Final answer:
Protein variations from a single gene can occur through alternative splicing, mutations such as single-nucleotide substitutions, and regulation of gene expression, with mechanisms like gene duplication contributing to the evolution of new protein functions.
Step-by-step explanation:
Variations in proteins can arise from a single gene through several mechanisms. One way is via alternative splicing, which allows a single gene to produce multiple protein variants by including or excluding certain segments of the RNA transcript during processing. Mutations, such as single-nucleotide substitutions, can alter the amino acid sequence, ultimately modifying the protein's structure and function, as seen with the substitution in sickle cell anemia. Moreover, gene expression regulation plays a role in protein variation as cells express different proteins based on necessity, despite having identical genes.
Gene duplication also contributes to protein diversity. It gives rise to genes with a similar sequence that can evolve independently, increasing the chances of a new protein forming and possibly leading to new phenotypic characteristics. While most genetic variants stem from point mutations impacting the genetic code, alternative splicing and gene duplication are significant drivers of protein diversity within organisms.