Determination of Genotypes by Family Studies
HUMAN blood groups are unique among inherited physical characteristics in that each blood group factor is transmitted by the mode of single-gene inheritance. This means that the inheritance of blood group factors is determined by a single set of allelomorphic genes. In contrast to this, physical characteristics such as skin color and eye color have a much more complex mode of inheritance and may require many genes in order to produce a specific inherited character. Because the inheritance of blood group factors is so comparatively simple, geneticists have found them a convenient group of characteristics to employ in various types of genetic studies.
A second characteristic of blood groups, which makes them particularly useful in the medical field, is that in the most usual form of inheritance the genotype of an individual can be determined as well as his phenotype. Thus, the type MN person has inherited a gene for M from one parent and a gene for N from his other parent. The genes for M and N are codominant and the heterozygote is readily detected by laboratory tests. A type M person is genetically homozygous for M and must have inherited a gene for M from each parent.
Unfortunately, there are exceptions to this simple inheritance of blood group factors by codominant genes, and when they occur it is necessary to utilize means other than the conventional laboratory procedures to determine genotypes in two of the most important blood group systems, i.e., the ABO and Rh-Hr systems. In. . .