620 GENETICS IN RELATION TO AGRICULTURE 



Recombination. The uniting of parental factors in individuals of the second or 

 later generations after a cross. 



Reduction Division. One of the last two divisions in gametogenesis, when ho- 

 mologous chromosomes are dissociated and pass into different gametes; the hetero- 

 typic division, meiosis, the mechanism of segregation. 



Recessive. The opposite of dominant. 



Reciprocal Hybrids. Hybrids the sexes of whose respective parents are reversed. 



Regression. In biometry, the avarage variation of one variable for a unit varia- 

 tion of a correlated variable. 



Reversion. The appearance of a distantly ancestral character in an individual, 

 as the production of 'purple-flowered sweet peas by crossing two whites. 



Scurs. Abortive horns. 



Segregation. The process by which genetic factors become dissociated to different 

 gametes by the mechanism of the reduction division. 



Sex-chromosome. The accessory chromosome which has come to be associated 

 with one or the other sex, or one member of a pair of morphologically or physiologically 

 distinct chromosomes which carry a factor or factors for sex. 



Sex-linked. Applied to factors located in the sex-chromosomes or to the charac- 

 ters conditioned by them. 



Sex -ratio. The proportion of males and females in a population. 



Sire. The male parent of a beast; generally with dam as the female parent. 



Soma. Body; especially in contrast with the germ or germ-plasm. 



Somatic. Pertaining to the body as contrasted with germinal which pertains to 

 the germ cells. 



Somatic Segregation. Appearance of genetically diverse tissues in the same indi- 

 vidual due usually to mutation in a somatic cell or, possibly, rarely to chromosome 

 aberrations, or, in a few questionable cases, to some unknown cause. 



Somatogenic. Originating in the soma. 



Somatogenesis. See Development. 



Species. That which is specialized or differentiated recognizably from anything 

 else of the same genus; collectively those individuals which differ specifically from 

 all the other members of the genus and which do not differ from one another beyond 

 the limits of (actual or assumed) individual diversity. 



Spermatozoon (pi. Spermatozoa). A mature male sex cell in animals. 



Spindle. The nuclear division figure. 



Sterility. Lack of ability to produce normal, living young; the opposite of fertility. 



Stereochemistry. A branch of chemistry which considers the spatial arrange- 

 ment of the atoms composing a molecule. 



Stirp. As contrasted with body or soma, the germ or germ-plasm. 



Strain. A group of individuals within a variety which constantly differ in one 

 or more characters from the variety type. 



Synapsis. Non-technically, the conjugation of maternal and paternal chromo- 

 somes preceding the reduction division. 



Taxonomy. The department of science which embodies the principles of classifi- 

 cation, especially systematic classification of organisms. 



Teratology. The phase of morphology concerned with the naming and classifica- 

 tion of abnormalities. 



Tetraploid. Quadruple the haploid or double the diploid number of chromosomes 

 characteristic of the species. 



Toxin. A poison produced in animal tissues. 



Triploid. Treble the haploid or once and one-half times the diploid number of 

 chromosomes characteristic of the species. 



