1. An initial stage in the preparation of cells or tissues for microscopical examination, in which the material is killed and preserved to prevent distortion, decay, and self-digestion (autolysis). Various chemicals (fixatives) can be used to penetrate the cells and preserve their structure either by denaturing the protein (e.g. picric acid, ethanol) or by tanning (e.g. acetic acid, formaldehyde). Plant specimens are often prepared for electron microscopy by fixing with Luft's fixative, a 1% solution of potassium permanganate. Common botanical fixatives include Navashin's solution (chromic-acetic-formaldehyde) and Carnoy's solution, an alcoholic fixative that rapidly penetrates the tissues and is especially suitable for hard materials such as seeds.
2. The attainment of a frequency of 100% by an allele in a population due to the complete loss, either by chance or by natural selection, of all other allelic forms of that gene. The likelihood of fixation occurring is greater in small populations.