Pet Health Care Article by Dr. Baum: It’s In The Genes
DNA testing is rapidly becoming a valuable tool for those breeders who seek to improve their respective breeds. Whether you are seeking to eliminate a harmful recessive trait, predict the likelihood of specific color patterns for a future litter or simply wanting to verify the paternity from the breeding that was done with frozen semen, this technology is for you!
Before getting into specific tests it is important to define some terms and explain some concepts. The words, genotype and phenotype are basic to your understanding. Genotype refers to your genetic makeup; phenotype is how those genes are expressed. Genes can be dominant or they can be recessive. Some traits are expressed as simple (controlled by the interaction of one gene pair only) or they can be expressed by the mixture of multiple genes. Genes are made of DNA.
Let’s look at a gene that is carried by about twenty percent of the French Bulldogs — the gene for Juvenile Cataracts. This condition, where the effected individual will generally develop cataracts in one or both eyes by the time they are three years old, is passed on as a simple recessive trait. The genotype of an affected individual is that both members of the gene pair must be the gene for forming cataracts. This can only occur if both parents carry this recessive gene. A carrier will always have one recessive gene and one dominant gene and because the dominant gene prevails in expressing the phenotype, this individual will not be affected. When two carriers are mated, the chance of having an affected offspring is one in four. There is also a one in four chance of having an offspring that doesn’t carry the faulty gene as well as a two in four chance of producing carriers. By identifying the genotype of potential mates it should be possible to eliminate this gene from the genetic pool within one generation by breeding only non- carriers.
Color patterns are inherited in a slightly more complex way. Instead of only having one pair of genes dictate the expression of a trait, color is inherited as a result of multiple pairs of genes called multiple alleles. Within this group of genes, certain ones seem to dominate the others but the influence of any one pair can be altered by the various combinations of the other gene pairs. Brindle is the most common color in the Frenchie gene pool. Two other versions of the same gene can show up. The most dominant version will result in solid black dogs, while the most recessive version is responsible for fawn. By knowing the genotypes of prospective mates it is possible to come up with probabilities regarding the color of the progeny. However, there never can be any guarantees!
The AKC runs a DNA testing program which is for verifying lineage only. (Interestingly, the tests do not identify a specific breed). Frequently used studs, producing more than seven litters in their lifetime are required to be registered. As technology has improved, and litters sired with frozen semen, or even fresh chilled semen, for that matter, are more commonplace, the DNA tests provide a safety net for all reputable breeders. For more information about AKC programs that will benefit your breeding program, simply visit the AKC website.
Very often I am asked, “At what age can these DNA tests be done?” Happily, for those of us who need immediate gratification, the answer is: as soon as they are born! An individual retains the same DNA from the moment they are born until they die- it does not change. The process for obtaining the DNA samples is really quite simple. Using the small plastic brush that is supplied by the by the genetic testing laboratory, the inside of the cheek is brushed gently to obtain the surface cells that will be used in the testing process.
As valuable as these DNA tests can be, at this time they are not a panacea for identifying all of the potential maladies that can afflict a particular breed. To date, no specific gene has been identified that has been associated with the occurrence of cleft palates, elongated soft palates, stenotic tracheas, vertebral abnormalities, hip dysplasia or hydrocephalus. Until that time, breeders will have to exercise good judgment if they are to avoid these conditions. In general, the more that you concentrate your genetic pool in the pursuit of one specific desirable trait, the greater the chance you have of allowing undesirable recessive traits to be expressed.
Inbreeding and line breeding are two practices that are frequently done to set a particular trait in a line of dogs. In reality, they are only extensions of the same thing, with inbreeding being the most extreme example of the two. In line breeding the individuals involved in the mating are more distant relatives while in inbreeding the relationship is more incestuous with very close relatives such as parent-offspring, or siblings being mated to each other. While line breeding is inherently less risky than inbreeding, there is always the issue of an unwanted recessive trait expressing itself. While there are complex algorithms that have been devised that can theoretically give you a risk factor with a certain mating, my general advice is—don’t line breed if there has been any prior expression of recessive trait within that line.
P.S. As some of you may or may not know, I am a crossword puzzle fanatic. So, I thought it was quite telling, that while I was writing this article, the November 18 edition of the New York Times Crossword had the following clue at 18 across: Promotes recessive traits, say.
The answer of course: Inbreeds.
Thank you to Robert Loechel of VetGen for his input.
Note: This article also appears in The French Bullytin, Vol. 29, No. 2