Phenotype Frequency
Definition
The proportion of a given population that expresses a particular antigen combination. Multiplying phenotype frequencies across independent loci gives the combined frequency of a multi-system profile, which is the key statistic in blood group evidence.
- Field
- Blood group population statistics
- Definition
- Proportion of a population expressing a given antigen combination
- Combination rule
- Independent-locus frequencies are multiplied together
- Application
- Combined match statistic in blood group evidence
Common questions
Why must the loci be independent before multiplying their frequencies?+
Multiplying frequencies assumes the systems assort independently in the population; if two blood group loci were linked or correlated, multiplying would overstate the rarity of the combined phenotype and mislead the statistic.
How does phenotype frequency compare in strength to a modern DNA match statistic?+
Blood group phenotype frequencies are typically far less discriminating, often only excluding a modest fraction of the population, whereas multi-locus STR profiling routinely reaches match probabilities many orders of magnitude smaller.
Does phenotype frequency vary meaningfully between population groups?+
Yes, antigen frequencies for many blood group systems differ across ancestral populations, so a statistic calculated with the wrong reference population database can misstate how common or rare a phenotype actually is.
Related terms
- Absorption-Elution
- A technique for detecting antigens on dried red cell debris. Specific antiserum is absorbed onto the stain; if the antigen is present,...
- Antiserum
- Serum collected from an immunised animal containing antibodies directed against a specific antigen or group of antigens. In species identification, antiserum raised...
- Blood Group System
- A set of red cell surface antigens encoded by one gene locus or by two tightly linked loci. The International Society of...
- Linkage Disequilibrium
- The non-random association of alleles at different loci within a population. Certain HLA allele combinations (e.g., HLA-A1 with HLA-B8 with HLA-DR3) occur...
- Probability of Exclusion
- The proportion of the population whose blood type differs from that of the stain and who would therefore be excluded as its...