The variation of a gene that occurs at the same place on homologous chromosomes is called an allele.

Here's a breakdown:

* Homologous chromosomes: These are pairs of chromosomes that carry the same genes, one inherited from the mother and one from the father.

* Allele: A specific version of a gene. For example, the gene for eye color has different alleles that determine whether your eyes are blue, brown, green, etc.

* Locus: The specific location of a gene on a chromosome.

So, alleles are alternative forms of a gene that occupy the same locus on homologous chromosomes.