The correct answer to the question is option 3: Alleles. In genetics, an allele refers to one of the different forms or varieties of a gene. Genes are segments of DNA that provide instructions for the development and functioning of an organism. Alleles can occur in pairs, with one allele inherited from each parent, and they can be either dominant or recessive.

Option 1: Genotypes refers to the combination of alleles in an individual. It represents the genetic makeup of an organism, including both the dominant and recessive alleles.

Option 2: Sib pairs refers to siblings that share the same biological parents. It does not specifically pertain to different varieties of a gene.

Option 4: Isomers are molecules that have the same chemical formula but different structural arrangements. It is unrelated to genetics and does not apply to the question.

Therefore, the correct answer is option 3: Alleles, as it accurately describes the different varieties or forms of a gene.