FROM PEAS TO PUPS - Topsfield Bassets

FROM PEAS TO PUPS:

BASIC GENETICS FOR DOG BREEDERS

by Claudia Waller Orlandi , Ph.D.

Part 3: Dominant and Recessive Traits

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.

Figure 1

EVERY DOG BREEDER’S “NEED TO KNOW” BASIC 4

CONCEPT	COMMENTS
CHROMOSOMES & GENES Hereditary components in every cell that determine how a dog will look and act.	-Chromosomes are made up of genes, which carry hereditary information -Chromosomes and genes are inherited by a puppy in related pairs, one member of each pair coming from the sire, the other from the dam. -Each parent passes on a random, chance assortment of chromosomes and genes inherited from his or her ancestors.
DOMINANT & RECESSIVE GENES Two types of genes whose interaction help determine which trait will be passed on to a new-born puppy.	- Dominant genes "win out" over recessive genes. - Use capital letters for dominant genes (ex. T for ticking). - Use lower case letters for recessive genes (ex. t for non-ticking). - Example, a dog inheriting T from the sire and t from the dam will have the gene pair Tt and will have ticking.
HOMOZYGOUS & HETEROZYGOUS Refers to whether members of a gene pair are alike (homozygous) or dissimilar (heterozygous). In- (line) breeding within first 3 generations increases homozygosity.	-Heterozygous gene pair example: Tt (members are dissimilar) -Homozygous gene pair examples: TT and tt (members are alike) -Breeder’s Goal: To "arrange" good genes in homozygous pairs to ensure that no matter which member of a gene pair a parent happens to pass on to a puppy, it will be a "desirable" gene.
PHENOTYPE & GENOTYPE How a dog looks on the outside is his phenotype. A dog’s genetic make-up is his genotype.	-How a dog looks on the outside does not always predict what genes he is carrying and what he will produce. -Example: A dog inheriting the gene pair Tt from his parents will have ticking, but because he "carries" the gene t for non-ticking he is capable of producing dogs with non-ticking when bred to a bitch carrying the Tt or tt gene pair. The non-ticking would occur in any puppy that by chance happens to inherit his sire’s t gene and a t gene from the dam.

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.

FIGURE 2

Genetically DOMINANT and RECESSIVE Traits In The Dog Does not include disease, coat color or abnormal factors
DOMINANT TRAITS	HEAD Low set ears Long ears Long head Wide ear leather Dewlap Dark eye Correct bite Black nose Short face	BODY Sternum Deep chest Straight top line Straight tail High tail set Good spring of rib Heavy bone Achondroplastic short leg with crook (correlates with big bone) Compact foot Short coat Weight Body height Poor shoulder angulation Poor stifle angulation Short, choppy gait	MENTAL Intelligence Shy and/or vicious temperament
RECESSIVE TRAITS	HEAD Pronounced parietal crest and occiput Large skull size Short ears Fine skull Light eye Bulging eye Overshot/Undershot Bite	BODY Good shoulder angulation Good stifle angulation Long, reaching gait Low tail set No feathering on tail Kinked tail Long coat Longer, straight leg (correlates with light bone)	MENTAL Mild, non-aggressive temperament Lack of intelligence

A FINAL WORD ON ANGULATION AND TEMPERAMENT

Several traits in the above chart are worthy of a closing comment. These are good shoulder and stifle angulation and good temperament, all of which are considered genetically RECESSIVE traits by most authorities. The shy temperaments and poor angulation often in evidence in the show ring today seem to support the theory that these traits are genetically dominant. Once these faults rear their ugly heads in a breeding program, a breeder needs to understand the concept of dominant and recessive genes as well as the effect of linebreeding to start decreasing their occurrence.

REFERENCES

Seranne, A. 1980. The Joy of Breeding Your Own Show Dog. Howell, New York.
Willis, M. B. 1989. Genetics of the Dog. Howell, New York.