A Genetic Primer for Breeders
by John B. Armstrong
What traits are inherited? | Genes and Alleles | Naming Genes | Dominance | Expressivity and Penetrance | Sex Linkage | Determining the Mode of Inheritance | Inbreeding | Notes | References
Most of you are undoubtedly aware that color and certain diseases such as progressive retinal atrophy (PRA) are inherited -- that is, passed down from one or both the parents. However, you may wonder how a trait that does not appeared in the dam's pedigree can suddenly turn up in a litter out of Ch. Jacob Hugelsberg. Is it inherited, or just an accident? Surely Jacob has been used so often that someone would have noticed if the problem came from him.
Just how much of a role does genetics play in health, general conformation and temperament? Probably you would like to know exactly what traits are inherited, but once somewhat starts talking about "partial dominance" or "expressivity" you get glassy-eyed. The objective of this essay is to explain some of the basics of inheritance to those of you who are largely unfamiliar with the principles of genetics -- hopefully without losing you in complex technical jargon.
What traits (or characteristics) are inherited?
The answer is almost all -- from temperament to size and coloring, as well as genetic diseases like PRA. (Infectious diseases are not, though the susceptibility to them may be to a greater or lesser extent.)
The occurrence of any particular characteristic depends on two factors: genetics and the "environment". "Genetics" refers to the encoded information (instructions), controlling all biological processes, that are carried within the cells of all living organisms. These encoded instructions are responsible not only for maintaining the continuity of a species (or breed) but also for many of the differences between individuals within a species or breed.
The environment also contributes to the differences between individuals. The relative contribution of genetics and environment is not the same for every trait. Some traits, such as color, are influenced very little by the environment. For others, such as temperament, the effect of the environment is much greater. Geneticists use the term heritability to indicate the proportion of the total possible variability in a trait that is genetic. However, except for the cases where genetics is the main source of variability, the heritability of a trait is difficult to establish, and may not be the same for different breeds. Therefore, I cannot tell you that the heritability of size is 70% genetic (or whatever it may be), so aside from understanding the basic concept, the notion of heritability has little to offer the breeder.
Before moving on to a more detailed discussion of genetics, I would like to take a brief look at what is meant by "environment", in the present context. For a puppy, the first environment it encounters is that of the mother's womb. Is the mother well nourished, healthy, and free from stress? How old is she? Is this her first litter? How big is the litter? Once the puppy is born, it experiences a new environment where it has to compete for food and attention. Litter size is still a factor. How much food does the puppy get? How much attention does it get from the mother, the breeder, and the eventual owner? Does it have a safe and healthy environment? Does it have other dogs to associate with? Etc., etc.
The gene is often called the basic unit of inheritance. A gene carries the information for a single step in a biological process, but most biological processes -- even the ones that may appear to be simple -- are made up of more than one step. Thus, one should not get the idea that a trait is determined by a single gene, but rather that the general rule is that many genes control a single trait. A good example is color. In some breeds, such as the Poodle and the Borzoi, there are a great variety of colors, so it should come as no surprise that this is the result of the action of a variety of genes. There are not only genes for making the different colored pigments, but also genes which control the distribution of the pigments both within the individual hairs, and over the entire body. (Other breeds may come in only one color. They have the same genes, but only a single allele of each.)
All animals have thousands of genes, but they do not float around loose in the cells. To make cell division and reproduction more manageable, genes are physically connected to other genes to form chromosomes. Most "higher" animals have two sets of chromosomes; one from the mother and the other from the father. So that the number of sets does not keep increasing from one generation to the next, the sperm and eggs get only one set. However, the mechanisms that assure this are not able to tell which chromosomes came from the mother and which from the father. Therefore, the set that is passed on in a particular egg or sperm is a mixed set. The number of possibilities depends on the number of chromosomes. Since dogs have 39 chromosomes in a set, the number of possible combinations is well over 1 billion! Therefore, the possibility of getting two litter-mates that have the same combination of chromosomes is extremely remote. (Incidentally, wolves also have 39 chromosomes in a set and can breed with domestic dogs. Foxes, however, have only 19 chromosomes and cannot.)
One of the 39 chromosomes carries genes that determine sex. In mammals, the chromosomes carrying the "female" genes is designated X and the one carrying the "male" genes is designated Y. An animal with two X chromosomes will be a female, while one with an X and a Y will be a male. (One with two Ys will be in serious trouble!) Genes other than those determining sex are also located on these chromosomes and are said to be sex-linked.
