DNA in Y2K
A Dog Breeders Guide to Surviving in the Century of Biotechnology
by Lisa Van Loo
On March 22, 1999, Time Magazine announced that the coming century would be the "Age of Biotechnology". With scientists nearing completion of mapping the human genome, and others hard at work doing the same for dogs, a wealth of new information will become available over the next few years. At times, this information may be bewildering; at other times, it may appear that the latest "discovery" contradicts the one made just before. This series of articles will describe the basic principles behind genetics, and will discuss the directions biotechnology may take the dog breeder in the upcoming millenium.
Introduction: Mendelian Inheritance
Gregor Mendel was an Austrian Monk, who in the middle of the Nineteenth Century did some planned experiments breeding peas and carefully charted his results. His explanations of the results he found are the basis for all modern genetic study. His hypothesis was that some essence in living things makes it possible for parent generations to produce offspring that are very close copies of their parents. Today we know about genes. We know how they work and interact with each other and the environment to produce the living things we see around us every day.
Some people have described dog breeding as an art, while others have described it as science. In reality, it is both. As an artist must learn how best to use his tools to create a new work of art; the dog breeder must learn how to use the tool known as genetics, to create a new work of art - the living puppy. Unlike an artist, however, the dog breeder does not have ready access to his tools - we cannot see genes, only their result. We can only guess at what genes may have contributed to the dogs we see. Because of this reliance upon guesswork, many breeders do not attempt to understand genetics. Although breeding dogs is often likened to gambling, a basic understanding of genetic principles will help one to interpret the results of a breeding and to be better able to predict what the outcome of any particular planned breeding might be.
Genetics is the study of genes. In order to understand genetics, one must know a little bit about how genes are made up, how they are packaged and how they move from one generation to the next. Genetics is a broad topic, covering many areas, from molecular chemistry to population diversity. This series of articles will focus on those topics in genetics that impact on the dog breeder. Many people are discouraged from learning about genetics because of all the complicated terminology involved. Any new terms will be defined in the body of the article, and a list of terms appears at the end of the series.
A gene is a section of DNA (Deoxyribonucleic acid - a special type of molecule that contains all of a dogs genetic material). Each gene is programmed to produce a certain type of protein. Think of a string of beads. Each bead represents a gene. Now, imagine two strings of beads lying next to each other, both very long, and containing thousands of beads, with each bead on one strand paired up with a bead on the other. Imagine the double strand of beads is tightly twisted along its length. This is what DNA looks like. This tight twisting is what makes the DNA able to fit into the tiny cells. With a few exceptions, every cell in the body has this DNA in it, in the form of chromosomes. Dogs have 39 pair of these chromosomes in each cell, with the exception of the egg and sperm cells.
If every cell in the body has the same DNA, how is it that the cells are different? Why are brain cells brain cells, for instance, and not liver cells? During the time the puppy is developing in the dam, a process called cell differentiation is taking place. Through a complicated process involving different types of triggers, different genes on the chromosomes are switched on, or switched off. This way, when it is time for the baby puppy to make brain cells, the brain cell genes "switch on" and brain cells are made at the proper time and in the proper place for the brain to be.
One of the exceptions to the pairs of chromosomes is in the germ cell (egg or sperm) itself. Germ cells only contain single strands of beads - not double strands. This may seem strange, until you remember that one strand from the sire (father) will match up with one strand of the dam (mother) to make a normal, double strand in the puppy. If the egg and sperm each had double strands, the puppy would have quadruple strands - twice as many as it needed!
It is not possible to determine which half of each chromosome will end up in the sperm or egg. This randomness in dividing and "packaging" the DNA is called segregation; only one gene from each pair will be found in any one egg or sperm. This means that the genes will pair up in new combinations when the sperm and egg unite to make one cell with a full complement of chromosomes.
This separate reshuffling of genes is Natures way of ensuring that there is enough different genetic material around at any given time, so that if anything should happen to affect the population as a whole, some individuals are likely to survive to carry on the species. This is known as genetic diversity.
