Multiple allele

General
If there are more than 2 alternative alleles for 1 locus in a population - this is referred to as a multiple allele.
 * occurs in the inheritance of normal and pathological traits in humans
 * e.g. gene for erythrocyte antigen A blood AB0 system can occur in up to 15 alleles
 * similarly haemoglobinopathies- genetically determined by multiple alleles: the synthesis of Hb in humans is determined by 3 pairs of genes (polygenically determined), each of which has several alleles (polyalleles); thus, dozens of aberrant Hb types can arise

Allelic interactions
In a diploid cell, there are 2 alleles for a single gene. There can be 3 different relationships between these alleles, on which the final gene expression depends.

Incomplete dominance and recessiveness

 * the dominant allele does not completely suppress the recessive allele, the recessive allele is also partially expressed.
 * e.g. flower colour in a flower (A - red colour; a - white colour):
 * homozygote AA - red colour
 * homozygote aa - white colour
 * heterozygote Aa - pink color

Complete dominance and recessiveness

 * the dominant allele completely suppresses the expression of the recessive allele
 * e.g. blood group in humans (A - agglutinogen A is formed; 0 - no agglutinogen is formed):
 * homozygote AA - blood group A
 * homozygote 00 - blood group zero
 * heterozygote A0 - blood group A (agglutinogen A is formed)

Codominance

 * both dominant alleles are fully expressed in the heterozygote and do not affect each other
 * e.g. with human blood groups (alleles A and B are codominant to each other and dominant to allele 0):
 * heterozygote A0 - group A
 * heterozygote B0 - group B
 * heterozygote AB - group AB (both agglutinogens are formed)
 * homozygote AA - group A
 * homozygote BB - group B
 * homozygote 00 - group zero (no agglutinogen is formed)

Related articles

 * Alleles
 * Allelic interactions
 * Hemoglobin
 * Blood groups