... and Alleles
Most genes carry out their functions correctly, but some are altered by exposure to radiation (natural or man-made), certain chemicals, or even by accident when a cell divides. A gene may be thought of as a small program. There are many possible places in the program where an error (mutation) might be introduced. Many of these will have the same effect; the program will not function. Others may modify the action of the program. Some may appear not to affect the program at all. Since the latter produce no observable effect, we need not worry about them. All, however, regardless of their effect, change the information carried in the program, so that, strictly speaking, each is a different version of that program. In genetics we call each version an "allele". Technically, different versions, even if they produce the same effect, are different alleles, but we generally only worry about the ones that produce different effects and simply treat those that produce the same effect as though they were the same.
Though there are potentially a large number of alleles for each gene, by far the most common are those that prevent function entirely. Therefore, for many genes we only find the normal allele, often called the wild-type, and "no-function" (null) alleles. For some genes, we also get alleles that function partially or abnormally. However, no matter how many alleles there are in a population, an individual can carry only two -- one from the sire and one from the dam.
When the two alleles are the same, the individual is said to be homozygous for that gene. When the alleles are different, it is heterozygous.
There are rules for naming genes -- unfortunately not all geneticists use the same system. The one I will use here is common, but not universal.
A gene is named for the first mutant allele discovered. Thus, in the fruit fly (Drosophila) which normally has dark reddish-brown eyes, a mutant with white eyes was discovered many years ago. Consequently, the particular gene in which this mutation occurred is called "white" and given the symbol w. The mutant allele is designated w (notice that it is italicized), and the wild-type allele is designated w+. Another mutation, discovered later, has light yellowish-brown eyes and is called "eosin". However, it is also an allele of the same gene and is, therefore, not given a different letter designation. Instead, it is designated we. (This system reserves capital letter designations for dominant mutant alleles.)
The alternative system that you will more likely encounter is very similar, except we don't use a + sign to designate the wild-type allele. This can introduce an element of confusion. For example, gray coat color is not considered the normal (wild-type) color in poodles. However, as it is dominant, it is given the symbol G, while the wild-type allele is g.
The naming of genes can also be eccentric. The dilute gene results in a lightening of the basic color and, appropriately, is designated D. A second gene has a similar effect, and is called C (for color). However, the best known mutant allele of this gene is the one that results in albinos, so the gene really should be called A -- but this designation had already been used for agouti.
If, for a particular gene, the two alleles carried by an individual are not the same, will one predominate? Because mutant alleles often result in a loss of function (null alleles), an individual carrying only one such allele will generally also have a normal (wild-type) allele for the same gene, and that single normal copy will often be sufficient to maintain normal function.
As an analogy, let us imagine that we are building a brick wall, but that one of our two usual suppliers is on strike. As long as the remaining supplier can supply us with enough bricks, we can still build our wall. Geneticists call this phenomenon, where one gene can still provide the normal function usually met by two, dominance. The normal allele is said to be dominant over the abnormal allele. (The other way of saying this is that the abnormal allele is recessive to the normal one.)
When someone speaks of a genetic abnormality being "carried" by an individual or line, they mean that a mutant gene is there, but it is recessive. Unless we have some sophisticated test for the gene itself, we cannot tell just by looking at the carrier that it is any different from an individual with two normal copies of the gene. Unfortunately, lacking such a test, the carrier will go undetected and inevitably pass the mutant allele to some of its progeny. Every individual, be it man, mouse or poodle, carries a few such dark secrets in its genetic closet. However, we all have thousands of different genes for many different functions, and as long as these abnormalities are rare, the probability that two unrelated individuals carrying the same abnormality will meet (and mate) is low.
Sometimes individuals with only a single normal allele will have an "intermediate" phenotype. (For example, in Basenjis carrying one allele for pyruvate kinase deficiency, the average life-span of a red blood cell is 12 days, intermediate between the normal 16 days and 6.5 days in a dog with two abnormal alleles. Though often termed partial dominance, in this case it would be preferable to say there is no dominance.
To carry our "brick-wall" analogy a bit further, what if the single supply of bricks is not sufficient? We will end up with a wall that is lower (or shorter). Will this matter? It depends on what we're trying to do with the wall and possibly on non-genetic factors. The result may not be the same even for two individuals that have built the same wall. (A low wall may keep out a small flood, but not a deluge!) If there is the possibility that an individual carrying only one copy of an abnormal allele will show an abnormal phenotype, that allele should be regarded as dominant. Its failure to always do so is covered by the term penetrance.
A third possibility is that one of the suppliers sends us sub-standard bricks. Not realizing this, we go ahead and build the wall anyway, but it falls down. We might say that the defective bricks are dominant. Whether dominant abnormal alleles can really be thought of in these terms is not clear. A few probably can, but others produce their effects in other, poorly-understood ways.
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