Population analysis of the Staffordshire Bull Terrier breed by the English Kennelclub September 2015
Genetic analysis of the Kennel Club pedigree records of the UK Staffordshire Bull Terrier population has been carried out with the aim of estimating the rate of loss of genetic diversity within the breed and providing information to guide a future sustainable breeding strategy. The population statistics summarised provide a picture of trends in census size, the number of animals used for breeding, the rate of inbreeding and the estimated effective population size. The rate of inbreeding and estimated effective population size indicate the rate at which genetic diversity is being lost within the breed. The analysis also calculates the average relationship (kinship) among all individuals of the breed born per year and is used to determine the level of inbreeding that might be expected if matings were made among randomly selected dogs from the population (the expected rate of inbreeding).
Summary of results
The analysis utilises the complete computerised pedigree records for the current UK Kennel Club registered Staffordshire Bull Terrier population, and statistics were calculated for the period 1980-
2014.
Figure 1: a plot of number of registrations by year of birth, indicative of any changing trend in popularity of the breed, followed by the yearly trend in number of animals registered (and 95%
confidence interval).
Breed: Staffordshire Bull Terrier
Figure 1: Number of registrations by year of birth

Trend of registrations over year of birth (1980-2014) = 157.19 per year (with a 95% confidence interval of 75.40 to 238.97).
Table 1: census statistics by year, including sire use statistics.
Table 1: by year (1980-2014), the number of registered puppies born, by the number of unique dams and sires; maximum, median, mode, mean and standard deviation of number of puppies per sire; and the percentage of all puppies born to the most prolific 50%, 25%, 10% and 5% of sires.
year #born #dams #sires puppies per sire %puppies sired by most prolific sires
max median mode mean sd 50% sires 25% sires 10% sires 5% sires
1980 | 984 | 603 | 364 | 26 | 2 | 1 | 2.7 | 2.8 | 79.88 | 58.23 | 34.55 | 21.75 |
1981 | 3222 | 1057 | 572 | 70 | 3 | 1 | 5.63 | 7.72 | 83.4 | 62.63 | 41.46 | 29.27 |
1982 | 4287 | 1266 | 675 | 70 | 4 | 4 | 6.35 | 8.16 | 82.04 | 61.91 | 40.19 | 27.6 |
1983 | 5136 | 1477 | 819 | 91 | 4 | 3 | 6.27 | 8.3 | 81.83 | 62.11 | 40.38 | 27.82 |
1984 | 5761 | 1738 | 980 | 71 | 4 | 3 | 5.88 | 7.68 | 82.02 | 61.57 | 40.15 | 27.77 |
1985 | 6558 | 2056 | 1200 | 74 | 4 | 2 | 5.46 | 6.91 | 80.97 | 59.99 | 38.93 | 26.62 |
1986 | 6458 | 2072 | 1282 | 111 | 3 | 2 | 5.04 | 6.55 | 80.85 | 59.68 | 37.97 | 26.32 |
1987 | 6196 | 1974 | 1238 | 101 | 3 | 2 | 5 | 6.23 | 80.7 | 59.12 | 37.36 | 25.52 |
1988 | 6248 | 1941 | 1229 | 96 | 3 | 3 | 5.08 | 6.96 | 80.52 | 59.22 | 39.34 | 27.94 |
1989 | 7767 | 1754 | 1153 | 65 | 5 | 5 | 6.74 | 6.85 | 76.12 | 53.11 | 32.77 | 22.36 |
1990 | 6590 | 1452 | 996 | 63 | 5 | 5 | 6.62 | 6.55 | 75.78 | 53.2 | 32.85 | 21.88 |
