredity rather than the genes.

We can see that it is because of Crossing Over that no two pups of a litter can be exactly the same, in fact no two pups, or humans for that matter, are ever identical in genetic make up. The line exception being identical twins which are extremely rare in canines and being of the same sex cannot mate...Variation is most important to the breeder and the organism. Before domestication, variation was an important force which, when coupled with natural selection, brought great evolutionary changes....Indeed, without variation we would likely have no Shorthairs...no dogs at all...no life at all. The animals which inhabit the earth today are here only because of their variations which permitted them to successfully adapt to environmental changes down through the ages....Many have fallen by the wayside because they produced no variations which could adapt to these changes. Should anyone question the efficiency of variations and natural selection, he has no knowledge of evolution....Should he question the increased effectiveness of variation and human selection, he does not know the dog family....

The dog was, in all probability, the first animal domesticated. Domestication did not eliminate natural selection although it reduced its importance and substituted human selection in its place. It is primarily human selection wich has produced the great variety of pure bred dogs we have today. Who will deny the effectiveness of selection when seeing a wolf standing between a tiny Chihuahua and a big St. Bernard?....There are other forces which contribute to variation and thus make selection more effective but none are more constant than crossing over.

It will be seen that if the genes are Homozygous crossing over is ineffective...we get no variation. From A-a we can get a-A and eventually all the combinations, AA, Aa, aa but if we start with A-A or a-a and mate the same way, no amount of crossing over is going to provide any variation. A thinking person will say that if we should obtain complete homozygosis, we will have no variation and will have reached a standstill and be capable of no further progress. That is a fact....and when we reach that point, we will have to sit around and wait for some favorable mutations before advancing further.....Of course, we will never reach that ideal, so it should give no cause for worry. We say this casually but is this homozygosis within the realm of possibility? What are the odds against it?

Let's do a little guessing and see just how remote that perfection is....how many possible combinations of genes there are in the GSP. Mans genes have been variously estimated from 10,000 to 100,000. Just for the sake of discussion let us say that the Shorthair has 20,000 genes in his 78 Chromosomes. We will say that there are 10,000 or half, that are properly aligned to start with, homozygotic. They are the genes which make him a dog and particular breed of dog. He may share 6000 in common with the wolf, his ancestor. Now all we have to about are the remaining 10,000 genes. That doesn't seem like many....it is possible that there are Shorthairs which have even 15,000 homozygotic genes. If there is such a dog he is 50% above the breed average and be assured you have heard his name because he is outstanding. But how about the average GSP for which we have 10,000 genes to align to perfection. How many combinations are possible in 10,000 pairs? Disregarding linkage, there are 210,000!! Let's work with 103000 it's a slightly smaller figure but easier to handle. How much is 103000? It is "1" followed by three thousand zeros, enough to fill a couple of these pages. I can't grasp such a figure. The earth may be 75,000 years old. How many SECONDS is that? Only 1015. Light from some of the more distant celestial bodies require 1,000,000 light years to reach our planet.....Light travels at 186,000 miles per SECOND OR 669,000,000 miles per hour. 660,000,000 mph for a million years only comes to 1017 miles. A really paltry sum compared to 103000! Can you think of anything which might even remotely approach a "1" followed by 3000 zeros? Can we make a Shorthair completely homozygous? The smallest particle of any element is its atom. The largest mass we can imagine is this old plant, earth. It is about 25,000 miles around and about 8000 miles though. If every single atom in the total mass of the earth was another earth itself and has as many atoms in it as our earth has...the total number of atoms in 1050 earths would only be 101000 atoms! This figure is 30 times less than 103000. Can we make the Shorthair completely homozygous? If we could....what a dog! With some slight grasp of the inconceivability of 103000, we need never fear that our individualistic Shorthairs will ever become stereotyped and fixed. We will never have a rubber stamp Shorthair.....

We must think of crossing over as presenting us with a new variation in each pup....a challenge to keen observation and thoughtful selection. many of the variations will be undetectable to the most observant and would still be lost with the most vigorous testing, yet complete testing will disclose many of the finer points which would otherwise be lost. It is the cumulative effect of these many very minor variations which can push the Shorthair to new heights of versatility and perfection even in this generation. If we watch closely, the genetic pennies, the evolutionary dollars will take care of themselves.

MAPPING

Crossing over has permitted the geneticist to map the chromosomes of the most commonly studied organisms. More than 500 genes have been mapped in the fruit fly. That is, to pinpoint the lineal location of specific genes without ever having seen a gene. The mechanics of mapping is relatively simple. Less complete than the round wrinkled ratios.

If we pick up a "beads for genes" strand and pull it 'till it breaks and we keep doing this over and over again (it will confirm the suspicions of any onlookers as to our sanity) we will find that where it breaks is a matter of chance. We will also find that the further apart any two beads (genes) are the more likely they are to be on separate pieces of the broken strand. Conversely, the closer two beads the less likely is the break to come between them. Thus the frequency of crossing over given the relative position of the various genes on the chromosome. The genes at extreme opposite ends of the same chromosome cross over so often as to assort almost independently. They have but limited linkage.

