Figure 2.

(a) Probability and (b) level of expression of MHC-I nucleotide alleles in relation to allele frequency in house sparrows. Classical (orange, circles, solid line) MHC-I alleles were more likely to be expressed and expressed at a higher level when rare, while the probability and level of expression of non-classical (blue, triangles, dashed line) MHC-I alleles were independent of allele frequency. Lines represent fitted means with 95% confidence intervals shown. Raw values in (a) represent the proportion of birds expressing each allele, while raw values in (b) represent the mean expression level for each allele.