Horizontal Gene Transfer (HGT) - a.k.a. Lateral Gene Transfer (LGT) - is the transfer of genetic material (i.e. DNA) from one organism to another, but via routes other than the normal inheritance from parent to offspring.

Evidence for HGT was first reported in the 1920s, when UK biologist Frederick Griffith noted that different strains of pneumococci bacteria could somehow exchange their virulent tendencies (ref.). At the time, the existence of genes and DNA were not known.

Since then, multiple DNA-based investigations have confirmed that HGT has occurred in multiple organisms - not just bacteria, but also plants, reptiles, fish and mammals (including humans). Unlike inherited genes, these genetic exchanges can, and do occur between completely different species,

In simpler organisms, HGT is known to be very common. In bacteria for example, it's now thought that HGT is the principle mechanism for the transfer of antibiotic resistance from one bacterium species to another.

In more complex multicellular organisms, HGT is far less common, but nevertheless it only has to happen once to permanently change the genetic code of an organism and all of its subsequent offspring. In 'evolutionary time' - for example over hundreds of thousands of years - it becomes fairly likely.

In most cases, the exact process for HGT in complex organisms remains unknown. But it has been confirmed that viral, bacterial and fungal infections - possibly involving transposons ('jumping genes') - are sometimes involved. Parasitic infections, and even bites from insects can also be HGT vectors. Other possibilities include horizontal transfers due to symbiotic hosting (e.g. gut bacteria).

The discovery of HGT means that the traditional 'tree of life' diagrams commonly used to illustrate the evolution of species are often very dramatically over-simplified, and (especially for lower level organisms) need to clearly show possible 'horizontal' gene transfer as well as 'vertical' (i.e. inheritance).

An added complication is that the total extent of HGT which has occurred in the past is not known with any certainty.

Although HGT has been regarded as a driving force in the innovation and evolution of genomes, especially in prokaryotes, its extent and impact on the evolutionary process and phylogeny of organisms or species remains controversial"

Source :Proc Natl Acad Sci U S A. 104(11): 4489–4494.

Further reading : See Horizontal gene transfer in evolution at Wikipedia.

HGT might go some way to explaining why numerous organisms have been found to carry genes which code for proteins that seem to have no use for that organism*. They may have been horizontally transferred from some other species where they were relevant.

* Note : As an example, humans carry two genes, CYRN1 and CYRN2, that code for the protein cyritestin - which has been shown to have no function in humans. (ref.)

Also see :DNA / RNAplugin-autotooltip__plain plugin-autotooltip_bigDNA / RNA

The vast majority of human DNA lies outside of the genes within the cells. Of this, recent work has discovered that 85% of these stretches of DNA appear to make RNA - which, for the most part, has yet-to-be-determined functions.

A study published in the online journal

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