Explain the concept of multiple alleles.

(A) $ABO$ blood grouping provides a classic example of multiple alleles.
Multiple alleles refer to the existence of more than two alleles for a single gene within a population. While an individual can only possess two alleles at a time,the presence of three or more alleles can only be observed through population studies.
Occasionally,a single gene product may produce more than one effect. For example,starch synthesis in pea seeds is controlled by one gene with two alleles ($B$ and $b$). $BB$ homozygotes synthesize starch effectively,resulting in large starch grains. In contrast,$bb$ homozygotes have lower efficiency,producing smaller starch grains.
Upon maturation,$BB$ seeds appear round,and $bb$ seeds appear wrinkled. Heterozygotes $(Bb)$ produce round seeds,suggesting $B$ is the dominant allele. However,the starch grains in $Bb$ seeds are of intermediate size. If starch grain size is considered the phenotype,the alleles exhibit incomplete dominance.
Therefore,dominance is not an autonomous feature of a gene or its product. It depends on the gene product,the resulting phenotype,and the specific phenotype chosen for examination when a single gene influences multiple traits.