Breeds which have few breeding animals can suffer a loss in the amount of genetic variation present within the breed. What this means is that less genetic variation exists between animals in the breed. This can be useful to an extent in that it increases the overall homogeneity of the breed, such that most animals within a breed are likely to look, behave or perform similarly. The down side to this genetic homogeneity is that the breed is less able to adapt to new selection pressures, either environmental or human-mediated. For instance, less genetic variation might mean that the breed is less able to withstand a new disease strain, because the genes which may have provided disease resistance have been lost.
Another possible consequence of small numbers of breeding animals is an increase in the level of inbreeding. In genetic terms, an inbred animal is more likely to have inherited two alleles at any gene (see sidebar: A Primer in Genetics) that are identical. As such, the probability that an animal will have inherited two copies of a 'deleterious' allele is increased. A classic example would be Severe Combined Immunodeficiency (SCID), a recessive disorder of concern in Arabian horses. Affected foals are unable to fight infections and die within a few months after birth. In order to be affected, a foal must inherit two copies of the SCID allele. Since the probability of inheriting any two identical alleles is increased for inbred animals, the frequency of horses born with SCID is higher in an inbred population than in a non-inbred population.
In general, the consequences of excessive inbreeding include a greater occurrence of genetic diseases, a decline in reproductivity and overall hardiness, and at worst, extinction of the breed.
As part of a global project organized by the Food and Agriculture Organisation of the United Nations, we are using DNA markers to identify genetically isolated breeds as a high priority for conservation. Our research project focuses on the Canadian Horse (see sidebar: Le Cheval Canadien) with the intention of providing a realistic framework applicable towards conservation efforts directed at other endangered breeds. An important part of the Canadian historical scene for over 300 years, the number of Canadian Horses has declined to the point of creating concern about the level of genetic variation and inbreeding within the breed.
For our study, we are making use of current molecular genetic techniques to compare the amount of genetic variability within rare and more common breeds in order to establish if rare breeds have suffered from a critical decline in genetic variation. Once we have determined the amount of genetic variation within our target breed, we can make recommendations as to whether or not the stud books should be opened in order to increase the amount of variation within the breed. The next step is to work out the genetic relationships between breeds so that we can establish which breeds are most closely related to our target breed. Should the opening of the stud books be considered necessary, we can then suggest that this be done in a limited fashion, allowing only those breeds which are genetically similar to our target breed to be contributors.
To date, we have analysed DNA from 903 horses from 11 breeds of horses common in Canada, including the Canadian Horse. These data were generated for development of parentage testing systems by Mann Equitest Inc. of Guelph, Ontario. The eleven breeds included in this analysis were (the short form and number of horses analysed for each breed is given in brackets): Appaloosa (APP, 65); Arabian (ARA, 93); American Saddlebred (ASB, 55); Belgian (BEL, 54); Canadian (CAN, 33); Miniature Horse (MIN, 126); Morgan (MOR, 98); Quarter Horse (QHH, 95); Standardbred (STB, 98); Thoroughbred (TBD, 94); and Tennessee Walking Horse (TWH, 92).
We used the DNA data (see sidebar: Microsatellite Markers) to compare both the overall level of genetic variation in different breeds, as well as the level of inbreeding. According to preliminary analysis, the level of genetic variation in the Canadian horse is about midrange for the 11 breeds surveyed, indicating that the Canadian Horse has not suffered a loss in genetic variation due to it's small population size. The level of genetic variation in the 11 breeds is shown as the black bars in the graph in figure 1. The white bars depict the average level of variation of individual horses within the breed (the average occurrence of two identical alleles at any gene). This can be used to give us a measure of the overall level of inbreeding in the breed. The lower the white bar in relation in relation to the black bar, the more highly inbred the breed.
From Figure 1, we can see that the
Canadian horse does not appear to be more highly inbred than more popular breeds. Of note
is the relatively low level of individual variation in Arabian horses (compared to what
one might expect given the amount of within breed variation depicted by the black bar),
suggesting a higher average level of inbreeding within the breed. As the Arabian breed
does not suffer from small numbers, the greater degree of inbreeding may be a result of
line breeding practices. Our second analysis involved the use of the same DNA data to work out the genetic relationships between the 11 breeds, based on the degree of genetic similarity between breeds.
From the figure, we see that breeds
known to be closely related (such as the Thoroughbred and Standardbred) cluster together,
while the Belgian, Miniature and Canadian are more distantly related to the remaining
breeds, which are of race and riding type. This shows us that the Canadian, bred in
isolation for so long, does still appear to be genetically distinct from the popular race
and riding horse breeds, reinforcing the need to consider the conservation of this breed a
high priority.