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Breeding Bull Terriers

Practical Genetics for Bull Terrier Breeders and Owners

Health Seminar presented to the Bull Terrier Club of America, October 10, 2002

Jerold S Bell, DVM, Tufts University School of Veterinary Medicine

As a breed, the AKC registered Bull Terrier population has remained relatively constant over the last fifteen years. Individual dog registrations were at 1,155 in 1985, making the Bull Terrier the 65th most populous breed in registrations. While registrations grew to 1,310 in 1990, the breed rank dropped to 66th most populous, due to the rising popularity of other breeds. By 1995, AKC registrations were 1,096 dogs, and a breed rank of 79th. The breed registrations reached a low of 941 (84th) in 1998, and have returned to 1,072 (80th) in 2001. These numbers represent a stable population.

There are ways to measure the genetic diversity and health of a population. One method is to measure the average inbreeding coefficient for a breed. The inbreeding coefficient is a measurement of the genetic relatedness of the sire and dam of a dog. All individuals inherit pairs of chromosomes, one from the mother, and one from the father. On the chromosomes are genes; so all genes come in pairs. If both genes in a gene pair are the same gene (for instance, “aa” or “AA”) the gene pair is called homozygous. If the two genes in a gene pair are unlike (for instance, “Aa”) the gene pair is called heterozygous. If a dog appears on both the sire and dam’s side of the pedigree, it increases the inbreeding coefficient of the dog. The inbreeding coefficient also provides a measurement of homozygosity.

The non-variable gene pairs in a breed are all homozygous, and allow the dogs to breed true. This is why two Bull Terriers bred together always produce a Bull Terrier, and not a chicken. The variable gene pairs in a breed produce the variation you see between dogs in the breed. The inbreeding coefficient gives a measurement of the total percentage of variable gene pairs that are expected to be homozygous due to inheritance from ancestors common to the sire and dam. It also gives the chance that any single gene pair can be homozygous.

When computing inbreeding coefficients, you have to look at a deep pedigree to get accurate numbers. I prefer to use 10-generation pedigrees, which require a computerized pedigree database to compute. Looking at the historical pedigrees of Bull Terrier breeding dogs (males and females that have five or more registered offspring), we find that for dogs born in the decade 1970-1979, the average ten generation inbreeding coefficient was 23.11% +/- 6.04%. For Bull Terriers born 1980-1989, this number is 21.54% +/- 5.69%. For 1990-1999, the average inbreeding coefficient is 19.01% +/- 6.23. These numbers have not increased over the years, and have slightly decreased. They show that your breed is utilizing the diversity of pedigree background available, and not breeding itself into a corner of the gene pool. This suggests that your breed has acceptable overall breed-wide diversity.

There appears to be a long-standing issue among Bull Terrier breeders about whether American Bull Terriers need a constant infusion of British Bull Terrier blood (imports) to maintain the quality of the breed. In discussing this issue, I have asked whether there is a different breed standard in the UK, or some other factors that would make British dogs more desirable than American bred dogs, and I am told that this is not the case. With the established acceptable breed-wide genetic diversity of American dogs, importing dogs is not a necessary practice. The selection of American or British dogs for breeding should be based on individual characteristics, based on the needs in your breeding program.

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