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 cynologists (those who study dogs) with a window into the past.

The 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: A Window Into The Past

Since mutations that occur to DNA over time are inherited by future generations, geneticists (who study the science of genes, heredity, and the variation of organisms) can infer the evolutionary history of organisms by comparing their DNA sequences.

Mitochondrial DNA is located in mitochondria, discrete structures within cells that perform a specialized function known as organelles. Below, this schematic shows the components of a typical animal cell (Messer Woland and Szczepan, 1990).

(1) Nucleolus
(2) Nucleus
(3) Ribosome
(4) Vesicle
(5) Rough endoplasmic reticulum
(6) Golgi apparatus
(7) Cytoskeleton
(8) Smooth endoplasmic reticulum
(9) Mitochondria
(10) Vacuole
(11) Cytoplasm
(12) Lysosome
(13) Centrioles

Right, this photograph taken through a microscope (known as an electron micrograph) shows how an actual mitochondrion appears.

Below, mitochondria are surrounded by two membranes, an outer membrane and inner membrane (see numbers 2 and 1, respectively). The outer membrane encloses the entire structure and allows certain molecules and ions to move in and out of the mitochondrion. The inner membrane encloses a fluid-filled matrix (below, number 4) that contains enzymes that partipate in certain chemical reactions. Cristae (below, number 3), or folds in the inner membrane that project into the matrix, create internal compartments within the mitochondrion that can be clearly seen in the electron micrograph, above. The cristae provide more surface area for chemical reactions to occur within the mitochondria. The primary function of the mitochondria is to convert food molecules into a usable energy source, although mitochondria play an important role in many other metabolic tasks needed to maintain life.

Many cellular functions, such as the process of replicating DNA, need ATP (adenosine triphosphate) produced by mitochondria as their energy source. Mitochondria are found in most eukaryotic cells, which are the most structurally complex known cell type. A typical eukaryotic cell contains about 2,000 mitochondria, which occupy roughly one fifth of its total volume. When the energy needs of a cell are high, mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive.

Mitochondrial DNA exists as a small number of circular molecules which are independent of the DNA located in the cell nucleus. At fertilization of an egg by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the offspring. Because the sperm has only about 100 mitochondria compared to 100,000 in the egg, as the cells develop more and more of the mitochondrial DNA from the male is diluted out. Hence less than 0.01% of the mitochondrial DNA is paternal.

Because mitochondrial DNA is inherited primarily from the mother, it is easier for scientists to discern evolutionary interrelationships among various species that are believed to have a common ancestor. A biologist who studies the origin and descent of species, as well as their change, multiplication, and diversity over time, is known as an evolutionary biologist. Since mitochondrial DNA primarily reflects the history of females in a population, studies must include analysis of nuclear DNA, which is more dificult to analyze, if the studies are to provide a comprehensive evolutionary history of a population that may include traits primarily passed from generation to generation by the males.

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. [more] [back]