THE USE OF GENETICS IN BREEDING CHAMPIONS
PART 4: MENDELAIN LAWS (CONTINUED)
In part 3 we covered the essence of the Mendelian Laws. Don’t underestimate their relevance. As you will see in this part, we can use them in our efforts to breed racing champions. But then again, do not think that genetics or any other system can provide you with guaranteed success. If this were true, pigeon breeding and racing would have been really boring. The best we can do is to improve the probability of breeding champions. But let’s start by playing devil’s advocate first.
ISSUES THAT COMPLICATE THE INTERPERTATION AND USE OF MENDELIAN LAWS
It looks like we won’t get very far in our efforts to breed even a nice colour pigeon, let alone a champion racer if we were to rely only on Mendelian Laws. In the first generation (F1) it is still quite simple – the strongest (dominant) colour/trait will probably win.
Phenotypes vs genotypes. The first question that comes to mind when one studies the Mendelian Laws as they apply to racing pigeons is this – if all the examples given are of visible (phenotype) traits, such as eye and feather colour, then what about the invisible (genotype) traits, such as perseverance, ability to orientate, sense of urgency, speed? Furthermore, how do we identify them to begin with? Do they react the same as the phenotype traits? (By the way- what we see as phenotype, or external traits are as much genoptype as the ones we cannot see. The only difference is that they are the reflections of what happened in the genes!) And if we are involved in probabilities, how many traits are there and what are the odds that the genes will act the way we would like them to? How much control do we really have over Mendel’s Laws? If chromosomes are the carriers of traits, and if a pigeon has 80, how many possible combinations of chromosomes are there? The number of genes on one single chromosome can be a few hundred on the smallest chromosomes to thousands on the larger ones. Imagine how many combinations we would be able to make from each chromosome if we were to consider them in terms of genes? Even though we actually deal with allele pairs, the number of possible permutations still remains huge.
The role played by generations. The F1 generation of one pair of pigeons is the F2 (and F3, F4, F5, …) generation of other pigeons (their ancestors). So, the offspring will inevitably display the colours of their ancestors in some ratio. And to complicate matters even more, we know that pigeons inherit many more than just one trait from their parents, grandparents, etc. Fortunately (or not?) these traits are passed on independently.
Remember that genes do not mix. If a particular trait is inherited, no matter how many generations are involved, the gene carrying the trait will still be the same. This is what makes it so difficult to breed out an unwanted trait, especially if the trait is recessive, because you can’t see if the trait is present in the pigeon by just looking at its colour. (Sometimes you can!) The results of a dominant to recessive breeding depends on whether the pigeon (genes) that appears to be dominant carries the recessive trait, which you will not necessarily see. The only way to determine of the unwanted trait is present is by genetic testing, which will require a laboratory and some sophisticated equipment, or test breeding, which can be a time-consuming exercise.
Qualitative traits. Tests on the inheritance of simple external factors, colour being the most obvious one, are easy to carry out and interpret. It is quite another matter when we are dealing with alternative qualitative traits. Interpreting the results of research carried out in this area is difficult since these traits depend not on one gene but on several. The performance of a pigeon is bound up with numerous factors, on the one hand structural ones (such as the quality of the wings, feathers, aerodynamics, musculature, respiratory system and water resistance) and on the other functional ones (such as the work of the heart and of all the sensory organs involved in the direction-finding mechanism) but also with psychological factors (such as resistance to thirst, courage, drive to get back to the loft, etc).
To these must be added the variety of races (distances, weather conditions, etc) which means that even when prizes are won (the only tangible feat) we do not necessarily obtain an exact measurement of the pigeon’s performance capability.
Homozygous vs heterozygous. In pigeon breeding it is important to know whether a particular pigeon showing a dominant phenotype is homozygous for the dominant allele or heterozygous, especially if the recessive homozygote has an undesirable trait. The recessive genotype may be simply feathered feet, which is not acceptable to many breeders, or, more seriously, it may be a lethal or semi-lethal trait. There are many traits between the two extremes, such as tendency to obesity, a weak back, lack of will power to return to the loft, lack of a sense of urgency, lack of ability to orientate, etc.
Breeders of racing pigeons need a system of screening genotypes to prevent carriers of an undesirable allele from having progeny. Two potential sources of information exist which may determine the probability of heterozygosity – the pigeon’s pedigree and its progeny. However, there is never complete assurance that a particular pigeon is homozygous for a dominant allele, that is, not a carrier of the recessive allele.
