The ability to draw inferences as to the origin of semen in a sexual assault case is obvious. The power to do this in traditional serology was limited, and depended mainly on ABO and PGM typing. The PGM1 locus is expressed in semen and vaginal secretions, and the methodology and interpretation used in its typing are exactly the same as for blood.
ABO typing is somewhat different. Almost everyone has at least trace levels of antigen in their body secretions that correspond to their ABO blood type. However, about 80% of the population has very high levels of these antigens in body secretions. These persons are described as secretors. The remaining 20% of the population are described as non-secretors and have concentrations of the antigens that are too low for normal detection. The quantitative difference is genetically determined. The gene responsible has two alleles, a dominant form, Se, and a recessive one, se. Thus SeSe and Sese persons are secretors and sese persons are non-secretors.
The A, B, and H antigens are polysaccharides. They are found on RBC surfaces as lipo-polysaccharides and in secretions as glycoproteins. The biochemical genetics involves four genes as shown in the table below.
Biochemical Genetics | |||
Gene | Expression | RBC genotype | Secretions genotype |
Le, H and secretor | Dominant H allele in presence of dominant Se allele converts Lea substance to mixture of H, Lea and Leb substances | Depends on A and B alleles | Leaand Leb substances present |
Le, ABO A allele together with H and Se | A blood group, Lewis a and b | A, H, Le(a-b+) | Lea, Leb , A, H |
Le, ABO B allele together with H and Se | B blood group, Lewis a and b | B, H, Le(a-b+) | Lea, Leb , B, H |
Le, ABO O (silent) allele together with H and Se | O blood group (No A or B blood group), Lewis a and b | H, Le(a-b+) | Lea, Leb , H |
Le, H and sese | A, B or H depending on ABO gene | A. B. H depending on ABO gene, Le(a+b-) | Lea only |
Le, hh and Se | no ABH or Lewis | No ABO activity (Bombay phenotype) | Lea only |
lele, H and Se | ABO depending on ABO gene alleles, no Lewis | A, B, H (depending on ABO alleles), Le(a-b-) | A, B, H (depending on ABO alleles), no Lewis |
lele, H and sese | A, B, or H depending on ABO gene | A, B, H depending on ABO gene, Le(a-b-) | No ABH, no Lewis |
lele, hh and Se | No ABH or Lewis | No ABO activity (Bombay phenotype) Le (a-b-) | No ABH, no Lewis |
Note: Secretors all have an Le allele, an H allele and an Se allele. For simplicity, AB heterozygotes are not listed in the Table – they will have A and B. | |||
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