b. Antigen Loci.
(1) The presence or absence of ABO antigens on red blood cells is
dependent on two gene sites located on separate chromosome pairs. At one locus, on
an unidentified chromosome, is the gene H. The second gene locus is found on
chromosome 9; this is the locus for ABO determining genes. At the H locus there are
only two recognized alleles, the active H gene and the h gene. No product has been
demonstrated for the h gene; therefore, it is considered an amorph.
(2) At the ABO locus, there are three major alleles; A, B, and O. However,
as stated earlier, phenotypic expression of the A and B antigens are dependent on
activity at the H locus. The H gene produces a transferase essential for the expression
of A and B antigens on red blood cells. This transferase is responsible for the
attachment of the sugar L-fucose to the terminal D-galactose of some carbohydrate
chains (oligosaccharide chains) on the red blood cell membrane. Without L-fucose
being present on the terminal D-galactose, other sugars may still be attached to these
chains. The A transferase (N-acetyl-galactosaminyl transferase) and the B transferase
(galactosyl transferase) attach immunodominant sugars to the carbohydrate chain
where L-fucose has been added. If the individual is group O, there are no A or B
specified transferases and the H activity remains unchanged. The O gene does control
production of a protein which can be immunologically detected, but this protein has not
been shown to have impact on red blood cell antigenicity.
c. Difference Between Genotype and Phenotype.
(1) Phenotypes are those observable characteristics whether that particular
gene is present in a single dose (heterozygous) or in a double dose (homozygous). The
reactions seen when testing with antiseras are considered observable characteristics.
(2) The H gene must be present for the expression of the A or B gene. H
substance (antigen) is acted upon by the A or B gene and the majority of the H
substance (antigen) is converted to A or B antigen. In rare instances, a person lacks
the H gene and is homozygous h/h. As a result, these persons will test as group O
even though the A and B gene may be present. This blood group is known as Bombay.
(3) The O gene is an allele to the A and B genes. The O gene produces no
observable effect so it is said to be amorphic. When a person inherits the O gene with
an A or B gene, the A or B will be expressed, indicating that at least one A or B gene is
present. However, antisera testing will not demonstrate whether the person is
homozygous or heterozygous. Family studies are used to determine zygosity.