DEFINITIONS 297 



Mutant. An individual possessing a genotypic character differing from 

 that of its parent or those of its parents, and not derived from them by a 

 normal process of segregation. 



Mutate. To undergo a change in genotypic character independently of 

 normal segregation. 



Ovule. Female sex cell with its immediate surrounding parts. 



Ovum. Egg cell. 



PI, Po, etc. The 1st, 2nd, etc. generation of the parents. 



Palea. The upper of the two bracts immediately enclosing each floret in 

 grasses. 



Panicle. A compound inflorescence with pediceled flowers usually loose 

 and irregular, as in oats, rye, proso, etc. 



Pedicel. A stalk on which an individual blossom is borne. 



Peduncle. The primary stalk supporting either an inflorescence or a 

 solitary flower. In grasses the uppermost internode of the culm. 



Pericarp. The matured wall of the ovary. 



Phenotype. The apparent type of an individual or group of individuals, 

 i.e. the sum of the externally obvious characteristics which an individual 

 possesses, or which a group of individuals possess in common; contrasted 

 with genotype. 



Presence and Absence Hypothesis. The hypothesis that any simple 

 Mendelian difference between individuals , results solely from the presence 

 of a factor in the genotype of the one individual, which is absent from that of 

 the other. Presence and absence of unit-differences as a convenient method 

 of describing the results of genetic experiments should be carefully distin- 

 guished from the presence and absence hypothesis. The method is purely 

 objective and entirely free from hypothetical implications. 



Probable Error. A measure of accuracy for results obtained by statistical 

 methods. The chances are even that the true value lies within the limits 

 marked by the probable error. 



Probable Error of a Single Determination. S. D. X 0.6745. 



Probable Error of a Difference. The square root of the sum of the 

 squares of probable errors of the two results, or the probable error of a 

 single determination multiplied by the \/2. 



Pubescent. Hairy in a general sense; in special use, covered with short, 

 soft hairs. 



Pure Line. A group of individuals derived solely by one or more self- 

 fertilizations from a common homozygous ancestor. Sometimes erroneously 

 applied to groups of individuals believed to be genotypically homogene- 

 ous (a homozygous biotype or a clone) without regard to the method of 

 reproduction. 



Recombination. Union of parental factors in individuals of the second or 

 later generations after a cross. 



Reduction Division. That in which homologous chromosomes separate 

 preparatory to formation of gametes. 



Repulsion. Such a relation between two genetic factors that both are 

 not, as a rule, included in the same gamete, referring especially to cases in 

 which the factors in question give rise to obviously different characteristics; 

 also called "spurious allelomorphism." 



