Which of the following best describes alleles?

Alleles are best described as alternate forms of a single gene. This definition focuses on the genetic concept that for any given gene, there can be multiple variations, known as alleles, which can influence specific traits. For example, in the context of eye color, the gene responsible for color may have different alleles that produce brown, blue, or green eyes, depending on the variant inherited from each parent.

The other options address different biological concepts: the first option refers to chromosomes, which are structures composed of DNA that contain many genes. The third choice relates to antibodies and their relationship with antigens, and the fourth pertains to specific cells involved in the immune response. While all of these concepts are important in biology, they do not accurately capture the definition of alleles as variations of a gene.