Definition of genetics

Polymorphism was defined by Ford as 'the occurrence together in the same habitat of two or more discontinuous forms, or "phases", of a species in such portions that the rarest of them cannot be maintained by recurrent mutations'. Polymorphism of complement components can be studied at four different levels:

1. by allotyping of native complement components in serum (the phenotypic level);

2. by defining their subunit composition (the sub-phenotypic level);

3. by establishing the population and formal genetics (frequency of allotypes, and frequency and segregation of the respective genes/alleles);

4. by gene mapping and DNA sequencing to reveal the gene structure and define sequence variants (the genotypic level).

For this purpose, a complex methodology including electrophoretic and isofocusing techniques with functional or immunologic detection of individual components, serology, quantitation, Southern blot and restriction fragment length polymorphism (RFLP) analysis, polymerase chain reaction (PCR) amplification and sequencing of DNA is applied (Table 1). By these means, insights into the evolution of polymorphisms and deficiencies and structure-function relationships, as well as into genetically determined disease susceptibilities, are gained. The analysis of complement polymorphisms at the DNA level has provided an even more complex picture by revealing further variants, which subdivide alleles defined at the protein level. DNA sequence analysis of protein allotypes of a number of complement components has demonstrated that most sequence variations are not confined to short stretches of genomic DNA, but rather are scattered throughout the entire gene; therefore it appears to be technically difficult completely to replace protein typing methods by direct DNA typing using the PCR methodology. A combination of typing methods, both at the protein and the DNA level, should be applied to obtain complete analysis for complement component polymorphisms, as well as for the detection of complement deficiencies.

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