Basics of heredity
Heredity refers to the transfer of biological characteristics from a parent organism to offspring. Leading up to our modern day understanding of heredity came the work of Charles Darwin, an English naturalist who achieved lasting fame for his theory of evolution. In his 1859 book, On the Origin of Species, Darwin theorized that changes in a population's inherited characteristics (or traits) from generation to generation could be explained by natural selection, whereby organisms with beneficial traits (ones that helped the organism to survive in its environment and reproduce) tended to have more offspring, and in doing so, passed more copies of their beneficial inheritable traits on to the next generation. Although his theory depended on the presumption that traits were inherited,
he was unable to explain the mechanism for heredity.
Gregor Mendel, known as the father of modern genetics, studied the inheritence of traits using pea plants
between 1856 and 1863. Mendel's ideas, first published in 1866, proved that inheritance patterns of certain traits in pea plants obeyed simple statistical rules. The importance of Mendel's concept of a fundamental unit of heredity was not understood until his work was rediscovered in the early 20th century, more than 15 years after his death.
By the 1940s, experiments pointed to DNA (deoxyribonucleic acid)--in the form of large molecules known as chromosomes--as the carrier of genetic instructions for the development and functioning of living organisms.
Studies that analyze DNA can help provide information about evolution, providing us with a window into the past.
Cynologists (students of cynology) concern themselves with canine evolution, breed development and differentiation, canine behaviour and training, and canine history. The discipline has virtually only existed since the last quarter of the nineteenth century, when purebred canine registries were organized, beginning with the first--The Kennel Club in the United Kingdom--founded in 1873. The basis for the study of cynology lies in the science of heredity.
genetics of a breed
The carriers of biological information from parent to offspring, which interact with each other to influence physical development and behavior are known as genes. Genes are located on long strands of DNA, organized in cells as chromosomes. In living organisms, DNA usually exists as two long strands that are twisted together in the shape of a double helix, right. The backbone, which is made of sugars and phosphate groups, holds each strand together. Attached to the backbone of each strand are four different bases, known as adenine, cytosine, guanine and thymine. The DNA double helix is held together by hydrogen bonds between the bases on each strand, whereby adenine on one strand only bonds with thymine on the other strand, and cytosine on one only bonds with guanine on the other. It is the sequence of these four bases that encodes genetic information.
The gene pool of a population is the complete set of genetic material found among the living members of that population. Differences in the traits among individual organisms are due to genetic differences, or genetic variation.
Generally, the larger the gene pool, the more genetic diversity (or variety in physical and behavioral traits) within that population.
Cell division is essential for organisms to grow, and when a cell divides it must replicate (or make an exact copy of) its DNA so that the two daughter cells have the same genetic information as the parent cell.
Over time, errors can occur during replication, causing a change in the sequence of the gene's bases known as a mutation.
Mutations create variation in the gene pool by causing new traits to appear over time. Individuals possessing the most advantageous traits for survival are more likely to survive longer to reproduce than those with unfavorable traits. Favorable mutations thereby accumulate in the gene pool and become more common in later generations, resulting in evolutionary change (Darwin's theory of evolution). Conversely, mutations deleterious to survival are removed from the gene pool. This process by which favorable inherited traits propagate throughout a reproductive population is known as natural selection.
Selective breeding (or artificial selection) is the process by which man intervenes, choosing which animals are allowed to breed based on whether or not they exhibit specific desirable traits, whereby desirability is defined by man. Selective breeding generally narrows the gene pool in favor of certain specific traits, increasing the likelihood that the desirable traits will be passed down from generation to generation.
A breed of dogs is a group that exhibit very similar appearance and behavior qualities that man has defined to be desirable. Dogs have been selectively bred for specific characteristics for thousands of years. Initially, the selections of which animals to breed were made based on useful behavior, such as hunting ability. In modern times, many dogs have been increasingly selected for breeding based on their physical appearance alone, for purposes of showing.
