Evidence of the phenotypic expression of a lethal recessive allele under inbreeding in a wild population of conservation concern

Amanda E. Trask, Eric M. Bignal, Davy I. McCracken, Pat Monaghan, Stuart B. Piertney, Jane M. Reid

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1. Deleterious recessive alleles that are masked in out bred populations are predicted to be expressed in small, inbred populations, reducing both individual fitness and population viability. However, there are few definitive examples of phenotypic expression of lethal recessive alleles under inbreeding conditions in wild populations. Studies that demonstrate the action of such alleles, and infer their distribution and dynamics, are required to understand their potential impact on population viability and inform management responses.2. The Scottish population of red-billed choughs (Pyrrhocorax pyrrhocorax), which currently totals <60 breeding pairs and is of major conservation concern, has recently been affected by lethal blindness in nestlings. We used family data to show that the pattern of occurrence of blindness within and across affected families that produced blind nestlings was exactly 0.25, matching that expected given a single-locus autosomal lethal recessive allele. Furthermore, the observed distribution of blind nestlings within affected families did not differ from that expected given Mendelian inheritance of such an allele.3. Relatedness estimates showed that individuals from affected families were not more closely related to each other than they were to individuals from unaffected families that did not produce blind nestlings. Blind individuals tended to be less heterozygous than non-blind individuals, as expected if blindness was caused by the expression of a recessive allele under inbreeding. However, there was no difference in the variance in heterozygosity estimates, suggesting that some blind individuals were relatively outbred. These results suggest carriers of the blindness allele may be widely distributed across contemporary families rather than restricted to a single family lineage, implying that the allele has persisted across multiple generations.4. Blindness occurred at low frequency (affecting 1.6% of observed nestlings since 1981). However, affected families had larger initial brood sizes than unaffected families. Such high fecundity of carriers of a lethal recessive allele might reflect overdominance, potentially reducing purging and increasing allele persistence probability.5. We thereby demonstrate the phenotypic expression of a lethal recessive allele in a wild population of conservation concern, and provide a general framework for inferring allele distribution and persistence and informing management responses.
Original languageEnglish
Pages (from-to)879-891
Number of pages13
JournalJournal of Animal Ecology
Issue number4
Early online date21 Mar 2016
Publication statusPublished - Jul 2016

Bibliographical note


We thank everyone who helped with fieldwork, particularly Sue Bignal for her ongoing efforts on Islay, and David Jardine, Mike Peacock and Neil Metcalfe for providing information from Colonsay. We thank all landowners and farmers who allowed access to nest sites. We thank Scottish Natural Heritage, and particularly Rae McKenzie, for their ongoing support, and thank the Royal Society for the Protection of Birds for their generous logistical support for AET and JMR. We thank Tom Pennycott and John Mould for undertaking veterinary post-mortems of blind nestlings, and Islay Veterinary Surgery for their help. AET was funded by the Natural Environment Research Council and Scottish Natural Heritage, JMR was funded by the Royal Society, and DMcC was funded by the Scottish Government Rural Affairs & Environment Portfolio Strategic Research Programme 2011–2016, Theme 1: Ecosystem Services & Biodiversity.

Data accessibility

Data on occurrences of blindness in affected families are available in the Supporting Information. All other data associated with this article are available at the Dryad Digital Repository http://dx.doi.org/10.5061/dryad.57t89 (Trask et al. 2016).


  • conservation genetics
  • disease aetiology
  • extinction
  • genetic disorder
  • heterozygote advantage
  • inbreeding depression
  • inheritance
  • mutation
  • Peter's anomaly


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