1991 | 5850 | 1262 | 890 | 71 | 5 | 4 | 6.57 | 6.29 | 75.69 | 52.75 | 31.79 | 21.06 |
1992 | 5184 | 1111 | 777 | 65 | 5 | 5 | 6.67 | 5.92 | 74.71 | 51.45 | 31.02 | 20.14 |
1993 | 5629 | 1183 | 802 | 69 | 5 | 5 | 7.02 | 6.88 | 75.93 | 53.26 | 32.65 | 21.67 |
1994 | 6159 | 1320 | 854 | 58 | 5 | 5 | 7.21 | 6.83 | 76.81 | 54.18 | 32.57 | 20.96 |
1995 | 7198 | 1474 | 951 | 78 | 6 | 5 | 7.57 | 7.71 | 76.69 | 54.47 | 33.7 | 23.16 |
1996 | 8461 | 1693 | 1036 | 111 | 6 | 5 | 8.17 | 9 | 76.91 | 55.79 | 34.77 | 23.37 |
1997 | 9114 | 1800 | 1127 | 92 | 6 | 6 | 8.09 | 8.74 | 76.7 | 55.09 | 34.74 | 23.66 |
1998 | 9677 | 1884 | 1192 | 104 | 6 | 5 | 8.12 | 8.16 | 76.39 | 54.69 | 33.55 | 22 |
1999 | 9957 | 1935 | 1246 | 75 | 6 | 6 | 7.99 | 8.42 | 77.14 | 55.81 | 35.24 | 23.51 |
2000 | 10068 | 1985 | 1248 | 122 | 6 | 5 | 8.07 | 8.78 | 76.31 | 54.65 | 34.17 | 22.95 |
2001 | 10292 | 2007 | 1307 | 92 | 6 | 6 | 7.87 | 7.6 | 75.1 | 52.65 | 31.9 | 21.09 |
2002 | 10708 | 2070 | 1319 | 193 | 6 | 6 | 8.12 | 9.65 | 76.37 | 54.88 | 34.41 | 23.52 |
2003 | 11215 | 2163 | 1323 | 153 | 6 | 5 | 8.48 | 9.68 | 76.94 | 55.41 | 34.94 | 23.87 |
2004 | 12399 | 2302 | 1405 | 175 | 6 | 6 | 8.82 | 10.48 | 76.41 | 55.18 | 34.72 | 24.01 |
2005 | 13074 | 2445 | 1493 | 123 | 6 | 5 | 8.76 | 10.06 | 77.4 | 56.4 | 36.01 | 24.59 |
2006 | 12699 | 2348 | 1484 | 130 | 6 | 6 | 8.56 | 10.14 | 76.45 | 54.97 | 35 | 24.32 |
2007 | 12181 | 2239 | 1362 | 161 | 6 | 6 | 8.94 | 11.66 | 78.17 | 57.68 | 37.09 | 25.91 |
2008 | 10484 | 1955 | 1244 | 136 | 6 | 5 | 8.43 | 9.8 | 76.86 | 55.22 | 35.05 | 24.12 |
2009 | 8733 | 1625 | 1027 | 180 | 6 | 6 | 8.5 | 11.38 | 77.37 | 56.48 | 36.56 | 25.92 |
2010 | 8366 | 1578 | 969 | 147 | 6 | 6 | 8.63 | 12.52 | 78.54 | 58.28 | 38.57 | 27.87 |
2011 | 7093 | 1385 | 839 | 139 | 6 | 5 | 8.45 | 11.94 | 80.69 | 60.03 | 39.74 | 28.65 |
2012 | 6137 | 1194 | 742 | 127 | 6 | 6 | 8.27 | 11.24 | 80.22 | 59.39 | 40.08 | 28.97 |
2013 | 5840 | 1123 | 676 | 232 | 6 | 6 | 8.64 | 13.56 | 81.03 | 60.86 | 41.51 | 29.9 |
2014 | 4626 | 863 | 540 | 128 | 6 | 6 | 8.57 | 12.92 | 78.6 | 58.82 | 41.07 | 30.42 |
Generation interval: the mean average age (in years) of parents at the birth of offspring which themselves go on to reproduce.
Mean generation interval (years) = 3.30
Figure 2: a plot of the annual mean observed inbreeding coefficient (showing loss of genetic diversity), and mean expected inbreeding coefficient (from ‘random mating’) over the period
1980-2014. ‘Expected inbreeding’ is staggered by the generation interval and, where >2000
animals are born in a single year, the 95% confidence interval is indicated.
Figure 2: Annual mean observed and expected inbreeding coefficients

Estimated effective population size: the rate of inbreeding (slope or steepness of the observed inbreeding in Figure 2) is used to estimate the effective population size of the breed. The effective population size is the number of breeding animals in an idealised, hypothetical population that would be expected to show the same rate of loss of genetic diversity (rate of inbreeding) as the breed in question. It may be thought of as the size of the ‘gene pool’ of the breed.