The opposite of diversity is known as bottlenecking. Bottlenecking occurs whenever a population becomes very small, so that much inbreeding occurs. Bottlenecking occurs in purebred dogs for a number of reasons.
Without diversity in a gene pool (the entire genetic material available in a breed), it becomes very difficult to select mates which complement each other with respect to virtues, and offset each others faults. It also becomes difficult to eliminate hereditary defects. Nobody can invent genes where none exist. We can only work with the genes that are present in the gene pool.
Now that we know how chromosomes divide and arrange themselves into sperm and egg, then are united to form the new puppy, lets look at how individual genes behave, and how individual traits (characteristics that influence appearance and behavior) may be passed from one generation to another.
Imagine that one pair of genes controls basic color. One allele (one of a pair or group of genes that may occupy an individual site on a chromosome, called a locus) codes for black color, one gene codes for brown. Black color is dominant over brown in the retriever breeds. Dominance simply means that when a dominant gene of a pair is present, it is the gene that is expressed - it is the gene whose results (in this case, hair color) we can see. Genes are usually designated by letters. In this case, well use the letter B. A capital letter indicates the dominant form of the gene. A recessive gene is one which does not show expression unless both genes of the pair are recessive. It is a "hidden" gene, masked by the presence of the dominant gene. The recessive form of a gene is by convention designated by a lower-case letter. For our example, the dominant gene will be called B, while the recessive gene will be called b. When one of each gene type is written, it is usual to place the capital letter first, as in: Bb.
If both beads at our imaginary locus on our string of beads are B, that dog has both of those color genes in the dominant, black form. The dog would be black colored. His genotype (what his genes are) would be BB. His phenotype (how the dog looks to the eye) would be black. If this dog were mated to a female who also had the BB genotype, their offspring would have the following genotype:
This diagram is called a Punnett square diagram. It shows how the sires and dams genes separate individually into the egg and sperm, and all of the possible combinations when one gene from the sire combines with one gene from the dam. The squares in the middle of the diagram represent the possible outcomes of mating two individual dogs with these gene types. As you can see from the example, all of the puppies from this particular mating will be BB, and will appear black to the eye. When both genes of a pair are identical (i.e. BB or bb), this is known as homozygosity. A dog with the gene pair BB is homozygous dominant, and will look black, while one with bb is homozygous recessive, and will look brown.
When a pair of genes is not identical (i.e. one gene dominant, one recessive, or Bb in our example) this is called heterozygous. Heterozygous just means that the two genes of the pair are different from each other. It is also what is known as a carrier. A dog with the Bb might be said to be black with a brown factor. Factor is yet another way of saying gene. Some dogs are advertised as "brown-factored black" or "black, carries brown factoring". This simply means the dog appears black, but when bred, he/she has produced some brown puppies, thus showing he/she carries the gene for brown color.
Lets look at some more examples:
In this example, the dam is homozygous dominant and is black colored. The sire is heterozygous dominant; he carries one gene for black color, one gene for brown. When we look at his appearance, however, he is black colored, too. The resulting diagram shows that all of the puppies are black colored, as well, but half of them are homozygous for the recessive brown gene. This example shows how a recessive gene can be carried down from one generation to another, sometimes for many generations, without any affected (brown colored) dogs showing up.
In this example, a bitch, homozygous dominant for black color, is bred to a dog which is homozygous recessive for brown color. The mother is black colored, while the father is brown. All of the puppies will be heterozygous. All have one copy of the gene for black color; thus, they will all be black colored, because that is the dominant gene of the pair and it is masking the recessive. The recessive gene for brown color has not gone away, even though we cannot see brown color in any of the puppies. It is simply hidden, and can reappear in subsequent generations. Remember that each gene from the pair will separate, and go into individual sperm or eggs. So, each of these puppies will have some sperm (or eggs) with the B gene, and some with the b gene, but not both. Lets see what happens when two heterozygous dogs are bred together.