Before a type of dog is recognized as a true breed, it must be demonstrated that mating a pair of dogs with certain characteristics usually produces offspring that have the same characteristics as the parents. The dogs originally chosen by man as examples of a breed are often of unknown parentage and are referred to as foundation stock.
Once consistent offspring can be demonstrated to result from man's selective breeding, a dog breed registry (an organization that tracks the parentage of dogs) will recognize a breed standard, a description of a hypothetical specimen of that breed to which offspring should conform. Dogs selectively bred to conform to the breed standard are said to be purebred. When the lineage (parentage) of a purebred is recorded with the dog breed registry, the dog is said to be pedigreed.
DNA: explaining the variation among today's breeds of dog
For thousands of years, dogs changed little. Then something fascinating happened. In 19th century Europe, there was a sudden explosion of dog breeds. The Victorian Era (the period of Queen Victoria's rule between 1837 and 1901, left) gave rise to the Industrial Revolution, railways, photography, electric lights, Darwin and an emerging leisure class. The whole Western world became obsessed with perfection and design, which they applied to architecture and gardens until they looked "just right". And then, it was the dog's turn. Dogs became a status symbol of the new middle class, and experimentation aimed at producing custom-made canines became a kind of middle class hobby. By the middle of the 19th century, we saw the emergence of the dog fancy: pet dog ownership, dog showing, dog breeding and dog sports. People began evaluating dogs for desireable looks and behavior and controlling who these dogs were breeding with, isolating the breeds from each other. It was that process that led to the proliferation of the modern breeds that we have today. Starting with the original, primordial, working-type dogs that existed, man set out to improve populations by selective breeding and "created" dogs with large floppy ears, curled tails, pug noses and short legs.
The 19th century saw the creation of hundreds of breeds as a result of man's obsession with perfecting canine form and function. Retrievers are known to chase and retrieve an object, almost indefinitely. Terriers will chase anything small that moves, digging tirelessly after vermin. Collies are habitual herders. Pit bulls, gentle around people, will savagely attack other dogs. Still today around the globe, people breed sniffing dogs to seek out and find everything from trapped people to illicit drugs to explosives.
In biology, the genome of an organism is its whole hereditary information that is encoded in its DNA. Within biochemistry, the study of chemical processes in living organisms, are methods known as DNA sequencing that can determine the order of the four bases (adenine, guanine, cytosine, and thymine, or AGCT for short) in DNA.
We know that some mutations, or changes in the order of the four bases, can change the proteins made by the cells that in turn lead to changes in cell function and ultimately physical appearance. Early breeders who engaged in selective breeding had no idea they were mutating genes. They saw a short longish dog and mated it with another short longish dog until they got a consistently short long dog, amplifying traits each time a new litter was conceived.
Dogs are believed to have more physical and behavioral variations that any mammal on this planet. Such astonishing diversity is possible because of the special nature of a dog's DNA, exceptional in the animal kingdom.
We can think of DNA as the basic instruction manual for every living organism. Coiled up in each cell's nucleus, DNA is made up of four chemical components, ACGT. The dog has 2.4 billion of these chemical components, whose sequences make up thousands of genes, that together specify all the components of a dog. Researchers at the National Institutes of Health (NIH) have deciphered the dog genome, the full compliment of the dog's genetic information, only to find that all domesticated dogs are genetically similar, 99.8% similar to be precise. A mere 0.2% difference in the genome is responsible for all the variation in size, shape and character among dogs. NIH researchers, for example, believe that a small variation in a single gene, the IGF-1 gene, influences a dog's size, determining whether the dog will be a 10-inch tall Chihuahua or a 42-inch tall Great Dane (right).
If dog DNA is so similar among breeds, then how do we explain the rapid rise of so many dog types in the last 200 years, the blink of an evolutionary eye?
