LESSON 2
IMMUNOGENETICS
Section I. BLOOD GROUP IMMUNOGENETICS
2-1.
BACKGROUND
a. Although prior workers established the heritability of the blood group antigens,
the origin of immunogenetics as a discrete subject is usually credited to Little, Cole, and
Irwin, who not only established the genetic determination of blood group antigens of
fowl but also went further to use these antigens as markers in carefully controlled
matings.
b. Formerly, geneticists were limited to investigations of traits such as body color
or wing shape in fruit flies, or clinical disorders in man. These, however, are usually
difficult to analyze on a molecular level and often suffer from pleiotropism, that is,
multiple effects of a single gene. For this reason, the emergence of immunogenetics
gave promise of more closely approaching the action of the gene. In fact, early papers
went so far as to state that antigens were direct gene products, a hypothesis codified in
the dictum "one-gene-one antigen." It is apparent now, however, that only protein
antigens or those in which the antigenic site resides in protein, such as the Rh antigens,
are closely related to an RNA messenger coded by nuclear DNA.
2-2.
GENETIC TERMINOLOGY
a. Genes are the basic units of heredity. There has been little doubt since the
description of the double helix by Watson and Crick in 1953 that the information
necessary to encode biologic structures and processes resides in deoxyribonucleic acid
(DNA), a high molecular weight polymer containing the sugar 2-deoxy-ribose,
phosphate, and 4 nucleotide bases (adenine, thymine, guanine, and cytosine). For our
purpose, it is sufficient to conceptualize genes as linearly arranged units on
chromosomes, arrayed in the nucleus as strings of beads. Any particular gene,
however, will be situated normally at a fixed position or locus on a particular
chromosome. Because chromosomes occur in pairs (in humans, there are 23 pairs),
gene loci must also be paired. One exception to this rule in mammals and other sexually
reproducing animals is lack of paired material between portions of the sex
chromosomes, X and Y. The Y chromosome is generally much shorter than the X and
lacks most of the gene loci of the X chromosome such as the locus for the Xga blood
group.
MD0845
2-2