Inbreeding seems to be something like a myth in dog breeding. Whilst some of the breeders use inbreeding or line breeding in a do-or-die-way to improve the quality of their dogs as fast as possible others avoid inbreeding as a matter of principle believing that all genetic problems will be solved by decreasing the inbreeding level alone.

Despite those somewhat emotional approaches inbreeding--from the population geneticists point of view--is nothing else than a breeding system with advantages and disadvantages that have to be considered. To have the chance to efficiently deal with it, it seems necessary to be informed about those genetic mechanisms that cause the advantages of inbreeding as well as the disadvantages.

When dealing with the association between inbreeding and reproductive traits in dogs therefore we have to answer some questions.

1. What Does Inbreeding Mean

Inbreeding is defined as mating of two individuals that are closer related than two randomly chosen individuals of the respective population. The degree of genetic relation between two individuals depends on the number and the position of common ancestors in their pedigree and can be measured by calculating the "coefficient of inbreeding" (COI) which was established by Sewall Wright in 1922. The COI is based on the calculation of combined probabilities and expresses the expected percentage of identical genes an individual inherits from one or more common ancestors of its parents (Falconer, 1984).

It has to be kept in mind that the COI only represents a probability value that varies with the number of generations considered when calculating it. Moreover the COI is the same for full sibs although they can strongly differ in their individual degree of homozygosity.

2. What are the Consequences of Inbreeding

The basic consequence is the increase of the percentage of homozygote genloci. The practical consequences depend on the quality and the function of those genes that become homozygote. If these are genes that are associated with desirable traits inbreeding leads to improvement respectively fixation of those traits in the population. If these are defect genes that cause hereditary diseases inbreeding leads to increased prevalence respectively increased severity of those diseases. Furthermore the increase of homozygosity has some unspecific consequences. As the increase of homozygosity is always associated with an increase in the gene frequencies of those genes that become homozygote more often, simultaneously the gene frequencies of the allelic genes decrease and sometimes those allelic genes disappear from the population. This change in gene frequencies results in a loss of genetic diversity on the population level as well as on the individual's level. It usually is associated with an unspecific decrease of fitness also known as "Inbreeding depression". Fitness in the context of population genetics is defined as the proportion the offspring of a single animal contributes to the following generation. Fitness therefore includes attributes of disease resistance and vitality as well as attributes of reproductive performance. The symptoms of inbreeding depression mainly are caused by an impaired adaptability against harmful environmental influences. Especially attributes with low heritability and therefore high reactivity against environment are prone to be influenced by homozygosity.

There exist an almost innumerable number of papers that deal with associations between inbreeding and fitness in different species. Although the actual influence of a defined inbreeding level differs from population to population due to the individual genetic burden of the population, the individual selection pressure and the individual quality of the environment the overall accepted position is that an increasing level of homozygosity leads to decrease in fitness (Kristensen and Sorensen, 2005).

3. What is an Increase of the Inbreeding Level Caused By

There are four main factors that unavoidably lead to an increase in the level of inbreeding and therefore to an increase in homozygosity:

 Genetic drift in small and genetically isolated populations.

 Intentional inbreeding strategies by breeders that focus on fast improvement of desirable traits.

 Intensive selection in favour of desirable traits or against undesirable traits.

 Overuse of special sires (popular sires).

4. What has Inbreeding Depression to do with Insufficient Reproduction

Failure in reproduction principally can be due to problems in three different biologic units: The father, the mother and the offspring.

Regarding the above-cited effects of inbreeding there can be different reasons of reproductive problems when looking at those three biologic units.

 Looking at the father: Reduced vitality and motility of the sperms and reduced resistance of the male against infections of the reproductive tract can be effects of unspecific inbreeding depression. Homozygosity of defect genes can cause defects of sexual organs.

 Looking at the mother: Also in females reduced resistance against infections can cause reproductive failure. But also hormonal imbalances due to unspecific inbreeding depression or defects in genitals may have an impact on fertility.

 Looking at the offspring: Increased mortality in all phases of the early life from fertilisation to weaning can be due to homozygosity of defect genes as well as due to reduced resistance against environmental influences.

5. What about Inbreeding and Fertility in Dogs

Although a lot of scientific evidence exists concerning the association between inbreeding and fertility in different species there is only little scientific evidence available on that subject in dogs.

 In 1982 Wildt et al. compared an inbred group of Foxhounds with a not inbred group and found lower conception rate, smaller litter size as well as smaller number of sperms in the inbred group. Motility of sperms and ejaculate volume as well as volume of the testes also was different between the two groups although failing the significance limit.

 In 1987 Schmidt et al. investigated in the association between COI and litter size in four dog breeds. Only in one of those breeds, the Short Haired Dachshund, the COI of the mother was associated with the number of raised puppies. In the other investigated breeds no such association could be proved. But it seems remarkable that the COI of the latter breeds was significantly lower than that of the Short Haired Dachshund.

 Dahlbom et al. (1997) investigated in fertility parameters in male Irish Wolfhounds. Although the Irish Wolfs frequently exhibited low libido, small and soft testicles and poor semen quality compared with control dogs of other breeds, no association of that traits with the COI could be established. Remarkably the overall COI of the Irish Wolfhounds investigated was quite low.

