Gene Position Effect

Gene Position Effect 

the influence of the position of the genes in a chromosome on their observed activity. The phenomenon was discovered by the American geneticist A. Sturtevant in 1925.

The gene position effect is observed in structural chromosomal rearrangements, or translocations; as a result of such rearrangements, genes may be transferred from the chromosomes’ active zones (euchromatin) to the inactive zones (heterochromatin) and become inactive, and vice versa. When translocation brings back a euchromatic gene from the heterochromatin to any point in the euchromatin, the given gene is reactivated. The reversibility of the position effect is used as a means of demonstrating that a given genetic change is due to the position effect rather than to genetic mutation. As a result, the chromosome puffs in the euchromatic segments disappear, and the synthesis of deoxyribonu-cleic acid and ribonucleic acid is disrupted; the heterochromatin is activated upon being transferred to the euchromatin and becomes cytologically indistinguishable from the latter.

The gene position effect may cause simultaneous disruption in the activity of several euchromatic genes following the gene that immediately adjoins the heterochromatin; the influence of the heterochromatin, starting from the locus of rearrangement, is always in the direction of the nearest euchromatic gene and is gradually weakened as the distance between the euchromatic and heterochromatic genes increases (polarized distribution effect). A phenomenon that has been the subject of intensive study is the “mosaic” position effect, which is phenotypically manifested in mosaicism—that is, the appearance of altered somatic cells mingled with normal ones.

The molecular mechanism of the position effect is unclear. The effect is presumably based on morphological change in the translocated segment of the chromosome. Further studies of the gene position effect may well clarify the mechanisms of genetic regulation in eukaryotes.

V. V. VELKOV