This last example shows how some breeders have been surprised by a recessive gene in their line, whether a good gene or a bad gene. Both parents appear black. We cant look at their genes to see if they are carrying the recessive brown gene. In fact, both parents may have been the result of many generations of breedings like example #2 above, where a carrier is bred to a homozygous dominant. While all of the dogs look black in all the litters, in fact, half were carriers, and passed the brown gene on to the next generation.
When two of these carriers are bred together, sometimes a sperm cell with the recessive gene in it "finds" an egg with the recessive gene, and a homozygous recessive puppy results. This is a brown colored puppy. Imagine the breeders surprise when the brown sport crops up in an otherwise black litter, possibly from generations of black breedings. Since we cant see a dogs genes, how can we tell if a dog is carrying a recessive gene? We can do a test mating.
A test mating (sometimes called test-breeding or a test cross) is done to see if a particular dog carries a recessive. Remember, the only way we can see a recessive is if a puppy has two copies of the recessive gene - one copy from the sire, one from the dam. In our example, the dam is brown. Both copies of her color gene are lower-case b; she can only pass recessive genes to her puppies, as she does not have a copy of the gene for black. The sire is black colored, but when he is mated to the brown bitch, approximately half of the puppies will be brown colored. Thus, we can now "see" the sires genes - he is a carrier for the brown factor, as well as the gene for black.
In Chesapeake Bay Retrievers there occurs an inherited eye disease called PRA (Progressive Retinal Atrophy). The disease progresses, or develops over time, until the dog becomes blind. The gene for the form of PRA in Chesapeakes, prcd (progressive rod-cone degeneration) has been studied extensively. It is known that the prcd gene in Chesapeake Bay Retrievers is a simple recessive. The gene for a normal retina is dominant. It follows the same rules of inheritance as the brown gene does in our example above. Thus, a dog with two dominant genes at the prcd locus will not be affected, nor will it be a carrier. Such a dog can only pass on normal genes to its offspring. If a dog has one copy of the gene, that dog has a normal phenotype and will never develop the disease. However, since this dog also carries the recessive form of the prcd (PRA) gene, he or she is a carrier and would produce the gene in approximately half of its germ cells. A dog with this type of genetic makeup (one dominant gene, one recessive gene) is said to be heterozygous recessive for the prcd gene. A dog with two copies of the prcd (PRA) gene (homozygous recessive) will likely develop the disease at some time in its life. It will also pass the gene on to 100% of its offspring. Please see chart at end of article for an explanation of prcd gene inheritance in Chesapeake Bay Retrievers.
These examples illustrate simple Mendelian inheritance. These basic genetic principles apply to all living things. From the smallest bacterium to an elephant, the rules of dominance and recessiveness, homozygous and heterozygous, genotype and phenotype all apply.
In the next article, more complicated principles of inheritance will be described, including how genes can influence each other. Also described will be how environment can play a part in inheritance.
PRCD Gene Inheritance in Chesapeake Bay Retrievers
How to Read This Chart
The Parents genotypes are shown in the white squares, sires genotype on the top, dams along the left side. The nine shaded center squares represent the expected genotypes of any puppies resulting from mating a sire with one genotype to a dam with the same or a different genotype. To see what puppies would be expected to result from a mating between a carrier sire and an affected dam (for instance) go to the column on the sires side labeled carrier. Trace down the column until you reach the box that intersects with the row labeled "affected" on the dams side. The shaded box at the intersection shows the expected genotypes of puppies resulting from such a mating.
This chart represents statistical probabilities. The percentages show what the ratios would be from a large number of breedings, resulting in 100 or more puppies from those breedings. Individual breedings may not give the expected ratio, but the chart shows all the potential genotypes from any particular mating. Thus, when breeding an unaffected to a carrier, the breeder should expect some of the puppies to be carriers themselves, though this may be more or less than _ the litter.
P = gene for normal retinas
p = gene for prcd form of PRA
© 1999 Lisa Van Loo