Genetics: An Overview
Genes operate as the building blocks of creation. Often conceptualized as beads on a string, each individual gene contains information about a specific characteristic and will be either recessive (in the dog's genetic make-up but not physically visible) or dominant (physically visible in the dog).
The strings on which the gene pairs are found are called chromosomes. Every dog inherits one half of a chromosome pair, and therefore one set of genes from each parent. Chromosomes are always found in pairs, with each gene always located at a specific address, or allele, on the chromosome string.
When each gene of a pair is identical, that pair is called homozygous (pure for a given trait). If the genes of a specific pair are different, that pair is called heterozygous (not pure for a given trait). A homozygous pair of genes can be either dominant or recessive (DD or rr). A heterozygous pair consists of one dominant and one recessive gene (Dr). The dominant gene of such a pair, if it contained information for an outward physical trait (coat, eyes, etc.) would be visible. The recessive gene would be hidden, but there nevertheless.
From a genetic point of view, every dog is actually three dogs: shadow, substance and the combination of both. The shadow dog is the total of the recessive genes that are unseen, but are ready to become apparent in future generations. The substance is the living, breathing dog you can see and touch (the phenotype). The third dog is the gene-complex dog -- the total collection of its complete gene collection, both dominant and recessive (the genotype).
The importance of pedigrees can be appreciated in the light of the "three" dogs in one concept. Patterns of genetic activity can be observed and decisions made that will allow for the greatest opportunity for a desired result. For example, breeders know that dominant traits do not skip generations. They also know that recessive traits may skip one or more generations and must be inherited from both the sire and the dam to be visible. Knowledge about genetic activity allows breeders to encourage desirable traits while avoiding those considered to be undesirable.
In most situations, information contained in genes does not change. The only way for information contained within a gene to change is through mutation. Mutation keeps the gene pool of any species from becoming stagnant and is the driving force of evolution. In most cases, these mutations occur because of random accidents that occur during DNA synthesis. Since most mutated genes are recessive, however, they can remain hidden in the gene pool for generations before becoming evident.
Of course, not all mutations are beneficial -- some are even lethal. The process of natural selection tests the mutation, allowing it to continue only if it helps the animal to better cope with its environment. If it does not, the animal will usually not be strong or resilient enough to reproduce successfully.
As an example, suppose shorthaired dogs living in a cold environment experienced a mutation that led to longer, heavier coats in their puppies. These heavier coated puppies would be more likely to survive and reproduce, passing the longhaired gene to their offspring.
On the other hand, if those same shorthaired dogs experienced a mutation that produced hairless puppies, they would probably not survive long enough in the cold environment to pass the hairless gene to their offspring.
The process of natural selection can be thwarted, however, when humans become involved in selective breeding. For example, if humans provide warm environments for the hairless puppies, those puppies would avoid facing the consequences of the cold climate. Protected by humans, they could survive to reproduce and pass their hairless gene to their offspring.
Basically, all pedigreed dogs have come into existence in this manner. They are a product of selective breeding -- a process that has allowed for the development of a wide variety of dog breeds. Different people like different characteristics. They breed dogs with attributes they find pleasing in order to "create" an animal that with each generation, comes closer to fulfilling their concept of canine perfection.
Example of how recessive and dominant genes combine:
A pair of black Labrador Retrievers may produce some yellow puppies as well as the expected black puppies if both parents carry the recessive yellow gene. In this case, each parent would have both the black and the yellow color gene. The black color gene is dominant and the yellow color gene is recessive.
BLACK/yellow (female parent) x BLACK/yellow (male parent) = puppies with genetic make-up that can be BLACK/BLACK, BLACK/yellow or yellow/yellow.
Both the BLACK/BLACK puppies and the BLACK/yellow puppies would be black.
The recessive yellow gene would be hidden in the BLACK/yellow puppies. They would be just as black as their BLACK/BLACK siblings.
Puppies that had inherited the recessive yellow gene from both parents (yellow/yellow) would be yellow and would not have the ability to produce black puppies unless they were mated to a dog that carried the dominant black gene (BLACK/yellow or BLACK/BLACK).