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

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. 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.
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. 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. 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.