SHARING AT THE MAJOR HISTOCOMPATIBILITY COMPLEX AFFECTS THE SECONDARYSEX-RATIO IN DIFFERING WAYS

We analysed the effect of HLA loci on the secondary sex ratio, and inv estigated whether allele sharing between parents and between mother an d 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 signifi cantly 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 me chanisms.