A group of scientists at UT Southwestern combined extensive genetic data from different dog breeds and data on the shapes of dog skulls using computer programs developed by evolutionary biologist Dr. John "Trey" Fondon and have offered an intriguing explanation for why humans were able to transform dogs so easily1. Dr. Fondon's team collected blood samples from over 90 different breeds of dog and sequenced the DNA in an effort to find out what mutations, or changes in the DNA, are responsible for making the different breeds of dog look different. The researchers noticed something fascinating: specific regions in the dog's long strings of DNA code that are prone to mutation, called tandem repeat sequences. These tandom repeats are like a single word repeating many times within a sentence, for example, A-C-A-C-A-C-A-C-A-C. They have identified what they believe to be a genetic mutation mechanism that is responsible for the dog's rapid evolutionary changes.
As an organism grows, cells divide into two, theoretically producing two identical cells where there was previously only one. The genetic code within those cells must be precisely duplicated, so that both of the resulting cells will be identical, able to perform identical functions within the organism. Over time, mutations happen in these tandom repeat regions when "errors" occur and units in the repeating genetic sequence are accidentally added or subtracted (left) by the proteins responsible for "reading" and "copying" the letters in the long strings of genetic instructions. These mutations are then inherited by future generations. Deletions or additions in the genetic code that result in changes in the proteins made by cells can alter how the cells function, and those functions can alter the physical appearance of the animal. For example, by examining the tandom repeats in the gene that influences skull development in the dog, scientists found that the length of this repeat correlated with the amount of downturn or upturn in the dog's snout. Examination of the same genetic region from wild coyotes and wolves revealed variations in repeat lengths, but these animals do not have nearly the wide range of variation in repeat lengths that domestic dogs do, and, consequently, they don't exhibit the range in physical variation in muzzle length.
Researchers concluded that mutations occuring in these tandom repeats explain why we have been able to change the dog from generation to generation so easily. Two factors are primarily at play.
First, mutations in tandem repeat sequences occur much more frequently than in other areas of the DNA--up to 100,000 times as often--and are much more likely to result in significant morphological changes (or changes in physical appearance) in the organism. Second, while all living things have tandom repeats in their genes, the dog has significantly more than most.
Is this high proporation of tandom repeats in dogs the result of man's selective breeding efforts, or did it pre-date domestication? To answer this question, the scientists expanded their study to other canids, or members of the canidae family of mammals that include dogs, wolves, foxes, and coyotes. What they found was that wolves have it, coyotes have it, red foxes and gray foxes have it. But as soon as you take one more evolutionary step out and sequence a bear, a skunk, or a raccoon, it's gone. The absence of tandem sequences in these other mammals may explain why we can change the appearance of a cow somewhat, but it essentially always looks like a cow. A cow doesn't develop short stubby legs or a curly tail.
As humankind began to want a dog with a particular look or temperament, we unwittingly began to mold the dog. With the dog able to birth a litter twice a year, the prevalence of very highly mutable tandem repeats in the dog's DNA code made rapid change possible. With the creation and perfecting of new breeds came compteition, in the form of the dog show, competition that further encouraged the proliferation of more purebred dog types, hundreds of them. With the pure breed came rules that we call breed standards, a code of perfection, a specific description of what each particular breed should look like and how it should behave.
The dog was bred to our specifications, for better and worse. And man's tampering and rule-following has had unintended consequences. This massive eugenics experiment (the use of selective breeding in pursuit of the improvement of dogs' hereditary traits) has led to a steep rise in genetic diseases. Without new genetic material being introduced, you get a very high level of recessive genetic problems being expressed. We bred into the bull terrier an egg-shaped head and sturdy frame. Somewhere along the lineage we bred a tendency toward obsessive compulsive behavior, too.
Can we repair what we created?
Isolating defective genes has led to DNA tests that can detect the presence of cetain of these undesirable genes. For diseases for which DNA testing is available, testing allows breeders to screen for defective genes, with the potential to eradicate disease as dogs with unhealthy traits are excluded from the breeding population.
1Source: University of Texas Southwestern Medical Center At Dallas. "Research Points To New Theory Driving Evolutionary Changes." ScienceDaily 24 December 2004. 17 December 2008 <http://www.sciencedaily.com /releases/2004/12/041219192823.htm>