Xg antigen system
The XG antigen is a red blood cell surface antigen discovered in 1962 by researchers at the MRC Blood Group Unit.
The PBDX gene that encodes the antigen is located on the short arm of the X chromosome. Since males normally have one X chromosome they are considered hemizygotes. Since women have two copies of the gene and could be heterozygotic for the presence or absence of the functioning gene they could express the functioning protein on just some of their red blood cells.