A linkage group is a collection of genes that are close enough in a genome to be inherited together. The transcription and translation of genes in the group may or may not be linked together, and their exact activity depends as much on the organism they encode as on their proximity to each other. In simple cells lacking a nucleus called prokaryotes, the genes that make up operon units can be considered linkage groups. Eukaryotes, more complex cells, do not have an equivalent structural grouping in their genomes but despite a lack of operons they can contain many linkage groups. Both prokaryotes and eukaryotes pass on linkage groups largely through recombination.
Recombination is one process by which variability can increase in a genome, and consists of deoxyribonucleic acid (DNA) pieces breaking and reconnecting at entirely new places. When a linkage group is involved in recombination, all the genes within the group tend to stay together, so all of the genes' activities are relocated at the same time. This movement can be to a different place on the same chromosome or to a different chromosome altogether. Usually, nucleic acid movement by recombination doesn't disrupt a linkage group's function. Linkage groups can be broken apart during recombination, but the probability of that happening is fairly low.
Since the genes in a linkage group tend to stay together, they can also be studied together. Linkage groups can be used to map out a genome in terms of both structure and function, identifying regions that may be implicated in disease or regions that are inherently unstable because of their genetic composition. A number of genomes have been mapped in this way, including multiple important food crops. Once a particular crop is mapped, the map can provide a template for selective breeding to improve crop quality or yield in a variety of ways.
Linkage group identification has many applications relating to human health, from identification of genetic diseases to analyzing the genes of dangerous microorganisms or parasites. Analysis of linkage groups can also be used to identify new genes and genes that react differently to drugs or environmental stresses. Genes that confer resistance to drugs, pathogens, or pests can also be found in linkage groups, and depending on the circumstance can provide many drug targets or beneficial sequences to use in gene therapies. The discovery of linkage groups has created a plethora of new opportunities to analyze genetic sequences and use them to our advantage.