A group of interbreeding organisms of one species living in a certain geographical range at a given time. The nature of the population is determined by factors such as density, sex ratio, birth and death rates, emigration and immigration. See Biological evolution, Cohort, Community, Epidemiology, General theory of biological classification, Heterogeneity, Polymorphism, Population genetics, Population …
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Polypeptides
A chain of amino acids with or without a defined secondary structure. A secondary structure consists of segments of polypeptides that form a characteristic pattern that extends in one dimension. The bonding of two or more polypeptides gives rise to proteins, which are then folded into a specific shape for a particular protein. Two example …
Pons
Situated in brain stem, the pons serves as a relay station from the forebrain to the cerebellum with tracts that carry signals to another important relay station, the thalamus. It also has a vital function in the regulation of respiration (both inspiration and expiration), as well as being intimately involved in REM sleep. Its functional significance extends …
Polynomial
A mathematical expression containing three or more terms, denoted by letters. It has the general form: f(x)=anxn +an−1xn−1 +…+a2x2 +a1x+a0, where a0, a1 etc. are constants (real numbers) and n is the highest power of the variable, termed the degree or order of the polynomial. A polynomial of degree one (i.e., whose highest power of n is one) is called a linear …
Polynomial analysis of variance
Analysis of variance that describes the relationships between dependent and independent variables using polynomials of first and higher order. See Analysis of variance (ANOVA), Polynomial
Polyneural to mononeural innervation
First observed in neonatal muscles in 1917. Starting early in prenatal life, one spinal motoneuron synaptically innervates more than one fiber in skeletal muscle (polyinnervation). Subsequently, supernumerary or collateral axons are retracted so that one motoneuron innervates one muscle fiber (monoinnervation), a process that takes place mainly prenatally, but also after birth depending on the muscle. …
Polymerization
A chemical reaction in which two or more relatively simple molecules combine chemically to form larger molecules containing repeating structural units of the original molecules. The small molecules are monomers (molecules that react to other molecules of the same or different compound to form a larger molecule such as polymer) and the larger ones are …
Polymorphism
In genetics, it refers to the co-existence of a number of distinctly different types in a population. An important category of discontinuous variation within species, it consists of two sorts: genetic polymorphism and non-genetic polymorphism, although in practice it is difficult to distinguish between them. An example of the former is human blood groups, and caste …
Polygynous households
Commonespecially in sub-Saharan Africa, they are family units in which one man ismarried to more than one woman. Polygynousextended households were probably a recurring feature of family life duringmost of human evolution. Persistent questions about such households in contemporarysociety included, for example, whether they involve more interpersonal cooperation (e.g., among co-wives), are beneficial for women and for the developing infant …
Polygenes
Genes at more than one locus on a chromosome. Their variations in a particular population can have a combined effect on a discrete phenotypic character. The individual genes have a small quantitative effect, but together produce a wide range of phenotypical variation. Also called, multiple factor quantitative genes. See Behavior genetics, Gene, Chromosome, Epistatic/epistasis, Phenotype, …