190 



FUNDAMENTALS OF CYTOLOGY 



differential factor; under other conditions the environmental factor may 

 be differential; under all conditions both factors share in determining 



what the organism does. It should be e\'ident 

 that the same principle holds for inorganic sj's- 

 tems. Thus the behavior (curved flight) of a 

 pitched baseball depends upon the combined 

 influences of a constitutional factor (rotation) 

 and an environmental factor (air resistance). 

 The decisive role of differential factors in the 

 chromosomes is strikingly shown in gynan- 

 dromorphic insects, in which portions of the body 

 differing in the number of A"-chromosomes differ 

 also in sex and sex-linked characters (Fig. 137). 

 Sex-linkage. — An interesting example of the 

 inheritance of a nonsexual character depending 

 upon differential genes located in the sex chro- 

 mosomes is that of Daltonism, a type of color- 

 blindness that prevents some people from 

 distinguishing properly red from green. The defect occurs in relatively 

 few individuals in affected families and appears less frequently in women 

 than in men because it is both recessive and sex-linked. The chromo- 

 somal basis for its inheritance is illustrated in Fig. 138. 



Fig. 137. — A Drosophila 

 gynandromorph. The left 

 side is female and has two 

 X-chromosomes in its nu- 

 clei. The right .side is male 

 and has only one X-chro- 

 mosome as a result of the 

 loss of the other X in an 

 early embryonal mitosis. 

 {After T. H. Morgan et al.) 



Fig. 138. — Diagram illustrating the inheritance of red-green color blindness, when (A) 

 the father, or (B) the mother, or (C) both parents manifest the defect. 9, female. 

 cf , male. Recessive (defective) gene indicated by dot above X-chromosome carrying it. 

 Individuals manifesting the defect indicated by triple outlines. 



The gene responsible for the defect when in the recessive state is 

 located in the X-chromosome ; hence females may have either one or two 



