Sharing at the major histocompatibility complex affects the secondary sex ratio in differing ways.

We analysed the effect of HLA loci on the secondary sex ratio, and investigated whether allele sharing between parents and between mother and child, or child homozygosity, affected the viability of male embryos, which are generally less resistant to unfavourable conditions during pregnancy. The sharing conditions at the B and DR loci showed significantly differing effects: HLA-B seemed to favour female births, while, in pregnancies subsequent to the first, HLA-DR seemed to favour male births. Both HLA-B and DR loci seemed to work through immunological mechanisms.