69 THE AMERICAN NATURALIST [VOL. XLIX 



Pure line (Not included in the dictionary.) 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 homogeneous (a homozygcus biotype or a clone) without 

 regard to their method of reproduction. 



Repulsion. (Not given a genetic definition in the dictionary.) 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. ' ' 



Sex-limited inheritance. (Not defined in the dictionary.) The associa- 

 tion of the determiner for any unit-character, with a sex-determiner, in such 

 a manner that the two determiners are either generally included in the same 

 gamete, or that they are generally included in different gametes. This 

 method of inheritance is also called " sex-linTced" inheritance by Professor 

 T. H. Morgan and his students. 



Segregate. ' ' To become separated from the rest ; specif., of Mendelian 

 hybrids, to separate, by a numerical law, into dominants, hybrids and re- 

 cessives. ' ' 



Segregate. With reference to Mendelian unit-characters, to become sep- 

 arated through the independent distribution of the genetic factors before or 

 at the time of the formation of the gametes. 



The dictionary definition goes too far; the formation of domi- 

 nants, hybrids and recessives depends not alone upon the fact that 

 the factors segregate, but that the segregated factors recombine. 

 The word ' ' segregation " receives a fairly satisfactory definition. 



Unit-character. (Not included in the dictionary.) In Mendelian in- 

 heritance a character or alternative difference of any Tcind, which is either 

 present or absent, as a whole, in each individual, and which is capable of 

 becoming associated in new combinations with other unit-characters. 



I have made no systematic study of the definitions of technical 

 terms in other related fields, but have noted incidentally that 

 there is no recognition in the New Standard Dictionary of the 

 generally familiar usage of the words "meristic" and "sub- 

 stantive" as applied to types of variation. 



G. H. SHULL 



