Which statement best defines an allele?

An allele is defined as a variant form of a gene. Genes are the basic units of heredity and are made up of sequences of DNA that code for proteins or RNA. Each gene can exist in different versions, which are called alleles. These variations can result in different traits or characteristics. For example, a gene that determines flower color in a plant could have a red allele and a white allele. The presence of different alleles is fundamental to genetic diversity within populations, influencing traits passed from parents to offspring.

In contrast, the other options provided do not accurately encapsulate what an allele is. A physical structure within a gene refers more to the components of DNA, like nucleotide sequences, rather than the concept of alleles. A sequence of amino acids describes the structure of proteins, not genes or alleles themselves. Lastly, the entire genetic makeup of an organism refers to its genotype or genome, which encompasses all the genes and alleles present, rather than defining what an allele is.