Genetics of Hovawart Coloration

Author: Vendula Šlechtová

Author's remarks: The following text is a short summary of an original article that has been published in a journal called "Svět psů" ("World of Dogs"), August 2008, Issue 8, pp. 64-66.ISSN 1211-2976, in Czech.

Recently I have found in a discussion of a Czech hovawart webpage an article concerning colours in hw, where the author wrote own speculation about the genetics of hw coloration. In those times I joined in with my own, off course wrong idea based merely on the basic knowledge of principles of genetics. The question of hw colours did not let me sleep since those times, I was searching for some information in Czech as well as in foreigh literature and here I provide at least brief explanation that could be interesting not only for breeders of this beautiful race but also for the owners are searching for the reply same like I did.

Basic Principles of Heredity

In the beginning I would like to mention few sentences about the basics of Mendelian genetics on the most simple way. Therefore, those who know these principles can simply skip the following paragraph to the next one.

The physical as well as the character features of an individual are results of cooperation enviroment and genes inherited from the parents. A unit, which is responsible for a certain structure or function of a organism is gene and set of all the genes in organism is called genom. Each feature of a organism is controlled by an expression of one or more genes. Each gene can exist in one, two, or more variants called alleles inherited from the parents. One gene in one individual can have maximally two alleles; either same, or both defere (dog is a diploid creature, which means that each cell in each individuum, except gametic or some few other specialised cells that lack nucleus, contains two sets of chromosome, in other words, two alleles of each gene.) There may be several types of interactions among the alleles of a single genes. In this particular case, we have to understand only the relation dominant/recessive. The dominant allele (usually indicated in texts by a capital, let's say B) is expressed in each case when it is present in the genom in at least one copy. So called recessive allele (usually indicated by a small letter, e.g. b) can be only expressed if it is not overdominated by a dominant alllele, in other words if it is present in the genom two times (bb = recesive homozygot). So, the dominant allele will be expressed both, 1) if it is present in the genom 2x (BB = dominant homozygot), 2) or when present in one copy together with arecesive allele (Bb = heterozygot). The expression of the recessive allele will be supressed. Next phenomenon we need to explain here is epistasis. In short, epistasis is an interaction of two or more genes where one of them (epistatic gene) supresses expression of other one. If you are interested, you can learn more about dominance of alleles or epistasis.

Coat Coloration in Dogs

The principles of coloration inheritance in dogs are the same for all the breeds. Rarely we can find all the possible colour variants, which is a consequence of selection - either natural in wild living canine carnivors, or artifitial that is a result of human effort to create a race with certain features. The scientific literature says that dog coloration is coded by ten genes. However, in case of hw, only two genes are incorporated: gene A (Agouti) and a gene E (Extension). The name of Agouti gene coriginates from wild coloration of Southamerican rodents of genus Agouti (e.g. lowland paca, Cuniculus paca) and in dogs it referes to wolf-like "wild" colour. This genes exists in five variants (alleles) coding different types of coloration. The alleles are listed below in decreasing dominance:

A^s – codes black pigmentation

A^y – restricts pigmentation, individuals are light

A – codes wild (agouti) coloration

a^sa – codes saddle-pattern coloration which is present in e.g. Beagles

a^t – codes black-and-tan coloration, well known in e.g. Hovawart, Gordonseter or Doberman Pinscher

Currently, Hovawarts can have three different colour variations: black, blond and black-and-tan. In this breed there are 2 different alleles og Agouti gene A^s and a^t, so here we do not have to take the other alleles of this gene into consideration. Recall the paragraph about the relation of dominant and recessive alleles. Taking into account that A^s is dominant above all other alleles of Agouti gene, the individual will be black in any case where there is at least one A^s allele present in the genom. In other words, in case that the individual is a heterozygot or dominant homozygot for this gene. The expression of other alleles would be supressed. Since the a^t allele is recessive in respect to A^s, the black-and-tan coloration can be present only in individual with recessive homozygotic combination a^ta^t. In combination A^sa^t, the expression of marks (black-and-tan coloration) would be supressed.

