Molecular Genetics

Carbohydrate chains can serve as molecular markers on the genetic chromosome.

Facts

• Indiscriminate transfusion of blood is dangerous. Severe immune reactions can occur when the donor's blood is incompatible with the recipient's immune system.
• Four types are recognized in primates: O, A, B, and AB. Compatible transfusions are indicated at right by "yes", incompatible ones by "no". Within-type transfusions are compatible.
• Glycoproteins and glycolipids on cell surfaces differ in terminal structures depending on blood type. The three structures are diagrammed below. AB cells have both A and B structures.

  

Interpretations

• The part of a chromosome determining a character is called a locus. Each of the distinguishable types of information at a locus is an allele of that locus. The blood group locus in humans is on chromosome 9.
• O, A, and B are the principal blood group alleles. The O allele is recessive. A and B alleles are codominant, meaning that the phenotype determined by these alleles is not masked by the presence of other dominant alleles. AB individuals, heterozygous at the blood group locus, produce both A and B type chains.
• Differences in carbohydrate molecules can result from genetic differences at certain loci. The differences can be used as molecular markers. Carbohydrate differences are most often scored using allele-specific antisera, in this case, anti-A and anti-B sera.
• Two subtly different enzyme activities, a galNAc transferase and a gal transferase, are encoded at the same locus by different alleles.
• The allele not encoding an active enzyme, the O allele, is the recessive locus.

Further information

• ABO is the symbol for the blood group antigen gene.
• Frequencies of the ABO alleles vary from population to population.
• O individuals are identified as not reacting with anti-A or anti-B. Most can make the precursor depicted above. That precursor is the "H"-antigen.
• ABO typing is used in legal identity determination cases, but is not as useful as other methods.
• The presence or absence of the Rh factor is another blood polymorphism.
• Karl Landsteiner received the Nobel Prize for his discovery of the ABO system.
• Direct assay for a variety of enzymes can also reveal polymorphisms in populations that are useful as molecular markers.
• Protein antigens also serve as polymorphic markers.