Population analysis of the Rhodesian Ridgeback breed Genetic analysis of the Kennel Club pedigree records of the UK Rhodesian Ridgeback 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 Rhodesian Ridgeback population, and statistics were calculated for the period 1980-2014. 1
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: Rhodesian Ridgeback Figure 1: Number of registrations by year of birth Trend of registrations over year of birth (1980-2014) = 30.59 per year (with a 95% confidence interval of 22.91 to 38.28). 2
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 109 48 28 13 2.5 1 3.89 3.38 81.65 56.88 32.11 11.93 1981 366 76 46 46 6 1 7.96 10.12 84.43 63.66 43.44 24.86 1982 375 82 41 46 5 3 9.15 11.31 83.2 64.27 43.47 24.53 1983 485 92 54 35 6 1 8.98 8.45 84.54 61.03 28.66 18.76 1984 582 121 60 119 5.5 1 9.7 16.37 85.91 68.04 44.33 32.13 1985 602 125 69 58 6 1 8.72 10.26 82.89 59.14 39.7 23.42 1986 635 133 72 83 6 7 8.82 11.25 81.1 60.63 37.8 27.09 1987 683 143 87 44 5 4 7.85 8.54 80.97 60.03 38.8 22.11 1988 687 138 99 37 6 4 6.94 5.39 76.13 49.2 27.07 17.03 1989 768 117 87 52 7 7 8.83 8.38 75.91 52.21 32.94 21.09 1990 693 112 78 67 8 1 8.88 8.45 76.19 50.79 29.29 19.19 1991 729 101 68 78 8 8 10.72 10.44 76.68 52.54 30.04 18.79 1992 614 88 60 51 9 10 10.23 8.75 74.76 50.81 30.29 19.54 1993 675 91 60 71 8 7 11.25 12.25 80.74 60.3 36.89 22.37 1994 847 119 74 39 8 8 11.45 8.23 74.26 51.48 26.09 16.65 1995 844 121 68 54 9 7 12.41 10.55 78.79 54.27 30.69 16.47 1996 934 135 86 42 9 1 10.86 8.83 79.55 54.82 29.01 14.99 1997 1101 147 100 48 9 10 11.01 8.84 76.02 51.95 29.06 17.8 1998 991 127 87 43 9 9 11.39 8.71 75.58 52.47 28.25 15.44 1999 1068 156 105 103 8 1 10.17 11.67 80.62 56.93 32.87 20.6 2000 1352 184 116 55 9 1 11.66 10.25 78.99 55.55 31.36 18.71 2001 1011 138 90 54 9 1 11.23 11.23 79.33 57.67 35.01 24.23 2002 1223 164 107 110 9 1 11.43 12.92 79.89 55.76 33.52 22.49 2003 1249 174 108 83 8.5 7 11.56 11.57 79.34 55.64 32.43 20.34 2004 1551 219 122 95 9 1 12.71 13.01 80.46 59.06 32.88 20.57 2005 1538 207 130 110 8 1 11.83 14.19 80.69 59.75 37.19 26.14 2006 1592 221 131 63 9 1 12.15 12.67 83.04 60.18 34.86 23.3 2007 1683 233 140 72 8.5 1 12.02 13.1 82.77 60.49 36.96 23.77 2008 1506 202 119 112 9 8 12.66 14.38 79.75 58.57 36.72 24.57 2009 1178 174 107 114 9 2 11.01 13.42 81.75 58.23 37.1 23.34 2010 1120 164 103 94 8 1 10.87 12.02 80.09 56.25 33.66 22.77 2011 1301 180 117 58 8 1 11.12 10.92 83.17 58.88 34.05 20.37 2012 1027 144 95 71 9 1 10.81 10.46 79.26 55.31 33.89 21.13 2013 1106 142 92 97 9 1 12.02 13.46 78.84 56.69 35.08 24.77 2014 865 111 60 77 11 8 14.42 13.36 76.76 53.64 33.18 20.58 3
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) = 3.99 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 4
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 = n/a NB - this estimate is made using the rate of inbreeding over the whole period 1980-2014 5
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 383.4 675 711.6 987.6 1277.2 1499.4 1083.8 Total #sires 141 247 213 277 331 389 297 Max #progeny 146 130 145 126 252 466 210 Mean #progeny 13.582 13.66 16.7 17.823 19.287 19.27 18.239 Median #progeny 6 8 9 9 9 10 10 Mode #progeny 1 1 1 1 1 1 1 SD #progeny 23.123 18.761 21.679 22.125 29.648 36.219 27.807 Skew #progeny 3.6363 3.268 2.8338 2.3861 4.184 6.8893 4.0163 Total #dams 296 494 381 491 651 779 570 Max #progeny 35 32 50 44 35 45 42 Mean #progeny 6.4696 6.83 9.336 10.055 9.808 9.6226 9.5035 Median #progeny 5 6 8 9 9 9 9 Mode #progeny 1 4 7 9 1 1 1 SD #progeny 5.0969 5.083 6.9147 6.7027 6.6436 6.7644 6.8345 Skew #progeny 1.8966 1.6216 1.9968 1.1854 1.068 1.2491 1.2663 Rate of inbreeding 0.025815 0.015524 0.01468-0.01162-0.0062 0.0089-0.01179 Generation interval 3.5201 3.6159 4.0413 4.0742 4.1514 4.0761 4.5401 Effective pop size 19.369 32.207 34.061 n/a n/a 56.179 n/a 6
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. 7
Comments There appears to have been significant fluctuations in the rate of inbreeding between 1980-2014. While this has resulted in no overall apparent loss of genetic diversity, the breed will have seen diversity contract and then be replenished (possibly through the use of imported animals). 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. 8