For some traits, the gene of interest has been discovered and the presence of desirable alleles can be directly assayed with a high level of confidence. However, if the gene of interest is not known, markers linked to the gene of interest can still be used to select for individuals with desirable alleles of the gene of interest.
For certain attributes, the gene in question has been pinpointed, enabling accurate detection. Conversely, if the gene remains unidentified, markers connected to it can be used to identify individuals with desirable variants of the gene of interest.