REVIEWING THE BASIC 4

In the first two parts of our series we covered the four major genetic concepts every breeder needs to understand: (1) Chromosomes and Genes, (2) Dominant and Recessive Genes, (3) Homozygous and Heterozygous gene pairs and (4) Genotype and Phenotype (Figure 1). Learning to "arrange" desirable genes in our breeding programs begins with understanding these four concepts.

EVERY DOG BREEDER’S "NEED TO KNOW" BASIC 4

WHAT’S THE CATCH?

Breeding would be simpler if genes consistently played by the rules. Genes, however, are not always predictable. Several phenomena that can affect the action of genes include (1) Incomplete Dominance, where a gene does not totally mask a recessive version (usually relates to temperament, intelligence, body height and length of leg); (2) Incomplete Penetrance, generally occurring in a heterozygous gene pair such as Aa, where the dominant gene A does not always show itself in a dog’s outward appearance; (3) Modifying Genes which combine with other genes, accentuating the effect of a trait or changing it altogether (control polygenetic traits such as shoulders, stifles and sternum); (4) Lethal Genes, which result in death of the embryo when they are passed on by both parents; and (5) Mutations, which occur when a cell is not exactly duplicated, leading to a variation of the original gene. Most mutations are destroyed at birth but those such as the short legs of the Basset Hound and Dachshund were viewed as favorable and deliberately selected for.

IF ONLY PUPS WERE MORE LIKE PEAS!

Our job as breeders would be greatly simplified if traits like good shoulder layback, temperament and reaching gait were controlled by single pairs of genes, such as those in Mendel’s plants. Unfortunately, this is not the case. Outside of straight-forward traits like coat color and length, which are determined for the most part by single gene pairs, most of the intricate traits we desire in our dogs are controlled by complexes of genes which are called polygenes. Polygenes usually combine mixed patterns of dominant and recessive genes. Although we do not know how many thousands of genes are involved in polygenetic traits such as skeletal structure and gait, geneticists feel that such traits follow a bell-shaped curve, with most animals falling in the middle (Willis, 1989). Based on this theory, two excellent individuals will usually produce slightly less outstanding offspring and vice-versa. Regarding size within a breed, most animals will be average sized, with excessively large or small animals being less common.

DOMINANT AND RECESSIVE TRAITS IN THE DOG

At this point it is necessary for dog breeders to understand two fundamental concepts: (1) aside from coat length and color, the traits of interest to us are for the most part polygenetic (controlled by many gene pairs), and (2) polygenetic traits are generally composed of a mixed bag of dominant and recessive genes. Although research in canine genetics lags behind that of other species, and some authorities disagree on whether certain traits are indeed dominant or recessive, Figure 2 lists those dominant and recessive traits agreed upon by a majority of geneticists and breeders (Willis, 1989; Seranne, 1980). Most of these traits are controlled by numerous gene pairs (polygenetic) and factors such as incomplete dominance and penetrance. For our purpose, the most important task is to familiarize yourself with the list in general and make note of traits you wish to improve in your breeding program.

A FINAL WORD ON ANGULATION AND TEMPERAMENT

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