Position Effect
The change in the expression of a gene accompanying a change in the position of the gene with respect to neighboring genes; the change in position may result from crossing over or from a chromosomal aberration.

Postzygotic Isolation
A form of reproductive isolation in which a zygote is successfully formed but then either fails to develop or develops into a sterile adult. Donkeys and horses are postzygotically isolated from one another; a male donkey and a female horse can mate to produce a mule, but the mule is sterile.

Pre-eclampsia (toxemia)
A condition of late pregnancy characterized by the mother having elevated blood pressure, protein spilling in her urine and swelling of her tissues. If not treated successfully it can lead to eclampsia or toxemia which can cause seizures and have severe consequences to the mother and, baby (ie. Stroke, death).

Pribnow box
A segment upstream from the start-point of prokaryotic structural genes to which the sigma subunit of the RNA polymerase binds; the nucleotides most commonly found are TATAAT.

Primary cancer
The site of the cancer that first invaded the individual and now may have metastasized to other organs but originated in the first organ (ie. Breast cancer that metastases to brain or lung).

Prezygotic isolation
A form of reproductive isolation in which the two species never reach the stage of successful mating, and thus no zygote is formed. Examples would be species that have different breeding seasons or courtship displays, and which therefore never recognize one another as potential mates.

Presymptomatic Testing
Testing to determine whether a particular condition will develop in the future. Presymptomatic testing for breast cancer involves analyzing the BRCA1 and BRCA2 genes.