Below an effective population size of 100 (inbreeding rate of 0.50% per generation) the rate of loss of genetic diversity in a breed/population increases dramatically (Food & Agriculture Organisation of the United Nations, “Monitoring animal genetic resources and criteria for prioritization of breeds”, 1992). An effective population size of below 50 (inbreeding rate of 1.0% per generation) indicates the future of the breed many be considered to be at risk (Food & Agriculture Organisation of the United Nations, “Breeding strategies for sustainable management of animal genetic resources”, 2010).
Where the rate of inbreeding is negative (implying increasing genetic diversity in the breed),effective population size is denoted ‘n/a’.
Estimated effective population size = 97.7
NB - this estimate is made using the rate of inbreeding over the whole period 1980-2014
Table 2: a breakdown of census statistics, sire and dam usage and indicators of the rate of loss of genetic diversity over 5 year periods (1980-4, 1985-9, 1990-4, 1995-9, 2000-4, 2005-9, 2010-14). Rate of inbreeding and estimated effective population size for each 5-year block can be
compared with the trend in observed inbreeding in Figure 2.
Table 2: by 5-year blocks, the mean number of registrations; for sires the total number used, maximum, mean, median, mode, standard deviation and skewness (indicative of the size of the ‘tail’ on the distribution) of number of progeny per sire; for dams the total number used, maximum, mean, median, mode, standard deviation and skewness of number of progeny per dam; rate of inbreeding per generation (as a decimal, multiply by 100 to obtain as a percentage); mean generation interval; and estimated effective population size.
years | 1980-1984 | 1985-1989 | 1990-1994 | 1995-1999 | 2000-2004 | 2005-2009 | 2010-2014 |
mean #registrations | 3878 | 6645.4 | 5882.4 | 8881.4 | 10936 | 11434 | 6412.4 |
Total #sires Max #progeny Mean #progeny Median #progeny Mode #progeny SD #progeny Skew #progeny | 2175 264 8.9085 4 1 18.244 6.755 | 3984 362 8.3396 4 2 16.135 8.8088 | 2951 238 9.9665 6 5 15.195 6.4092 | 3637 330 12.209 7 6 20.402 6.6906 | 4203 545 13.01 7 6 22.166 8.5941 | 4371 451 13.079 7 6 23.844 8.3024 | 2517 504 12.732 6 6 28.289 8.6592 |
Total #dams Max #progeny Mean #progeny Median #progeny Mode #progeny SD #progeny Skew #progeny | 4510 35 4.2962 3 1 3.3747 1.9797 | 7425 27 4.4749 4 3 3.2516 1.8405 | 4741 36 6.2035 5 5 4.4117 1.9372 | 6295 44 7.0542 6 5 4.9343 1.8274 | 7495 54 7.2957 6 6 5.0254 1.8501 | 7803 50 7.3267 6 5 5.2133 1.9948 | 4657 39 6.8815 6 5 4.8885 1.6709 |
Rate of inbreeding Generation interval Effective pop size | 0.016291 3.1446 30.692 | 0.006316 3.1284 79.163 | 0.009828 3.53 50.878 | 0.002389 3.4139 209.31 | 0.000744 3.4061 672.3 | 0.000473 3.2335 1057.1 | 0.015633 3.2589 31.985 |
Figure 3: a histogram (‘tally’ distribution) of number of progeny per sire and dam over each of the seven 5-year blocks above. A longer ‘tail’ on the distribution of progeny per sire is indicative of
‘popular sires’ (few sires with a very large number of offspring, known to be a major contributor to a high rate of inbreeding).
Figure 3: Distribution of progeny per sire (blue) and per dam (red) over 5-year blocks (1980-4 top,
2010-14 bottom). Vertical axis is a logarithmic scale.

Comments
The rate of inbreeding in this breed has remained relatively steady over the whole period. However, this appears to be close to the level thought to be sustainable.
It appears that the extensive use of popular dogs as sires has increased (the ‘tail’ of the blue distribution increasing in figure 3).
It should be noted that, while animals imported from overseas may appear completely unrelated, this is not always the case. Often the pedigree available to the Kennel Club is limited in the number of generations, hampering the ability to detect true, albeit distant, relationships.