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                                  

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.

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.