It is at about this point that someone is going to ask, "What to hell good is all this to the practical dog breeder?" I must answer quite frankly, "I know of no good (other than the fact that all knowledge is of value) at the moment, to any dog breeder." I do not say that such knowledge is of no value....only that I do not know its practical application to dogs. This is not because it does not apply to dogs but only that so little canine genetic data is available that we cannot make use of it. Some day this information may serve us as it does other breeders today. If, for example, the color blind and hemophilic recessives were on opposite ends of the same X Chromosome their crossing over would be frequent and their linkage would be seldom. This might well justify the mating of a bitch carrying these defective genes (if other genetic factors were favorable) since many of the resultant males would only be colorblind, which is of no great handicap, and would not even carry the hemophilic gene. On the other hand if the two genes lay side by side on the gene string it might be necessary to produce 5,000 males before finding one which was not both hemophilic and colorblind. Therefore the value of such knowledge, should we ever gain it, is obvious.

IN COMPLETE DOMINANCE

Up to this point we have considered the Dominant-Recessive relationship as complete. It merely makes the explanation easier. Complete Dominance is in fact relatively rare. However, it forms the basis for the study and understanding of genetics and without it the variations would be more difficult to comprehend.....Dominance can range from complete to none, with all the shades in between....Mating red & white snapdragons gives us pink snapdragons. In shorthorn cattle, red to whites give roans in F1. In Andalusian chickens the cross gives us "blue". Is it any wonder that early breeders thought of a blending theory? It is important to remember that this apparent blending is only in the Phenotype (appearance) not in the Geneotype (genetic makeup)....Another instance of the fallacy of "likes beget likes"......As further proof of the lack of genetic blending is the fact that the F2 work out to natural unblended phenotypes. This apparent blending and incomplete dominance are but two of many factors which make the ART OF BREEDING, THE MOST IMPORTANT SINGLE FACTOR IN SUCCESS. There is no way to tell....with calipers, slide rule or IBM.....the Genotype from the Phenotype....except "AN EYE FOR A DOG". At the same time, without an understanding of the science behind the blending an "eye for a dog" may lead down the wrong path as it did for so many in the years before the gene theory was known as fact.

Since A is rarely completely dominant, Aa should, and sometimes actually does, reveal itself to the discerning breeder. I am told, for example, that the WW, Wirehair, has just a little longer and a little more "wirey" hair than the Ww Wirehiar. It is apparently the attention to these rather minute details which gives the experienced breeder this sixth sense. The glaring faults, we can all see.....

EPISTASIS

The fact that dominance is not always complete, complicates selection....The dominant is the stronger of the dominant-recessive pair and if it can't be boss it can at least alter the Phenotype of the Recessive allele, if we are sharp enough to see it. A far more complicated dominant-recessive relationship exists between some genes which are not even allelomorphs (alternative forms of the same gene which influence the same developmental process of processes, but in different ways). A gene which alters either the direction or intensity of another gene which is not its allelomorph is said to be EPISTATIC, THE PROCESS IS EPISTASIS....How important the roll of this action is in breeding is unknown....if it were known, it could quite definitely influence our use or the effectiveness of various breeding methods. The epistatic effect, like dominance, is not complete at all times.

The Irish Setter sometimes has the genetic factor for a black coat although such is but occasionally seen. We know this not only from the occasional such offspring reported but also from the fact that the tip of the nose and the foot pads are black in such individuals as carry the black genes while those which do not possess this factor have brown noses and pads. The Irish Setters which possess the factors for black would be black were it not for the Epistatic A-gene which acts on the black changing it to red....A red and a white shorthorn cross which yields a roan offspring does so because the gene for red is not completely dominant to its recessive allele. The relationship between the epistatic A-gene and the black factor is similar except that they are not allelomorphic. The factor which assumes the "recessive" role in such a partnership....the one which comes out the little end of the horn....is said to be HYPOSTATIC. In the above instance, black would be Hypostatic.

It is highly improbable that all readers share my interest in genetics. It is probable that most readers share my interest in breeding methods based upon genetic fact; in breeding methods which will advance the Shorthair....Some genetic facts which have been discussed and others which will be covered may appear, on the surface, to be irrelevant to our Shorthairs. But without an understanding of at least the basic details....and these do not include the action of specific genes....a grasp of the reasons various mating plans act as they do is impossible. All of the elementary genetics covered here is solely to provide a basis for the intelligent discussion of the plans of mating which climax this series. The conclusions will rarely be questioned if he fundamental genetic knowledge is understood....

At the close of this series, a breeding plan will be outlined for the individual for several individuals and for the breed as a whole. That is to say that a genetic plan will be offered for consideration in each instance. You will still be called upon to provide the most important ingredient....the art.

Copyright 2001. Dr. James G. McCue, Jr. All rights reserved. Postscript: And his legacy lives on in the German Shorthaired Pointers of today. May they always be healthy and bred with forethought and planning.

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