Incomplete penetrance. Variable expressivity refers to a situation of inconsistency – the genotype for a trait is not always expressed the same in the phenotype. In contrast, there are some traits where the genotype for the trait may not be expressed in the phenotype at all – the pigeon would appear as a normal, rather than an affected, individual. The allele responsible for incomplete penetrance usually behaves in a dominant manner in breeding tests. A dominant allele should express itself in the phenotype of the pigeon. The gene for the particular trait is, thus, dominant, but has incomplete penetrance. That is, the genotype for the trait is not always expressed in the phenotype. Incomplete penetrance is possibly the result of some kind of “threshold” effect, where the product of the gene has to build up to a certain point before it can effect the development of the trait. Another theory is that it is due to some delay in the switching on or off of the gene so that the gene is not active at the time when the trait is sensitive to change.
Whatever the cause, incomplete penetrance further complicates the work of the pigeon breeder who seeks to understand the inheritance of a trait in order to develop an effective selection program for the breeding of top class racers.
Pattern and colour. The pattern and colour are completely different factors in a pigeon’s phenotype. Consequently they are inherited independently of one another. (Chalmers, 2004: 2.) Pattern is usually seen either as checkering or the lack of checkering. Patterns range from solid as seen in solid reds or blacks, through T-pattern black and red checkers, dark checkers, medium checkers, light checkers, to barred pigeons such as blue bars and silvers. (The definition of different colours is something pigeon fanciers will probably never agree about. For the purposes of this writing I took Silver to be a “Red Bar”, or a Mealy with bars.)
Inbreeding. Perhaps the most important problem with the application of Mendel’s Laws is that they are mostly relevant to inbreeding. Now I know that some experts will disagree, but read Mendel’s Laws carefully, the second and third Laws are based on pairing up a cock and hen from the same parents in the F1 generation. I accept that the laws can apply to unrelated pigeons, but then there are other factors coming into play that can and will disrupt the odds of obtaining a particular (required) result. This is brought out well in quantitative genetics. Granted, this is probably so merely because inbreeding provides us with a measure of control/knowledge of the traits involved in the equation. If we were to use unrelated pigeons it would become much more difficult to identify any logical pattern of heredity. It also makes it somewhat more difficult to determine if a particular visible trait is pure or recessive. This argument is corroborated by Van Vleck, et al, 1987: 9 in his explanation of the Law of Segregation (if I understood him correctly). More about this later.
External signs. One should not expect to be able to evaluate a pigeon by means of external signs (such as the circle of correlation, markings in the eye, the length of the wing, etc). Hereditary transmission in the pigeon is a rather obscure process and the results of breeding are therefore largely unforeseeable – and yet it is an accepted fact that quantitative traits are subject to the basic rules of heredity.
THE VALUE OF MENDELIAN LAWS
In spite of all the factors that confuse the issue, it would be a mistake to ignore Mendelian Laws, because there are definite advantages to be gained from taking note of them.
Firstly, Mendelian Laws provide us with norms against which to determine what is false. This is why it is important to include the colour of all pigeon’s on a pedigree. Knowing the colour of the previous generations does not necessarily enable us to know what the colour of their offspring will be, but it does rule out a number of impossibilities. It is, for example, almost impossible to breed a (homogeneous) blue bar from two homogeneous red pigeons. I say almost impossible because there is always the possibility of atavism and mutation spoiling the pattern, no matter how small the probability of this happening. (We will discuss atavism and mutation in more detail later.)
Secondly, the notion of linkage applies, even when unrelated pigeons are paired. (We will also discuss linkage in more detail later.) And the experienced fancier who knows his pigeons well will learn what colour pigeons from which pairs will be good racers or breeders or simply useless. I have been the victim of this, when a well-known fancier sold met a useless pigeon from one of his best breeding pairs. I only realized this when, some years later, I read in a book that this particular fancier knows his pigeons so well that he can tell from the chicks’ wing form and shade of colour if they will be worth anything in breeding or racing. In this respect linkage offers us a good tool with which to select our racers and breeders. Even though it is unlikely that one will be able to identify a large percentage of a pigeon’s traits externally, this provides us with valuable information on the internal traits if we know which external traits are linked to which internal ones. Most fanciers have noticed that pigeons that inherited certain external traits from their parents, such as a white tick at the corner of the eye, a white primary in one of the wings, white toe nails, a particular shade of red, invariably race or breed better than their brothers or sisters that did not inherit this external feature. However, be careful! It often happens, and I am sure some of you must have seen this in your own lofts, that the nest brother or sister of the ace racer turns out to be the one that inherited the ability to pass the good traits on to his or her offspring, while the ace racer does not have this ability.
