The Rh blood group is determined by a single gene with two
alleles-- positive and negative. The positive (Rh+) allele is
dominant, so persons who are Rh+/Rh+ or Rh+/Rh- are said to be
Rh-positive. Individuals with two Rh- alleles are Rh-negative.
Citation
Hall, Prentice "Biology" Upper Saddle River, New Jersey,
2002
Page 344 in human genes subtitle, blood group genes subsection ,
paragraph 2