268 



CHAPTER 30 



BRACHY T 



BRACHY T+ 



50% 



25% I ^p-i^ 



25% 



die 10^4 days 



MONSTER TT 



n 



days 



16 

 days 



^ 



birth 



BRACHY T+ 



NORMAL + + 

 (Size Reduced) 



FIGURE 30-5. Brochyiiiy in the house mouse. {Courtesy ofL. C. Dunn ; re- 

 printed hy permission of McGraw-Hill Book Co., Inc., from Study Guide 

 and Workbook for Genetics by I. H. Herskowitz. Copyright, 1960.) 



in metabolism (starvation of the Creeper eye) 

 or by means of (2) specific chemical messen- 

 gers (mouse pituitary hormone). Tissues, 

 being acted upon at a distance, can have their 

 competence changed for genetic reasons 

 (homozygous Creeper limbs). It was also 

 found that adjacent tissues, which interact as 

 induction systems, can have their diff'erentia- 

 tion modified by changes in their response to 

 inducing agents (nonresponsiveness of meso- 

 derm to presumptive notochord in homo- 

 zygous Brachy), and, although no example of 

 this was described, probably, also, by changes 



in inducing capacity. It should be realized, 

 however, even though differentiation during 

 the development of higher multicellular or- 

 ganisms involves intercellular interactions of 

 all the types mentioned, that some cellular 

 traits are produced solely through the intra- 

 cellular action of the genotype. This is clear, 

 for example, from the phenotypic effect of 

 mutants induced during embryology which 

 may be detrimental or lethal to the cells con- 

 taining them, and from traits, as in Drosoph- 

 ila, showing phenotypic mosaicism in direct 

 correspondence with genotypic mosaicism. 



