An alternate form a gene is called a (n) :

Allele

Explanation:

Alleles are alternative forms of a given gene and occupy the same locus on homologous chromosomes. When we refer to locus, we are talking about the position of the gene on the chromosome; When we talk about homologous chromosomes, we mean chromosomes that have genes for the same trait.

It is important to note that alleles are always represented by letters. The allele is represented by a lower case letter when it is recessive, that is, when expressed only in pairs (Ex.: aa). The dominant allele is represented by a capital letter and is expressed even in a single dose (Ex.: AA or Aa).