→ It is the presence of more than two alleles of a gene to govern same character.
→ E.g. ABO blood grouping (3 alleles: IA, IB & i).
→ In an individual, only two alleles are present. Multiple alleles can be found only in a population.
→ A single gene product may produce more than one effect.
→ Occasionally, a single gene product may produce more than one effect.
→ For example, starch synthesis in pea seeds is controlled by one gene.
→ It has two alleles (B and b).
→ Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are prdouced.
→ In contrast, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains.
→ After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled, Heterozygotes produce round seeds and so B seems to be the dominant allele.
→ But, the starch grains produced are of intermediate size in Bb seeds.
→ So, if starch grain size is considered as the phenotype, then from this angle, the alleles
show incomplete dominance.
→ Therefore, dominance is not an autonomous feature of a gene or the product that it has information for.
→ It depends as much on the gene product and the production of a particular phenotype from this product as it does on the particular phenotype that we choose to examine, in case more than one phenotype is influenced by the same gene.