Allele is a particular form of gene, and they usually occur in pairs. In a diploid cell nucleus, each homologues chromosome contains two alleles. If both the alleles are same in the individual, then the individual is said to be “homozygous,” and if the each allele is different, then the individual is said to be “heterozygous.”
As humans receive 23 chromosomes from the mother and another 23 from the father, the alleles may be slightly different. These alleles will express slightly different proteins.
Thus, the blanks can be filled with alleles; proteins.