An allele is a variant form of a given gene.
The term human blood group systems is defined by International Society of Blood Transfusion as systems in the human species where cell-surface antigens—in particular, those on blood cells—are "controlled at a single gene locus or by two or more very closely linked homologous genes with little or no observable recombination between them", and include the common ABO and Rh- (Rhesus) antigen systems, as well as many others; thirty-five major human systems are identified as of November 2014.
The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH).
The National Institutes of Health (NIH) is the primary agency of the United States government responsible for biomedical and public health research, founded in the late 1870s.
A phenotype is the composite of an organism's observable characteristics or traits, such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior (such as a bird's nest).
Serology is the scientific study of serum and other bodily fluids.