CHAPTI]R LXXIII 



INHERITANCE IN ORGANISMS 



GENETICS 



It is a fact old in human experience that resemblances appear between 

 parents and children, and it is to this fact that the adage "like father, 

 like son" refers. Unscientific man overlooked the differences since they 

 did not seem to him significant, but it has been learned that heredity 

 involves inheritance of both resemblances and differences. The science 

 which deals with inheritance is known as genetics. 



590. Organisms from the Genetic Viewpoint. — An organism exhibits 

 a number of inherited physical characteristics involving size, form, color, 

 character of body covering, structure of internal parts, and so on. In 

 higher animals these become very numerous. It is found, however, that 

 organisms may inherit and may pass on potential characteristics which 

 do not appear in the individual. It has been found that some visible 

 characteristics correspond to one factor in inheritance, some represent 

 two, and still others are the resultant of three or more. Every somatic 

 cell of a metazoan possesses potentially all of the characteristics which 

 belong to the organism, but some cells display certain of these, other 

 cells others of them, and all combined comprise the characteristics of the 

 animal. These characteristics may be other than physical, such as 

 abilities, modes of behavior, and even particular instincts, but these are 

 all based upon structure which is inherited. 



591. Determiners or Genes. — In Chap. XI it was noted that the 

 splitting of the chromosomes in cell division had been interpreted as 

 implying the equal division of units arranged in a longitudinal manner 

 along the chromosome thread, these units corresponding to the charac- 

 teristics of the organism. This interpretation has been supported by 

 many subsequent observations, and it is now accepted as a fact that the 

 split chromosome consists of two series of units, those of one daughter 

 chromosome being opposite those of the other, and each pair resulting 

 from the division of one unit in the chromosome of the parent cell. Thus 

 for each unit in one of the daughter chromosomes there is a similar unit 

 in the other. These hypothetical units are known as determiners, fac- 

 tors, or genes, and these form the physical basis for the visible charac- 

 teristics, which are known as characters. There is considerable difference 

 of usage with respect to these terms — determiner, gene, character, and 



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