Possible linkage between Xg and the locus for a gene causing mental retardation with or without hydrocephalus.

Hereditary hydrocephalus (Bickers and Adams, 1949) is now known, at least in some families, to be an X-borne condition (Edwards, 1961). A few families with the X-borne type of hydrocephalus have been tested for the Xg blood groups but with no hint of linkage between their two loci (Edwards, 1968; Jabbour, 1968). In a kindred recently reported by Fried (1972) a single X-bome gene has apparently caused mental retardation sometimes with and sometimes without hydrocephalus. The Xg groups of the family (Fig. 1) pose a difficult exercise in scoring for linkage. The deceased maternal grandparents, I.1 and I.2, of the propositus could have had one of three arrangements of Xg genotypes.