1. Deleterious recessive alleles that are masked in outbred 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.
- Conservation genetics
- Disease aetiology
- Genetic disorder
- Heterozygote advantage
- Inbreeding depression
- Peters' anomaly