Glossary | 327 



Penetrance a measure of the proportion of individuals homozygous for 

 a gene that show its phenotypic effect. 



Pericentric applied to inversions that include the centromere. 



Phenocopij a phenotypic change that simulates a genotypic change. 



Phenotype the resultant of the interaction of the genetic information 

 with the environment; loosely, the characteristics of an organism exclu- 

 sive of its genetic endowment. 



Phenotypic variance the total variance observed in a character. 



Phosphate bond {high energy) usually the anhydride linkage between 

 phosphate ions which may be an important source of energy. 



Phosphorylation the addition of a phosphate to an organic compound. 



Photolysis splitting of water with light as the energy source. 



Photophosphorylation the addition of phosphate to organic compounds 

 with light as the source of energy. 



Photosynthesis the processes involved in the elaboration of organic com- 

 pounds from inorganic compounds with light as the energy source. 



Phyletic evolution any change occurring sequentially in a single line of 

 descent. 



Phylogenetic pertaining to phylogeny, the evolutionary history of an 

 organism. 



Plankton the population of floating organisms in a body of water. 



Plasma membrane the outer boundary of the cytoplast, continuous with 

 the endoplasmic reticulum and surrounded by a cell pellicle or wall. 



Plastids organelles of plant cells, usually containing pigments and usu- 

 ally synthesizing soluble or insoluble carbohydrates. 



Pleiotropy the influence upon two or more characters, not obviously re- 

 lated, bv one or more alleles. 



Pleistocene the major epoch of the Quaternary period, beginning about 

 1 million vears ago and ending about 10,000 years ago. 



Pliocene the last epoch of the Tertiarv period, beginning about 11 mil- 

 lion years ago and ending about 1 million years ago. 



Ploidy the number of genomes in the zygote of an organism. 



Polar bodies the two or three meiotic products that do not develop into 

 eggs in oogenesis in animals. 



Polygenes those genes without obvious major phenotypic effect; numer- 

 ous factors affecting a characteristic. 



Polymorphism the presence in the same population and at the same 

 stage of development of two or more conspicuously different forms of 

 an organism in such proportions that the rarest of them could not be 

 accounted for by recurrent mutation alone. 



Polyp the sedentary form of a coelenterate. 



Polypeptide the substance resulting from the formation of long amino 

 acid chains, the acid group of one molecule combining with the base 

 of another molecule with the loss of a molecule of water. 



Polyploid containing more than two haploid chromosome complements 

 in the reproductive cells other than gametes, or presumed to contain 

 more than two genomes in such cells. 



