Marker: an alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. of a geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) that provides an easily observable phenotypeplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism.. Markers are usually cloned or least well mapped. They are used as genetic landmarks in various genetic experiments. In some cases, markers do not have easily observable phenotypesplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. and can only be detected using molecular methods (e.g., SNPs or SSRs).