The two copies or forms of a gene for one trait are called

The two copies or forms of a gene for one trait are called alleles.

Explanation:

Alleles of a gene are formed due to a mutation at same locus of chromosome. Alleles can be a) dominant,

b) recessive or

c) null alleles.

A dominant allele always expresses itself in the phenotype and masks the effect of recessive allele. The recessive allele could express itself only in the absence of dominant allele. A null allele is the one that lacks the normal gene function.