The Xg blood group system is unique in that it is the only blood group antigen which is controlled by a gene residing on the X chromosome. There are six possible mating types in X-linked dominant inheritance, such as the Xg system represents. One of these types is of particular interest in this presentation, even though it amounts to only 6% of possible matings, assuming the usual western Caucasian gene frequency of the Xg system. In this mating type, an Xg(a+) father, whose X chromosome is designated as +, mates with an homozygous Xg(a—) mother (Fig. 1). The offspring of such a mating, if female, will be incompatible with their mother, since they have received their father's Xg(a+) X chromosome. Male offspring are of course compatible, since they receive their mother's Xg(a—) X chromosome and their father's Y chromosome. It follows that any biologically significant materno-fetal incompatibility with respect to the Xg system might lead to preferential loss of daughter conceptions.

The present paper deals with data collected during the course of a study of twin pairs who were participating in an ophthalmologic investigation. The twins were classified as MZ or DZ on the basis of their blood types (ABO, MNS Rh, Kell, Duffy, etc.), physical likenesses, and dermatoglyphic findings. The results expected of Xga testing, assuming random mating and no selection, were calculated by Li's method (1955), using the gene frequency of 0.65 for the Xga allele.