Genetics tends to give me a headache, all those terms … And what about those words like co-dominance and incomplete penetrance? Sounds like the makings of one of those hot and heavy sexual fantasy books, doesn’t it?
Is it important for dog breeders to know at least the basics of genetics? Dog breeding has traditionally been a blame game …. many will blame the sire or dam if something turns up in a litter. Because you’ve never had it before … let’s use as an example long stifles where you’ve always had short stifles. You were known for your short stifles!! All of a sudden you get a litter of puppies with some having long stifles. It has to becoming from the sire, doesn’t it? Neither parent had long stifles though…. Long stifles are recessive which means you have just had the misfortune to breed the right bitch to the right dog to get those yucky long stifles!
Virtually every article on genetics starts with Gregor Mendel. You know that 19th Century Austrian monk and his pea plants that we studied in grade school biology class. I probably yawned my way through most of that class …. who knew it would become so important to me as a dog breeder? Anyway, let’s skip over that section, shall we? I’m yawning just thinking about those silly pea plants.
Every living thing is made up of cells within which are organized structures called chromosomes located on strands of nucleic acid twisted into a double helix called DNA. Basically, DNA is the code which acts like a blueprint to build and make function each cell. DNA code is made up of a series of 4 letters A (Adenine), C (Cytosine), T (Thymine), G (Guanine) representing proteins which are subject to certain rules; A always pairs with T and C always pairs with G. This coding when put together into a specific string of letters makes up a certain gene.
Each dog has 39 pairs of chromosomes; 76 autosomal (non-sex) and 2 sexual, located in the nucleus of a cell. One set of chromosomes (39) is inherited from each parent in a random pattern to make up the entirety. It is this randomness that makes for the expression of different traits in the offspring and why a sibling might differ greatly from another.
Genes are the set of inherited instructions which supply the directions to build the proteins which make the body function. The specific location of a gene on a chromosome is referred to as a locus or loci and this positioning can dictate how different genes interact.
A gene has two parts (allele) to it, one inherited from the father and the other from the mother. Genes can be either dominant which is the stronger information and will normally win out over the recessive gene though the information from the recessive gene remains. If the allele is dominant, only 1 allele is required to express the trait; if recessive then 2 allele are needed.
In a genetic formula a dominant gene is represented by a Capital letter and a recessive is represented by lower case letter. When both alleles are the same (example: BB) this is referred to as homozygous but when the alleles are different (example: Bb) it is referred to as heterozygous. In the case of a heterozygous pairing one of the alleles is usually dominant and the other recessive. The interaction of alleles is responsible for the traits found in an offspring.
Some examples of phenotypes which are dominant or recessive:
|Dominant Traits||Recessive Traits|
|Low set ears||High set ears|
|Short foreface||Long foreface|
|Coarse skull||Fine Skull|
|Dark eye||Light eye|
|Short coat||Long coat|
|Curly coat||Straight coat|
|Heavy boning||Light boning|
|Short stifle||Long stifle|
|High set tail||Low set tail|
|Good eye pigment||Wall eyes|
|Black nose||Dudley nose|
|Good mouth||Overshot or undershot mouth|
|Normal palate||Cleft palate|
Two myths in the world of dog breeding genetics are 1) that inbreeding causes genetic diseases and 2) that cross breeds are healthier. The mis-perception with respect to inbreeding is because it tends to reduce genetic variation within a breed, as does popular stud syndrome; both of which emphasize the presence of recessive genes some of which may be deleterious. On the other hand, crossed breeds still have the recessive genes but they may be masked by the outbreeding …. until two corresponding recessives meet up in the future often on the 2nd generation or F2 breeding, for example breeding the offspring of a Golden Retriever and Poodle breeding (aka Goldendoodle) together. In that 2nd generation you would see dogs that look more like Golden Retrievers and Poodles along with the accompanying health problems which one or other of the breeds have.
A defense of inbreeding may be that 1) they concentrate desirable genes and 2) inbreeding promotes homozygosity which in turn exposes deleterious recessive genes allowing the breeder to eliminate them through selective breeding; lacking any better method of identifying carriers of those undesirable genes such as DNA testing. Many breeders who continually outcross are often shocked when they find some genetic condition has “jumped” several generations and will often blame the condition on the sire or the dam, not recognizing the fact that their dog also carried that recessive gene and the trait didn’t appear until bred to a dog with the corresponding recessive gene.
Traits may include physical appearance, behaviors such as herding or retrieving instincts or predisposition to diseases. Some traits are simple and one pair of alleles may dictate certain characteristics an offspring may exhibit. Other traits are complex and are caused by the interaction of numerous alleles.
The observable appearance of a trait or the overall appearance of an individual is called the phenotype and the underlying genetic makeup of a trait or the overall genetic makeup of an individual is referred to as genotype.
OK I said I wouldn’t mention Gregor Mendel but here it is …. Mendelian principles relate to simple inherited traits which are determined by single genes; there are, however, exceptions to simple inheritance. These exceptions are referred to as non-Mendelian inherited patterns and include …. well-known to Cavalier breeders …. polygenic traits. Examples of polygenic traits might include body shape, coat color and coat patterns.
Alleles may interact in different complex ways and variations to simple dominant/recessive inheritance can be seen as follows:
Polygenic traits are the result of the combined cumulative action of alleles of multiple genes. In order to be expressed several different alleles must be present and so hereditary patterns are more complex. Environmental factors may also play a factor in the expression of a trait. A very simple example of this is human height – multiple genes go towards determining height but while a person may have the genes to be tall if they suffered from malnutrition during growth may end up being short. Many congenital diseases are the result of polygenic inheritance such as Mitral Valve Disease (MVD) and Syringomyelia (SM). Please note that the Online Mendelian Inheritance in Animals website of the University of Sydney cites both MVD and SM as multifactorial which indicates that there is also an environmental aspect to the expression of these diseases).
Pleiotropy is where a single gene may influence several phenotypical traits simultaneously. Some of these traits may have conflicting effects with some being beneficial and others detrimental to the animal. An example of pleiotropy is the M allele which produces the Merle pattern but also is associated with deafness and eye defects.
Incomplete dominance or intermediate expression is seen when one allele for a trait is not dominant over another, rather there is a blending of the trait. An example might be if you have a dog dominant for black and a bitch dominant for brown – by mating them together you would get a blending of the two colors – brindle.
In the case of codominance neither allele in a gene pair is dominant or recessive and two traits might be expressed. In dogs gum coloration is codominant so if the sire has dark gums and the bitch has pink gums you will get offspring with a combination of dark and pink gums.
Penetrance refers the proportion of a population which might carry a particular gene AND express the trait. Simply put if a trait has 95 % penetrance this would mean that 95% of those that carry the particular allele responsible for a trait would express the trait and 5% would not.
The degree of penetrance can vary from complete high to low. Complete penetrance would mean that every individual who has the gene would express the trait whereas low penetrance would mean the trait is only occasionally expressed in carrier individuals. Low penetrance is sometimes written off as thought of being caused by environmental influences. In the case of incomplete penetrance (generally refers to an autosomal dominant condition) a group may all carry an allele but some do not express the trait at all while others do.