Cattle have far more complex blood groups than humans and different breeds typically have different combinations of blood groups. Even within a specific herd, there is always a high degree of variation. Chillingham cattle don’t have any blood groups that are unknown elsewhere, and neither do they have obvious affinities with any other breeds. However, all the individuals that have been typed have exactly the same combination and this uniformity is unique in cattle.
Is this uniformity a result of natural selection favouring a particular combination of blood groups or is it just the effect of long-term inbreeding? As we don’t know what blood groups were present in the herd when it was founded we cannot tell for sure. But in the 1990s new work with inherited pieces of DNA which don’t actually perform any function (microsatellites) showed that these also were remarkably uniform in the herd – far more so than in any other wild mammal population. The microsatellites studied are pretty evenly spread across the chromosomes of cattle and this suggests that the entire genetic endowment of every animal is effectively the same.
These results are as would be expected from a highly inbred herd. And the key genetic question is – how can an inbred herd have survived and continue to thrive? Following from that, what should we do about it? We know from dozens of experiments and observations in farms and zoos, not to mention taboos in human society against marriage of close relatives, that inbreeding is usually detrimental. This is because normally if a harmful gene is inherited from one parent, it is likely to be masked by a normal gene inherited from the other. But under inbreeding the chance of receiving a harmful gene from both parents is higher (that’s because if the parents are related, they are likely to be genetically similar). In most lab experiments of prolonged inbreeding, populations die out because of this accumulation of harmful genes. But in a very few, maybe 5% of cases and especially if inbreeding is very gradual, by sheer chance harmful genes can be purged from the population because in any one generation of young only a few individuals are affected. It looks as though this is what has happened at Chillingham.
Further genetic work, currently under way, is suggesting that all Chillingham cattle are descended from a single cow, and also that there may be a lack of genetic variation relevant to disease resistance. This could mean that the herd would be particularly vulnerable to a novel disease challenge.