 In a study in Boxers (Beek et al., 1999) the authors found an influence of inbreeding level of the litter on puppy mortality especially due to infections.

 A study in a recently created new breed, the "Elo" (Kaufhold et al., 2005) proved an influence of the COI of the mother and the COI of the puppies on the mortality of the puppies up to the time of weaning.

 In a study in Dachshunds (Gresky et al., 2005) the litter size and percentage of stillborn puppies was influenced by the COI of the mother, of the father and of the litter.

Despite that small amount of dog specific scientific evidence on the association between fertility and inbreeding the findings are quite consistent at all. And due to the fact that the principal effects of homozygosity are not species-specific (Kristensen and Sorensen, 2005) one can suppose that a high inbreeding level means harm concerning fertility in dogs. Therefore an important measure to prevent fertility problems in dogs is the control of the inbreeding level.

6. How to Control the Inbreeding Level of a Population

The suitable methods of limiting the homozygosity level in practice are strongly associated with the causative factors mentioned above.

 Increasing the effective population size: From the population genetics point of view most dog breeds seem to be strongly endangered populations. Although it was claimed that a minimum size of effective population of 50 is enough to avoid inbreeding depression in the short term (Kristensen and Sorensen, 2005) even this postulation seems unrealistic in many of the dog breeds. Especially when looking at European breeding populations that have experienced a strong genetic bottleneck during the first half of the 19th century the actual genetic variance depends on the number of animals that have passed that bottleneck rather than on the actual number of breeding animals.

Increase of population size can be managed in different ways:

 As the size of the effective breeding population not only depends on the total number of dogs but on the relative number of males and females as is indicated in Table 1 the simplest way to increase the size of the effective breeding population is to use a greater number of males and to avoid the overuse of single males. A further advantage of that strategy is that the wide distribution of defect genes a single sire could be burdened with can be avoided.

 Immigration of breeding animals, usually of sires from other populations of the same breed or even from other breeds. Although in dog breeding the use of dogs of other breeds usually is out of question in some cases it may be the last hope for the survival of a population. Anyway one has to consider that each form of immigration bares the risk of immigrating undesirable genes as well.

 Considering genetic diversity of a breeding population also when planning selection strategies. This can be done by weighting selection criteria due to their impact on the breeding goal respectively on the health status. The classic method of calculating a selection index is the best suitable way.

 Considering the genetic relation between mating partners.

 Calculating the COI of the prospective offspring when planning a special mating. This can be done manually or by using one of the available software. When using software one have to keep in mind that the actual value of the COI depends on the number of generations included and therefore can show quite different sizes in the same animal when calculated in different ways.

Although some geneticists claim defined limits of COI that should not be exceeded, in my opinion this makes little sense at all as the consequences of inbreeding rather depend on the actual genetic situation of the respective population as on the value of the COI. The strategy therefore should focus on the choice of that matings that result in a preferably low COI but also regarding other desirable and undesirable aspects of the planned mating.

Actual research in molecular genetics possibly will result in methods to plan matings by heterozygosity of molecular markers.

Table 1. Effective population size (Ne) depending on the actual number of males(m) and females(f) calculated by Ne=4*m*f/(m+f)(Falconer, 1984).

References

1.  Beek, S. van den, Nielen A.L.J., Schukken, Y.H., Brascamp, E.W. (1999): Evaluation of genetic, common-litter and within-litter effects of preweaning mortality in a birth cohort of puppies. AJVR 60(9), 1106-1110.

2.  Dahlbohm, M., Andersson, M., Juga, J., Alanko, M. (1997): Fertility parameters in male Irish wolfhounds: a two-year follow-up study. Journal of Small Animal Practice 38, 547-550

3.  Falconer, D.S. (1984): Einführung in die quantitative Genetik. Eugen Ulmer, Stuttgart.

4.  Gresky, C., Hamann, H., Distl, O. (2005): Einfluss von Inzucht auf die Wurfgröße und den Anteil tot geborener Welpen beim Dackel. Berl. Münch. Tierärztl. Wschr. 118, 3/4, 134-139

5.  Kaufhold, J., Hamann, H., Distl, O. (2005): Populationsgenetische Aspekte der neu gezüchteten Hunderasse Elo. Berl. Münch. Tierärztl. Wschr. 118, 1/2, 67-75

6.  Kristensen, T.K., Sorensen, A.C. (2005): Inbreeding--lessons from animal breeding, evolutionary biology and conservation genetics. Animal Science 80, 121-133

7.  Schmidt, A., Müller, S., Stur, I.(1987): Untersuchung über den Zusammenhang zwischen Inzuchtgrad und Wurfgröße bei verschiedenen Hunderassen. Zeitschrift für wissenschaftliche Kynologie 26

8.  Wildt, D.E., Baas, P.K., Chakraborty, P.K., Wolfle, T.L., Stewart, A.P. (1982): Influence of inbreeding on reproductive performance, ejaculate quality and testicular volume in the dog. Theriogenology 17(4), 445-452