e. To demonstrate that dominance is a property of traits rather than a
manifestation of gene activity, consider again the A1 and A2 traits. Based upon
serologic evidence, an A1A2 heterozygote is phenotypically A1, for example, the A1 trait is
dominant to A2; however, analysis of the serum transferases reveals that the A1A2
heterozygote possesses both the A1 transferase and the A2 transferase. In respect to
transferase activity, then, the A1 and A2 traits are co-dominant; or to simplify matters,
let's consider the blood group AB. Both the A and B genes are inherited as
heterozygote genes, but both will be expressed, because they are co-dominate.
MODES OF INHERITANCE
a. Background. The pattern of inheritance exhibited by blood group antigens
depends upon the distribution of their alleles among the paired chromosomes. Most
blood group alleles are located on 1 of the 22 pairs of autosomes, those chromosomes
other than the sex chromosomes, X and Y. No blood group genes have been
discovered on the Y chromosome and only the Xg and Kx loci have thus far been
ascribed to the X chromosome.
b. Autosomal Dominant or Codominant Inheritance. An autosomal dominant
or codominant trait shows a very characteristic pattern that is easy to recognize. The
trait never skips a generation, and occurs with equal frequency in males and females.
Most blood group and histocompatibility antigens fit into this category.
c. Autosomal Recessive Inheritance. Traits inherited in autosomal recessive
fashion will also occur with equal frequency in males and females. To manifest the trait,
the individual must be homozygous for the gene (allele). The parents may or may not
display the trait. However, if the parent does not display the trait, they must be carriers
of the gene (heterozygous) for the gene whose presence is not phenotypically apparent.
When the recessive trait is rare, it typically is manifested in members of one generation
and not manifested in members of the preceding or successive generation. Blood
relatives are more likely to carry the same rare gene than unrelated people. It is easily
understood, therefore, that when offspring are homozygous for a rare gene and display
the trait, the parents are usually blood relatives (a consanguineous mating).
d. Sex-Linked Dominant or Codominant Inheritance. Sex-linked dominant or
codominant pedigrees show affected males with normal wives transmitting the trait to
daughters only. Some sex-linked dominants of interest in immunogenetics are the Xga
blood group, which is present on the X chromosome.
e. Sex-Linked Recessive Inheritance. A much larger number of affected
males than females is the first thing to look for. Affected males do not come from
affected fathers but from carrier mothers, who may often be recognized by having
affected brothers, fathers, or uncles. A good example of this is hemophilia A.