What do we call alternative forms of a gene?

Alternative forms of a gene are referred to as alleles. Genes are segments of DNA that code for specific traits, and alleles are the variations of those genes that can result in different expressions of a trait. For example, a gene that determines flower color in a plant may have a purple allele and a white allele. These alleles can contribute to the diversity observed within a species, as they can lead to different physical or functional traits.

The other terms listed do not describe alternative forms of a gene. Genomes refer to the complete set of genes or genetic material present in a cell or organism. Chromosomes are the structures that organize and contain genes, made up of DNA and proteins. Phenotypes represent the observable traits or characteristics of an organism, which result from the interaction of its genotype (the genetic makeup, including alleles) and the environment. Therefore, alleles specifically denote the different versions of a gene, making it the correct term in this context.