INBRED strains of mammals are among the most powerful biolog- ical models used in pharmacology, toxicology, physiology, immunology (Tumbleson and Schook 1996), xenotransplantation (Bach 1993) and, more recently, also in genome mapping of quanti- tative trait loci (Rabattu et al 1996, Georges 1997). However, they are difficult to establish mainly because loss of genetic variation is accompanied by inbreeding depression often leading to high mor- tality (Charlesworth and Charlesworth 1987, Thornhill 1993). Furthermore, to achieve genetic homogeneity, at least twenty gen- erations of full sib matings are required (Davisson 1996). It has therefore been thought impractical to inbreed larger domestic mam- mals. Yet, a herd of Large White pigs has been inbred at the Babraham Institute since the early seventies starting with a pair of half-sibs selected from a line homozygous at loci encoding serolog- ically (class I) and lymphocyte defined (class II) swine lymphocyte antigens (SLA), within the porcine MHC on chromosome 7. The pres- ent breeding stock (three boars and 11 sows) has a mean inbreeding coefficient of 0.699 (range 0.637 to 0.752) and constitutes between the 8th and 11th generation on the paternal side and between the 9th and 12th generation on the maternal side. Compared to commer- cially bred Large White, the inbred Large Whites grow slower (85 kg weight at age 20 weeks vs. 30 to 40 weeks, respectively), produce smaller litters (around 7 at 3 weeks age of the piglets) and appear more prone to abnormalities in the legs suggesting some inbreeding depression. However, the herd remains viable with few congenital problems, no increase in disease susceptibility and does not need to be kept in specific pathogen-free facilities. To minimize inbreeding depression only breeding animals with best overall con- formation, leg condition and litter size are considered. Furthermore, matings between relatives other than full sibs are preferred. The present stock derives from 61 matings consisting of 28 half brother x half sister matings, 21 matings between individuals two to four generations apart, eight brother x sister matings and four father x daughter matings. Although used for immunological studies (Pabst and Binns 1989), nothing was known about the extent of genetic variation in this herd. Therefore, representatives have been sampled and, together with controls, subjected to DNA analysis at multiple highly polymorphic minisatellite loci. Initially discovered in humans, minisatellites are abundant in the genome of most vertebrate and plant species (reviewed in Pena et al 1993), consist of tandemly repeated nucleotide motifs and poten- tially exhibit extensive variation in motif copy number and internal sequence (Higgs et al 1981, Jeffreys et al 1985a, Jeffreys et al 1990) giving rise to multiple alleles. They are the most powerful tools known to date for individual identification, parentage testing (Jeffreys 1987), linkage analysis and genome mapping (Nakamura et al 1987). By hybridizing cloned minisatellite probes with restriction digested genomic DNA on Southern blots, highly informative multi- locus ‘DNA fingerprints’ (Jeffreys et al 1985b) or locus-specific pro- files are produced. Recently, such probes have also become available for pigs (Signer et al 1996) three of which were used in this study. Blood samples were collected from 21 inbred and 15 control pigs which included 10 unrelated outbred Large Whites [nine from the European PiGMaP families (Archibald et al 1995) and one from Dalgety plc, Cambridge] and five Large Blacks, a rare breed as examples of ‘semi-inbred’ pigs. The minisatellites probes were: S0322 which hybridizes to at least 10 unlinked polymorphic loci, and single-locus probes S0324 (located in a pseudoautosomal region of the X and Y chromosomes) and S0326 (on chromosome 16). None of them is known to be linked to any MHC loci. Lymphocyte DNA from blood was extracted by proteinase K incu- bation, phenol/chloroform extraction and ethanol precipitation. Two micrograms per individual were digested with restriction endonu- clease AluI (BRL) and electrophoresis performed in 0.8 per cent agarose for 44 hrs at 60 V in 1x TBE (89 mM Tris borate, pH 8.2, 2.5 mM EDTA). After Southern blotting and UV crosslinking, the DNA fragments on the blot were hybridized in 0.5 M sodium phosphate, pH 7.2, 7 per cent SDS, 1 mM EDTA at 65°C overnight with 32 P- labeled probe (Feinberg and Vogelstein 1983). Post-hybridization washes were done in 2x SSC, 0.1 per cent SDS at 65°C (all probes) and in 1x SSC, 0.1 per cent SDS at 65°C (for S0324 and S0326 only), followed by autoradiography in the presence of an intensifying screen at –70°C for at least 2 days. The resulting DNA profiles were scored by eye and pairwise compared. Genetic distance (D) between the animals’ S0322 DNA fingerprints was calculated as D = 1 – [2n xy / (n x + n y )], whereby n xy = number of shared bands between individuals x and y, n x = number of bands scored in x, n y = number of bands scored in y. Mean levels of heterozygosity were esti- mated from DNA fingerprint data as described (Stephens et al 1992). DNA profiling reveals remarkably low genetic variability in a herd of SLA homozygous pigs E. N. SIGNER*, A. J. JEFFREYS*, S. LICENCE†, R. MILLER†, P. BYRD†, R. BINNS† *Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, †Department of Immunology, The Babraham Institute, Cambridge CB2 4AT SUMMARY Inbred strains of rodents have become indispensable for a wide range of biological studies. It has generally been accepted that genetic uniformity is unlikely to be achieved before 20 generations of brother x sister matings discouraging attempts to inbreed larger mammals. Nevertheless, pigs, homozygous for the swine MHC haplotype SLA b/b, have been inbred at the Babraham Institute for almost thirty years and used for immunological studies. Since the herd had not been studied at the DNA level, DNA profiling at multiple hypervariable loci was performed and surprisingly little genetic polymorphism and extremely high inter-individual resemblance were observed reminiscent of that observed in inbred strains of mice. © 1999 Harcourt Publishers Limited 0034-5288/99/050205 + 05 $18.00/0 © 1999 Harcourt Publishers Limited Corresponding author: E.N. Signer SHORT COMMUNICATION Research in Veterinary Science 1999, 67, 205–209 Article No. rvsc.1999.0310, available online at http://idealibrary.com on