Bouvier Health Foundation Education Series
Part 1: Genetic Terminology – A Reference Guide
Complied by Penny Anderson
In the next several issues of our Dirty Beards publication, Board Members of the Bouvier Health Foundation will share information from various sources to update and educate the fancy in current genetic studies affecting Bouvier & canine health. The first of these articles is an attempt to define some of the most common terms used in genetic research. These terms are by no means an all inclusive list – but will include the most commonly used terminology. We hope this provides you with a good start on our journey and, ultimately, will assist you in making better breeding decisions…so, let’s get started!!!
DNA (deoxyribonucleic acid) is quite simply the basic molecule of heredity. It consists of a long, double-stranded, nucleic acid molecule arranged as a double helix that appears to be wrapped around itself in a spiral formation. This building block is the main constituent of the CHROMOSOME. I think we all know that each dog begins from a single cell, but more specifically, the cell nucleus contains the strands of DNA known as these chromosomes. The DNA consists of sub-units, known as “bases” or “nucleotides.” There are four bases and their letter designations are Adenine (A), Guanine (G), Thymine (T) and Cytosine (C). There is a specific associated distribution of these bases on the double helix. If on one strand there is the base “A,” on the second strand, associated in the same position, there will be the base “T.” Likewise, if on the first strand, the base is “G,” then on the second, in the same position, the base will be “C.” However the order of these sequences along the strand is not defined, and there can be a large variation in the order that occurs. It is the variation in the orders of these genes that produce different amino acids combining to produce the proteins that are the products of genes making up this DNA molecule.
The cell nucleus of every dog contains 39 pairs of chromosomes (38 pairs of AUTOSOME chromosomes + 1 pair (or 2) SEX chromosomes). Basically, all chromosomes other than sex chromosomes are autosomal. There are two copies of every chromosome in each cell; these are called HOMOLOGOUS CHROMOSOMES. Both the sire & dam each contribute equally to their offspring, receiving one component of these 38 pairs + 1 sex chromosome. In other words, each puppy will receive 50% of its genetic makeup from each parent. However, the sex of their offspring is always determined by the sire, who has both “X” & “Y” chromosome verses the dam, who has two “X” chromosomes.
GENES are made up of DNA, and are the basic molecule of heredity. They are strung along the length of each chromosome. Just as chromosomes come in pairs within each cell, so do genes. One member of the gene pair comes from the sire and one comes from the dam. It’s these strands of DNA that carry the genes from one generation to the next. The location of a gene on a chromosome is referred to as its LOCUS. Genes carry the genetic information, or “code,” for protein products that influences the performance or appearance of individuals. The gene for a particular trait is generally located on the same place (locus), on a homologous chromosome.
For many of these genes, there are alternate forms segregating in the population. These alternate versions of a gene are called ALLELES. Two alleles of the same gene differ in their DNA sequence. In a molecular genetic sense, the difference can be as small as a single base pair in the gene. Here again, a dog can inherit only two alleles of a particular gene (one from the sire & one from the dam). These alleles may be the same or they may be different. “Like” alleles are referred to as HOMOZYGOTES – the dog is pure for this trait and must pass it on to its offspring. Alleles that differ are referred to as HETEROZYGOTES and, when bred, will randomly pass on either the dominant or the recessive gene to their offspring. Alleles produce differences in the proteins, and these differences impact the visible (PHENOTYPICAL) characteristics of the gene. Some simple examples of visible variations are coat color, length, texture, etc., in different puppies, in the same litter. The phenotype is what we actually observe when we are looking at the different possibilities associated with a trait.
The GENOME represents the entire complement of genes in a species. The dog genome was sequenced in 2005. The GENOTYPE, which is a subset of the genes within a genome, refers to those that influence a particular trait. For some traits, they may represent alleles at a single locus. Or they can be very complex, sometimes involving hundreds of genes. Many researchers believe (not counting for environmental factors) that individuals of all species & types, have a pre-destined likelihood of developing conditions based on their genetic makeup…it’s pre-programmed in the DNA.
