Although heterosis and inbreeding depression are both thought to be caused by recessive or partly recessive deleterious alleles (e.g. Charlesworth and Willis 2009), empirical comparisons of their genetic bases are scant.
These results confirm that the potential beneficial effect of exercise on nitric oxide-mediated pulmonary artery vasorelaxation is partly blunted by deleterious effects of hypoxia on l-arginine bioavailability.
However, the concomitant presence of soluble hsp60 might partly offset this deleterious response due to the specific interaction with ATP synthase.
Heterosis, in contrast, is caused by the heterozygous masking of mildly deleterious, partly recessive alleles that have become fixed within populations (Whitlock et al. 2000).
An increase in progeny fitness following between-population relative to within-population crosses is thought to be caused by the heterozygous masking (in the F1) of mildly deleterious, partly recessive alleles historically fixed by drift in the parental populations.
In support of our model of the proposed non-cytotoxic expression of DUX4 very early in myogenesis, the early-myogenesis transcription factors PAX3 and PAX7 can partly counteract the deleterious effects of overexpression of DUX4 in C2C12 cells [ 27].
If the allele itself has pleiotropic beneficial effects in females, or is tightly linked to a beneficial allele, this may partly counteract the deleterious effects in males, and delay its purging.
Although the genetic basis of inbreeding depression is still under discussion, it is currently accepted to be mainly due to increased homozygosity for (partly) recessive, mildly deleterious alleles (Charlesworth and Charlesworth 1987; Charlesworth and Willis 2009).
Inbreeding depression is due to segregating variation within a population for both highly recessive lethal and semi-lethal alleles that segregate at low frequency, as well as more mildly deleterious, partly recessive alleles (Simmons and Crow 1977; Charlesworth and Willis 2009).
This paper provides a foundation for future investigations using stronger prospective designs to examine if medication overuse headache is partly mediated by the deleterious effect some medications may have on sleep architecture and quality.