Thirdly, knowledge, or at least awareness, of Mendelian Laws can sometimes help us to determine the sex of our youngsters at a very early age. From previous observations you learn to know which one of the chicks is the cock and which the hen based on their respective colours, if you keep the same pair together for a reasonable period of time. This can also be worked out quite accurately if you know if the cock and hen’s colours are pure or recessive.
Offspring inherit traits from both parents, and not always in the same pattern. That is why top-class breeders often deliver as many, if not more, poor quality as good quality youngsters. Because of the huge number of possible combinations of genes, breeding is very much like dealing cards for a game – the chances of received a good deck of cards are probably less than receiving a losing hand. Our objective is to increase the odds that we will receive a good hand, but in the process it will always be necessary to weed out the “bad hands”. The trump cards and the bad cards come from the same deck of cards!
Even if we know and understand very little about genetics, there are some simple “facts” that one can remember and that can be used to confirm or refute a pedigree, to avoid certain traits, to discriminate between probable racers and breeders and to increase the probability of breeding top class racers. Here are a few such “facts”:
Mendelian laws can be used as tools to analyze the modes of inheritance for many traits controlled by a few loci with major effects. Unfortunately the average pigeon breeder is limited to a process of exclusion of what appears to be unlikely or impossible options. This process of exclusion should be made in a stepwise, organized fashion. The steps should be:
Figure 8: The process of exclusion.
The tabulation of breeding results is a major step in understanding their meaning. Tabulation based solely on phenotypes of the parents involved in mating and of the progeny is extremely useful in the initial stages of a study. It enables you to identify recessive phenotypes and begin to assign genotypes. Possible genotypes for other phenotypes can be developed along with the genetic hypothesis. Once you have established possible genotypes, predictions of expected progeny ratios from specific matings can be made and tested. The appearance of progeny phenotypes not predicted by the hypothesis invalidates the hypothesis, as do ratios that deviate excessively from expectations.
So why is colour important when, what we are actually interested in, is racing performance? Point is, when you obtain new breeding stock, regardless of whether you paid enormous amounts or not, you need to know with some measure of certainty that the pigeon will improve the genetic quality of your loft. Even just a little knowledge of the genetics of pattern and colour can often help to reveal the truth about parentage. If the pattern and colour are wrong, it is very obvious that the remainder of the genetics of these pigeons will also be wrong. The factors of pattern and colour, while not important in themselves or in the context of racing or breeding ability, are nevertheless helpful indicators of the truth of the whole genetic package that fanciers are being offered when they buy pigeons. Remember that this does not apply to external traits only, but also to internal traits. Internal (genotype) traits that are prone to intermediate heredity are body size, length of the beak, the form of the head and the strength of the bone structure. This is valuable information for breeding purposes, because, for example, now we know what will probably happen if we were to pair two big pigeons with one another.
In closing, we took the first step into the arena of genetics by looking at the Mendelian Laws. Just to recap:
Law 1: The Uniformity Law. Two pure breeding traits brought together will produce uniform offspring in the first generation (F1 generation).
Law 2: The Fission Law. If two pigeons that appear the same are paired, they will transfer their traits to the F2 generation (their grand children) in a particular ratio, and this ratio is determined by the chromosomes of the grand parents.
Law 3: The Independence Law. When paring two pigeons (P-generation) that differ in more than one trait, this (differing) trait will be passed on independently.
Remember what we are trying to achieve – we are trying to improve the racing and breeding ability of our pigeons, not their colour or whatever other external traits. So, we should identify these internal traits and focus on them, rather than on external traits. The only value that external traits can have is if we can establish that a particular external trait is linked to an internal trait that we need. To do this we need to pay careful attention to our pigeons. Spend time observing them, make notes, keep records, if possible on computer, of their traits, habits, peculiarities, performances, etc. Remember as much as you can. The more information you gather on your pigeons the better chances are that at some stage you will form a clear picture in your mind how to breed champions from your pigeons.
The most important trait to select for must always be racing performance, never visible features. By trying to improve the visible (phenotypical) features of your top class racers you run the risk of a serious loss in racing quality, and once this happens it will be almost impossible to regain the racing quality of the original racers that you had. If the visible traits of your racers improve, it will be a bonus, but it should never become your primary objective. Don’t focus on breeding show-pigeons; focus on breeding racers. You might impress visiting fanciers who handle your pigeons if you breed good-looking pigeons, but you will only gain their respect if your pigeons perform well in racing.
Dr Jaap Nel. Email: email@example.com.
<<BACK to Articles