Final answer:
A gene in which one allele takes a different form from another is described as heterozygous. This term refers to the paired alleles on homologous chromosomes that are different in a diploid organism.
Step-by-step explanation:
The gene in which one allele takes a different form from another is described by the term heterozygous. When an organism has two different alleles for a specific gene, it is heterozygous for that trait. Alleles are the different versions of a gene that can exist. Diploid organisms, like humans, have a pair of each chromosome and thus two alleles for each gene. If the alleles are the same, the organism is homozygous for the gene; if the alleles are different, the organism is heterozygous.
The phenotype is the observable traits expressed by an organism, which are influenced by its genotype, but phenotypes can be the same for both homozygous dominant and heterozygous genotypes on account of the dominance of one allele over the other. Therefore, when discussing an organism with two different alleles for a gene, we are describing a heterozygous genotype.