Much of the genetic principles in use today refer back to the work done by Gregor Mendel in the 1860s. He produced two laws based on his hybridization studies with peas. The first was the LAW OF SEGREGATION, where he states that factors (which are now referred to as alleles at a gene), exist in pairs, and that a random member of the pair is passed from parent to offspring. The second was the LAW OF INDEPENDENT ASSORTMENT, which refers to the joint inheritance of two or more characteristics. He also found that some genes overrule others. If in a pair, one allele masks the presence of a second allele in the expression of a phenotype, that allele is said to be DOMINANT. The allele being masked is said to be the RECESSIVE allele. With dominance, two genotypes, that being the one homozygote for the dominant allele and the heterozygote, result in identical phenotype. This also means that we cannot distinguish the genotypes of these individuals by looking at their phenotype – dominant genes affect phenotype even if only one allele is present on a chromosome pair. Recessive genes only work if there are two of the same alleles present on a pair of chromosomes.
It’s nice when everything works smoothly – cells replicate normally and the population is healthy & happy – but what more commonly occur are cell mutations. Mutations are heritable changes in the DNA sequence that, alter the genetic information carried by the cell, leading to a potential alteration of a genetically controlled phenotype. The impact of a mutation on a population is through their contribution to fitness for those animals that carry the mutations. To be honest, cell mutations are how the Bouvier (or any purebred breed) was developed. There are several types of mutations – their results can be favorable, neutral (no impact), or unfavorable.
Conditions may also occur when dominant or recessive genes do not behave as expected.
- INCOMPLETE DOMINANCE is when a gene does not totally mask the recessive gene; where the phenotype associated with each allele is visible. Incomplete dominance is used when the heterozygote is intermediate; but the individual products of both alleles can’t be identified; it is the expression of both alleles in the phenotype and produces a mixed or diluted outcome.
- INCOMPLETE PENETRANCE is the situation for which the phenotype associated with a particular genotype is not expressed fully, so that an affected individual may not display all the components of that genotype. However, even if the dog itself doesn’t show that phenotype associated with the genotype, it will breed “true” to its genotype. That is, the progeny are likely to show the phenotype if they inherit the allele not expressed in the parent. It is felt by some researchers that incomplete penetrance is sometimes due to compensating factors in the environment.
- EPISTASIS is the interaction of genotypes at different loci and can come in a variety of forms. It can include the situation where an allele or genotype at one locus actually masks the expression of genotypes at a second locus on a chromosome. A second form can be where an allele or genotype at one locus; just modifies the expression of genotypes at a second locus.
- PLEIOTROPY is the influence of a single gene on more than one different trait. However, the effect may improve one trait, while making another trait worse.
Certain genotypical traits can be SEX-LINKED – any trait controlled by genes located on the sex chromosomes (X or Y). Something carried on the “X” chromosome can be transmitted by a bitch to either a son or daughter, but a male carrying something on his single “X” chromosome can only pass it on to his daughter. A trait can be SEX-LIMITED, for which a trait can only be expressed in one sex. The trait may be expressed in one sex, but its inheritance is determined by genes transmitted by both sire & dam. A trait can also be expressed more frequently in one sex over the other – SEX-CONTROLLED trait.
However, more common, and much less predictable, are traits controlled by more than one pair of genes for which the variation in phenotypes is influenced by genes at many loci. These are referred to as POLYGENIC TRAITS. With polygenic traits, there are multiple chromosomes affecting the genotype. When considering a breeding pair, it is important to look back a minimum of three generations for potential effect on the offspring.
The unfortunate reality of canine genetics is that very few traits have been genotypically mapped, but work does continue and that’s where we, as the fancy, can help. The Orthopedic Foundation for Animal (OFA)’s CHIC/DNA Repository is a centralized facility where scientists can collect DNA samples of canines meeting specific test criteria. It eliminates delays in collecting DNA samples and makes it possible for researchers to have samples with specific criteria (eg. healthy 12 year old, etc.). The CHIC website has information on how you – through your veterinarian – can submit samples for all your Bouviers – blood samples being preferred by researchers, though cheek swabs are perfectly acceptable.
This first article is intended to shed some light on a basic trip to discover what cells are and how they affect our breeding, but this is only the beginning of a giant climb, known as GENETICS. In the next edition of Genetics 101, we will compile a glossary of terms that will further assist you in understanding scientific terms. Very simply, it’s important to remember that what we SEE or OBSERVE in a dog is its PHENOTYPE, but what truly controls the health & well-being of itself and offspring are their molecular makeup, known as GENOTYPE!!!