Heredity is the genetic transmission of qualities and potentialities, called traits, from parent to offspring. Hereditary traits are numerous and include eye color, coat color, and size. Health and temperament traits are also hereditary. Among the many undesirable hereditary traits that can be passed on are deafness, eye disease, epilepsy, undescended testicles, and temperament flaws such as shyness or aggression. Traits are passed from parents to puppies by means of genetic material: genes, chromosomes, and DNA.

Genetic Material

Genes are bits of hereditary information. Passed from parent to offspring, their job is to control the transmission of a hereditary characteristic by specifying the structure of a particular protein or by controlling the function of other genetic material. Dogs may have as many as 200,000 genes. Over time, genes can mutate, or change. Sometimes a mutation is beneficial, but other times it results in genetic diseases.

Chromosome pairs resemble each other in length and shape. The exception is the pair of male chromosomes, with one having an X-shape and a smaller one having a Y-shape.

Each gene resides at a fixed position on a chromosome. Chromosomes are large, complex molecules that are found at the center (nucleus) of every cell. They contain the genetic blueprint for an individual dog. Each chromosome contains a sequence of genes. Dogs have thirty-nine chromosome pairs (humans have twenty-three pairs), for a total of seventy-eight chromosomes. Of the thirty-nine pairs of chromosomes, thirty-eight are similar to each other. These matched chromosomes each contain matching types of genes, although the actual information on the two genes may vary. The sex chromosomes make up the last pair: an X and a Y (male) or two Xs (female).

Each parent contributes half of a dog's chromosomes. This occurs at the moment of fertilization, when the egg and sperm meet. Like a deck of cards being shuffled and dealt, some chromosomes are kept and others are discarded. It's purely chance that determines which chromosomes a puppy will receive. That's why puppies in one litter can all look or behave differently.

The genetic material that makes up chromosomes is called DNA, or deoxyribonucleic acid. DNA is the molecular basis of heredity and is constructed in the form of a double helix, which looks something like a spiral staircase. The purpose of DNA is to code genetic information for the transmission of inherited traits.

Genetic Transmission

Genes are paired on a chromosome and come in two types: dominant and recessive. A dominant gene can cause a trait to be expressed all by itself. A recessive gene, on the other hand, must pair up with another recessive gene before the trait it controls can be expressed. Coat color is one of the best examples of how this works. Let's say that you breed two black Labrador retrievers. The resulting litter contains black puppies and yellow puppies. That's because each parent carried a yellow allele, an alternative form of the gene that carries the black color. The yellow allele is recessive to the black allele, but since each parent contributed a yellow allele, and those yellow alleles were paired on a chromosome, the two were able to combine to produce yellow puppies. If a chromosome pair consists of a black allele and a yellow allele, then the black allele will be dominant, producing black puppies. So a dominant gene paired with another dominant gene or a dominant gene paired with a recessive gene will always produce, or express, a dominant characteristic, such as black coat color. Only a recessive gene paired with another recessive gene will produce a recessive characteristic, such as yellow coat color.

Chromosomes contain long chains of DNA. DNA carries the genetic information that determines a dog's structure, personality, and other features.