Figure 1.

Relationship between MHC-I genomic allele count and allele frequency in a population of house sparrows among (a) nucleotide alleles and (b) amino acid alleles. Most genomic MHC-I alleles are rare: the number of genomic MHC-I alleles decreased rapidly as alleles increased in frequency within the population (i.e. were found in more individuals and thus more common), meaning most alleles occurred at low frequencies in the population, while few alleles were shared among many individuals. The pattern was more marked (i.e. a significantly more negative slope) for classical (orange, solid bars, solid line) alleles, compared with non-classical (blue, striped bars, dashed line) alleles, and the difference between classical and non-classical alleles was larger among amino acid alleles (b), compared with nucleotide alleles (a). Lines represent fitted means with 95% confidence intervals shown. Bars represent raw counts of unique alleles.