Heredity is the means by which traits (distinctive features) are passed on to the next generation. Physical traits include such things as eye shape and color, body size, and coat color. Behavioral traits involve characteristics like herding or retrieving instincts. Some traits are not readily apparent, like blood type or predisposition to a disease. All these traits are inherited.
We talked a bit about chromosomes in a previous post. Chromosomes carry the genes that determine the traits that are inherited. Chromosomes are string-like ropes made up of thousands of genes. Chromosomes occur in pairs; one from each parent. Every gene has a partner on the opposite chromosome, and these partnered pairs of genes determine the traits of an individual. There are some exceptions to this rule. For instance, the female sex chromosome contains more genes than the male sex chromosome, so some traits are linked to the sex of the individual. Other genes might be composed of extra material created through mutation or insertion, and those might not always have a partner on the opposite chromosome.
But most genes do have partners. There are many different forms of a gene that can exist at any given location. These different forms are known as "alleles". For example, at one certain location on the chromosome, an individual could have a gene for sable, or for tan-point pattern, or for black. These are three alleles that might be found at that particular location. You will inherit only one allele from one parent, and one from your other parent.
How do we know how this all works? Let's take a brief look backwards at how our knowledge of genetics developed.
Way back in the 1700s, a man named Jean-Baptiste Lamarck developed some theories about how evolution occurred. Lamarck observed that organisms adapted to their environment. He believed that bodily features were gained or lost through use or disuse. He also believed in something known as "soft inheritance"...the idea that the effects of the environment on an individual's traits could be passed along to their offspring. While this idea has generally been discounted, science is now beginning to delve into the area known as "epigenetics". We now know that sometimes environmental factors (nutrition, toxins, radiation etc) can indeed affect the genes that are passed on to offspring! Lamarck may have been on to something after all!
In the 1850s and 1860's an Austrian monk named Gregor Mendel did experimental hybridizing on thousands of peas. Mendel showed that the inheritance of traits followed certain patterns and laws. He was the first to postulate that traits could be dominant or recessive, or in some-cases co-dominant. Mendel also developed the theory of independent or random assortment of traits. The importance of his work was not realized during his lifetime, but Mendel is now regarded as the father of modern genetics.
During this same time frame, Charles Darwin published his theory of evolution by natural selection. (Darwin knew nothing of Gregor Mendel or his research). Darwin's observations led him to the idea that organisms adapt and change through "survival of the fittest". Darwin's theory of natural selection is widely accepted today. In the early 20th century the ideas of Mendel and Darwin were combined to form the basis of genetic science and evolution.
We can see examples of how evolution occurs in an artificial, man-made manner by looking at the results of selection in domestic animals. Man can change a species form and function by selecting for certain traits that we find valuable. Traits less valued are selected against and do not survive. For example, we rarely see a thirty pound Pomeranian today....small size has been selected for and the genes that produce larger body size have almost disappeared. We also attempt to produce an almond-shaped eye, small ears, a high tail set and a relatively short back. Although this is a man-made selection process, and not "natural" selection, it's a good example of how evolution occurs.
But organisms are not simply the product of their genes. Scientists are finding that traits determined by genes can often be greatly influenced by environmental factors. Hip dysplasia is a good example of this phenomenon. The genes that predispose to hip degeneration can be influenced by the effects of nutrition or stress on the joint.
Another example is coat color which is determined by genes, but can be changed under the influence of the sun or hair dye.
A non-canine example of environment affecting expression of genes is the coat color of the Siamese cat. The Siamese cat has a form of partial albinism due to an enzyme that blocks melanin, but this enzyme that produces light coat color is inactivated by cooler body temperature. The coat color of the Siamese cat's tail, legs, ears and face (which is cooled by the nasal passages) remains darker than the rest of the body because these regions are not as warm as the rest of the body. The gene for blocking dark coat color depends on warmth (an environmental factor) to be activated and expressed.
We learned from Mendel that some genes are "dominant"; others are "recessive", and still others are co-dominant. But only a few traits are determined by a single gene. Most traits in complex creatures like animals are created by the actions of multiple genes. This is what we call a “polygenic” trait; one that is the product of many genes interacting together. Even a fairly simple trait such as coat color is produced through the interaction of multiple genes.
Traits like the size and shape of the ear, or the croup, or the front, are examples of polygenic features. This makes it difficult to predict with any certainty what sort of offspring will be produced from any certain mating. Remember, there are literally millions of different combinations of genes that each animal can possess. Selection of individuals who possess the traits you wish to perpetuate is important. This is actually more important than looking back at ancestors in a pedigree, because some genes that produce certain traits can be lost through the generations.
How can we stack the odds in our favor to produce the traits that are important to us in our breeding program? One tried and true method used by dog breeders has been inbreeding or linebreeding. This system involves breeding together dogs who are closely related or who descend from a meritorious common ancestor.
Inbreeding and linebreeding help to preserve certain valuable traits. However, inbreeding may also preserve undesirable traits at the same time. Another problem with inbreeding is that any hidden “problem” recessive genes that the admired ancestor possessed stand a relatively high chance of being doubled up and expressed in the offspring, because each parent has a higher than normal probability of sharing some common genes. These problems then become nearly impossible to eradicate from a line and in some cases from an entire breed.
Breeds that were founded on only a few ancestors or breeds that use a few popular sires extensively will almost always eventually develop a few problems that become characteristic of that breed. Another problem with inbreeding and linebreeding is that along with producing a more uniform type, the inbreeding process can also result in a lack of variability in the genes needed for optimal immune system function. Unfortunately, impaired immunity can mean a greater susceptibility to infection, higher rates of autoimmune diseases, lower birthrates and decreased lifespan.
Breeding outside the line, or “outcrossing” serves to introduce more genetic variability and improve health and vigor, but the downside is a lack of predictability of type. Most physical characteristics and even most diseases are produced through the interaction of multiple genes. Many disorders of the immune system such as autoimmunity or allergy may be caused by something as simple as lack of a variety of genes in the immune complex. Such problems can often be corrected in the next generation by outcrossing.
Usually an outcross must be carefully planned for certain features that the breeder wishes to introduce or to eliminate, and several generations are required to refine those features to reach a certain goal. This might require more patience than many of us possess, and there is no guaranteed pot of gold at journey's end. However, the rewards of improved health and vigor may make your “surprise” outcross experiment very worthwhile.
There are a few diseases produced by a single, identifiable gene, and some of these are able to be tracked by DNA identification of that gene or a closely located “marker”. Carriers of certain diseases may be identified in this way. Identification of problematic genes does not necessarily mean that we should eliminate the carrier from the gene pool. That animal probably has other very valuable genes to contribute as well. Judicious selection coupled with rigorous testing can result in the reduction of incidence of a genetic problem down through future generations once the genetic carrier status is readily identifiable.
The dramatic production "Inherit the Wind" chronicled the events of a teacher in the 1920s who was arrested and tried for teaching his students about the science of evolution and heredity. One great line from that play sums up our general knowledge of genetics:
"The man who has everything figured out is probably a fool. College examinations notwithstanding, it takes a very smart fella to say 'I don’t know the answer!'"