The study of genetics is an extensive, as well as intensive, field of interest.  The author will attempt to introduce the basics in this article.  Obviously, the subject pervades so many areas it would be impossible to cover them all.  There are many books available on the subject of genetics that covers every breed of dog, as well as some very general information.  The author recommends that you pursue this area of interest as the study of genetics is very important for anyone seeking to establish, or even improve, their bloodline. If you are not doing either one of these options, then you should not be breeding dogs.  There are currently over 6 million dogs without homes in the U.S.A. alone, please do not add to their numbers.  The responsibility starts with each one of us.

     One of the first people to scientifically define the general principles of genetics was the Austrian monk and naturalist Gregor Mendel (1822-1884).  He accomplished this through his study of garden peas.  Genetics answers the question of why one animal differs in size, shape, color, intelligence, or ability from another. There are some traits that are inheritable from the parents, other traits are due to the influence of the environment the animal is raised in.  For example, if the sire and the dam of a puppy demonstrate that they have a calm disposition and are not prone to fear biting, than it stands to reason that that calm disposition will be passed along to the offspring.  But what if that puppy grows up, is chained outside without human contact, the owner kicks and yells at that dog, and in general, mistreats that dog.  The dog bites a child that is teasing it with food. Whose fault is it?  The breeder's, or the owner's, the child's, or the dog's?  We will leave the answer to that question to the courts.  The point I'm trying to make is that genetics plays an important role in helping the breeder to strengthen the points that are desirable, the points that are needed for a dog to fulfill its purpose, and eliminating, or reducing, the incidences of weaknesses that are not wanted. By studying the science of genetics and reproduction, we are learning how to breed better animals, in some cases, saving animals from extinction, and sometimes, simply avoiding unwanted additions to our pet family.  The scientific, professional breeder needs to understand how genetics will affect the breeding program they have in place.  How choosing the sire and the dam can compliment, or how they can become a problem, to the breeding program. The environment that the dog is raised in is dependent on the breeder being diligent in his/her carefully choosing the people they sell to, and the buyers being willing to take on the responsibility of owning a dog, mixed breed or pure.
     The basic unit of inheritance is the gene.  The genes are made up of DNA (deoxyribonucleic acid) , and they are part of the components that make up the DNA molecules referred to as chromosomes.  Every species have a specific number of chromosomes that complete the DNA package for that species.  For canines, the total number of chromosomes is 78.
     The genes encode, or blueprint, an animal's anatomy and physiology on proteins that are manufactured by the body.  The complex processes of life are directed by an estimated 50,000 to 100,000 genes, each individual gene coded for different proteins.
     A cell reproduces by dividing and forming an exact duplicate of itself.  That duplication extends to the chromosomes as well.  Throughout the body cells are dividing and multiplying, and replacing others. This process is called mitosis.
     A second process of cells reproducing has to do with the reproductive cells only.  A special process is in place so that one half of the total number of chromosomes are carried by the male and by the female. This process is called meiosis.  There are two distinct types of reproductive cells; the sperm of the male, and the ova, or the eggs, of the female.  Collectively, the sperm and the ova are also referred to as gametes.  In the species we know as canine, 39 chromosomes are carried by the male, and 39 chromosomes are carried by the female. Once the sperm fertilizes the ova, the chromosome number of the resulting fertilized cell is 78.  That fertilized cell is also referred to as a zygote.
     Scientists using microscopes, and special dyeing processes that stain the colorless genetic material, photograph and create a print.  From a photographic print they can arrange the 78 chromosomes as a series of 39 pairs.  This array of 39 pairs of chromosomes prepared from the photographic print is known as a karyotype.  When studied, one of the most obvious pairs of chromosomes are the ones that determine sex.  The male chromosome pair are distinctly different from each other, therefore referred to as the X and the Y.   The two sex chromosomes of the female resemble the X of the male, and are essentially identical.  The female chromosome pair are both referred to as X.  During meiosis the members of every chromosome pair are split up as the gametes form.  All the female ova gametes have a single X chromosome.  The male gametes (sperm) are equally divided with X- and Y- bearing cells.  Should the female ovum be fertilized with an X- bearing sperm, a female puppy will develop.  An ovum fertilized by a Y- bearing sperm will form a male puppy.  So it is the sire alone that determines a puppy's sex.  The other traits will not always be determined by the sire.
     As for the other traits carried by chromosomes, since the chromosomes are paired it is important to note that only one of the two traits encoded into the chromosomes and genes can be transmitted from each parent to the offspring. Note also that there are thousands of different breeds of dog in the world, that they all have the same number, sizes and shapes of chromosomes within their DNA because the genetic information of all dogs is nearly identical.  The fact that you can cross between the breeds of dogs that differ in appearance and the offspring will be able to conceive. It is the coding on this genetic material that will determine whether the offspring grows into a Chihuahua or a Great Dane.