A Human Double Cross-Over

Abstract

AN X-linked blood group system, Xg, was described this year jointly by investigators in the United States and at the Medical Research Council, Blood Group Research Unit, at the Lister Institute¹. More recently it was shown, at the University of North Carolina², that while ‘classical’ hæmophilia (hæmophilia A) is closely linked to colour blindness, as was already known from the work of Haldan and others, hæmophilia B (ptc deficiency, Christmas disease) is very loosely linked to colour blindness, if linked at all. In English families the loci for Xg and for hæmophilia A have been found to be widely separated³. Though work in progress is showing no hint of linkage between Xg and colour blindness the two should, in theory, be within measurable distance of each other because colour blindness is close^4–6 to the locus for glucose-6-phosphate dehydrogenase deficiency (g-6-pd) and g-6-pd is within measurable distance⁷ of Xg.