OK, so what about the blond hovies? In nature, there is nothing so simple as it looks like on a first view. There is a little rub in the inheritance of HW colors. The expression of Agouti gene is influenced by an Extension gene. This gene controls the intesity of dark pigment in the skin. It is a supreme or epistatic gene in respect to Agouti. There are three forms of this gene, here listed again according to decreasing dominance:

E^br – contols the striped coloration as it is known e.g. in deutshe dogge or in boxer

E – codes presence of dark pigment in the skin

e – restricts the presence of dark pigment

In case of Hovawart, again, we can forget the E^br allele (stripes), which is not present in this dog breed. The E allele is dominant in respect to e. The dark pigmentation  given by a combination of the alleles of Agouti gene would be present only in the case, that there is at least one dominant E alllele of Extension gene present in the genom of a given specimen. In case that the individual is a recessive homozygot for the Extension gene (ee), the coloration would be light. Such kind of interation, where recessive combination of two alleles of one gene supresses the expression of dominant allele of another gene is called recessive epistasis. Let's have a look on a following example:

How is it possible that two black-and-tan parents can have besides black-and-tan puppies blond progeny? Let's recapitulate what we know about the Agouti gene: taking into account that the allele for marks is recessive and is expressed only in recessive-homozygotic combination this combination a^ta^t must be present in both parents and therefore all the cubs have the same combination a^ta^t, also the blond ones. That means that in this case the blond coloration of puppies is a result of an expression of an Extension gene in recessive-homozygotic combination ee. So both parents must have been heterozygots (Ee) for the Extension gene. Why? A blond recessive ee-individual can be produced only when both black-and-tan parents are heterozygots (Ee). In case that both parents were dominant homozygots (EE), all the cubs would be black-and-tan, same situation would appear when only one of the parents in heterozygot (Ee). See the Table 1.

Table 1

This table shows the possible genotypes of the parents for the Extension gene, resulting potential genotypes of their progeny as well as the probability of their occurance. (BT - black-and-tan, BL- blond). Both parents are black-and-tan, therefore they must be recessive homozygots (a^ta^t ) for the Agouti gene (the only combination that allows black-and-tan coloration).

 [Author's remark: Sorry, the table is still in Czech, I will exchange it for translated version as soon as possible...]

 The following table (Tab. 2) presents all possible combinations of alleles that influence the coloration in Hovawarts. Due to the huge amount od combinations, I have dropped my idea to present here all possible coloration in litters of all possible combinations of the parents. However, according to above mentioned exaple, with respect to the colours of breed-dogs and colours of their parents, siblings and progeny, everyone who has an interest, can try to deduce the genotypes and likelihood of potentially comming littre. 

 At the end, I would like to note that according to literature there has been also the saddle-pattern present among the first hovawarts. This coloration is coded by a^sa allele of Agouti gene. Hovewer, this colour was eliminated from Hovaward bread via the human selection. Theoretically, if the breeders would prefer only blond or black-and-tan individuals (situation in Czech Republic), we could lose the black coloration in the Hovawart breed. The black Hovies are not common in Czech Republic. On the other hand in a certain part of Germany we have faced an amazement of some Hovi owners above our black-and-tan friend. In this area, this coloration is more rare. In case that the breeders prefer the blond dogs, it could lead to elimination of the other colours. Although the alleles for black and black-and-tan coloration  would still persist in the Hovawart population, but their expression would be supressed by the recessive combination of Extension gene alleles. The blond Hovies can have only blond puppies, while combination of two black dogs can teoretically provide puppies of all colours, same stands for the combination of blond and black or blond and black-and-tan. Two black-and-tan Hovies can give birth only to black-and-tan or blond cubs, never to black ones. If you have a mood, have a look on webpages of diffrent kennels and compare the coloration of puppies with their parents and grandparents. There are several, almoust textbook examples among them.

References:

Dostál, J., 1995. Chov psů. Genetika v kynologické praxi. DONA, České Budějovice, 208 str. CZ

Dostál, J., 2007. Genetika a šlechtění plemen psů. DONA, České Budějovice, 262 str. CZ

Robinson, R., 1982. Genetics for Dog Breeders. Pergamon Press plc., Oxford, 264 pp. ENG

Robinson, R., 1988. Inheritance of coat colour in the Hovawart dog. Genetica 78 (2): 121-123. ENG