The association between genetic diversity and four traits related to offspring viability in great tits has been explored. It was tested whether significant heterozygosity-fitness correlations (HFCs) were due to a "general" (a genome-wide effect) or a "local" (only one or a few loci involved) effect. Maternal heterozygosity was positively associated with offspring size even after controlling for the average size of the parents. A significant negative relationship between parental genetic relatedness and hatching success was also observed. However, one locus (Ase18) had disproportionately large effects on the observed HFCs. Results suggest that this particular neutral marker may be linked to functional loci influencing fitness in this species and offer one of the most clear examples about the potential important role of local effects in observed HFCs. Information [at]ebd.csic.es: García-Navas et al (2014) Heterozygosity at a single locus explains a large proportion of variation in two fitness-related traits in great tits: a general or a local effect? Journal of Evolutionary Biology. DOI: 10.1111/jeb.12539