CHROMOSOMES AND SEX 393 



are located in the Z-chromosome, but certain lethal effects of these 

 genes result in atypical ratios. A gene for "abnormal" may be present 

 in either the X or the Y (Winge). 



Crossing-over is often rare or absent altogether in individuals hetero- 

 zygous for the sex factors. In Drosophila it is normally only in females 

 (XX) that crossing-over takes place: new combinations resulting from 

 this process occur in the eggs but not in the spermatozoa, a fact which 

 greatly simplifies genetic analysis in this organism. The lack of crossing- 

 over between X and Y is doubtless associated in some way with their 

 wide difference in constitution and function, particularly with the large 

 amount of genetically "inert" material in them.^^ Lack of crossing-over 

 in the accompanying autosomes as well indicates the action of general 

 crossover suppressors. The common fowl, in which the female is hetero- 

 zygous for sex (XY), shows crossing-over only in the male (XX). On 

 the other hand, the genetic evidence indicates that in the fish genera 

 Aplocheilus and Lebistes crossing-over occurs frequently between the X 

 and Y. Thus it was observed that a certain character showing "one- 

 sided masculine inheritance," presumably because of a gene in the 

 F-chromosome, suddenly began to behave as though the gene had been 

 transferred to the X. In a later generation it was apparently returned 

 to the F by a second crossover (Winge, 1923a). In the related genus 

 Platypoecilus, which is heterogametic in the female, there is evidence of 

 crossing-over between the X and F (Gordon, 1927). In many organisms, 

 crossing-over occurs regularly in both sexes. It seems probable that these 

 are forms with less highly differentiated sex mechanisms and that lack of 

 crossing-over in forms like Drosophila is of value in preserving the state of 

 sexual heterozygosity. 



In Drosophila the normal allelomorph for the mutant character 

 "bobbed" is carried in the F-chromosome. In Phytodecta any one of a 

 group of four allelomorphic genes (p. 310) may be carried by either the 

 X or the Y.^^ 



The general conclusions regarding the function of sex-chromosomes 

 are amply supported by the results of chromosome aberrations, partic- 

 ularly non-disjunction.^° In all such cases "an abnormal distribution of 

 the sex-chromosomes goes hand in hand with an abnormal distribution of 

 all sex-linked factors" (Morgan). 



S8 MuUer (19146, 1918), Muller and Painter (1932). 



^^ Drosophila: Stern (1926a). Phytodecta: Zulueta (1925; see Morgan, 1926a); 

 Galan (1931) on chromosomes. 



^^ Non-disjunction of the two X-chromosomes in a normal female is called "pri- 

 mary" non-disjunction; the separation of XX from Y, or XF from X, in an XX Y 

 female is called "secondary" non-disjunction. Non-disjimction is also termed 

 "reductional" or "equational" according as it occurs in a disjunctional or an equa- 

 tional meiotic division. The frequency of non-disjunction in Drosophila can be 

 markedly increased by X-rays (Mavor, 1921 et seq.; E. G. Anderson, 1924c). 



