Bits & Pieces

B its and P ieces

Thanks to Michael Bowling for locating this obituary of Carl Raswan. When Amanda was invited to talk to the ladies of our village about Tuwaisaan’s trip to Windsor it was suggested that they come and meet him. A tea party was arranged in the yard at Pearl Island Arabians and the star himself made an appearance. ~ Jenny Lees, about Tuwaisaan Thaathaa’s trip to the Windsor Horse Show to salute Queen Elizabeth in July, 2022. Tuwaisaan was a gift to the Queen from the ruler of Bahrain, and is at home with Jenny.

News on Inbreeding and Inbreeding Depression

There’s an interesting new study on inbreeding in Thoroughbreds, ‘Inbreeding depression and the probability of racing in the Thoroughbred horse’ by Emmeline W. Hill et al. in ProcB, that came out in June this year. While it found that more inbred horses performed worse as athletes, it also made a fascinating discovery (which is common sense, when one thinks about it).

Basically, runs of homozygosity (abbreviated ROH) varied in length according to how close the inbreeding was in the pedigree. “Generally, shorter ROH reflect distant inbreeding resulting from a common ancestor many generations back in the pedigree, whereas long ROH reflect a more recent common ancestor.”

In the Thoroughbreds sampled, the authors found “[l]ong ROH had a strong negative effect on the probability of racing, while short ROH had no effect, indicating that recent inbreeding rather than historic inbreeding is the cause of inbreeding depression for this trait in the population.”

From the discussion of the results (the authors use FROH as the abbreviation for the effects of inbreeding/inbreeding co-efficient):

“Indeed, here we show that inbreeding in the distant pedigree, measured as FROH_short, is not disadvantageous to the breeding goal. This observation is in agreement with an analysis of pedigree-based inbreeding in the Australian Thoroughbred population that suggested that the ancestral history coefficient of inbreeding, the number of times an allele has been identical by descent in an individual’s pedigree, has a positive association with racing performance and probably captures the effects of positive selection for favourable exercise-relevant traits over many generations. However, more recently shared common ancestors, indicated by FROH_long, have a considerable negative impact on the viability of a horse for racing and contribute to wastage in the population. Although not quantified here, it is likely that these long ROH contain a higher proportion of rare, deleterious alleles, which cumulatively cause the inbreeding depression observed.”

Which I find fascinating, because it would mean that as long as the ancestor(s) targeted by inbreeding are pushed back into the upper reaches of the pedigree, then the harmful effects of inbreeding are more likely to be limited — at least as far as Thoroughbred racing performance goes. Unfortunately, I can’t find any mention in the study of what constitutes “many generations back” (six generations? more?) and what amounts to “a more recent common ancestor”.

And while there are problems with extrapolating from a study about racing in Thoroughbreds to the health of the Arabian population, I do think it shows that judicious inbreeding is not inherently bad. So even working with a limited gene pool, western Arabian breeders have wiggle room. ~ Kate McLachlan