Concept:A trait is controlled by a pair of genes found at the same position on homologous chromosomes.
Explanation:Each parent contributes one gene to the pair.
These alternative forms of a gene are called alleles.
The pair together determines the trait expressed.
If both genes are the same, the pair is homozygous.
If the genes are different, the pair is heterozygous.
Therefore, the pair of genes controlling a trait is referred to as an allele pair.
Answer:A. an allele