214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



ciations of whole chromosomes. A second class of mutants are 

 caused by changes in the composition of individual chromosomes. 

 The members of synaptic pairs of chromosomes sometimes twist 

 round each other, break and reunite so that i^ortions of chromosomes 

 become interchanged ; this is known as " crossing over " ; or por- 

 tions of a chromosome may become detached and united to another 

 chromosome, which is known as " translocation " ; such changes in 

 the composition of chromosomes lead to many complicated mutations 

 which, for lack of time, cannot be described here. 



All changes in the numbers or constitution of chromosomes are 

 known as chromosome mutations, or better, permutations. Another 

 and perhaj)s the most important class of mutations are those caused 

 by changes in the positions or structures of the ultramicroscopic 

 genes which lie in the chromosomes. Such mutations have been 

 found in almost all animals and plants that have been bred in large 

 numbers under experimental conditions. The most used animal for 

 these experiments is the little vinegar fly, DrosophiJa mel an og aster. 

 Indeed in the field of heredity and evolution this is the most famous 

 animal in the world, and the man who has been the leader in its 

 study. Prof. T. H. Morgan, has recently received the Nobel Award 

 in recognition of the importance of his work. Scores, if not hun- 

 dreds, of different workers have been engaged in the intensive study 

 of this little gnat and they are sometimes facetiously called Droso- 

 philists or modern worshippers of Beelzebub, the god of flies. The 

 peculiar advantages of this animal for the study of heredity and 

 mutation are: (1) the ease with which it can be kept and bred in 

 great numbers in milk bottles, (2) the fact that a new generation 

 can be obtained every 12 days, (3) the large number of hereditary 

 characters that can be recognized superficially, (4) its relatively 

 small number of chromosomes, 4 pairs, that can be readily distin- 

 guished one from another, (5) finally more than 500 mutations have 

 been found in some 25 millions of these animals that have been 

 studied during the past 25 years. These mutations affect every part 

 of the fly, such as color and form of body, wings, eyes, bristles, length 

 of life, viability, liability to disease, etc. By several ingenious meth- 

 ods, which time does not permit me to describe, it has been possible 

 to locate the particular genes that have undergone mutation in par- 

 ticular chromosomes and even in particular regions of those chromo- 

 somes, so that chromosome maps have been constructed giving the 

 locations of these mutant genes in the different chromosomes.^ 



^ Since this lecture was given, very important discoveries liave been made by Painter 

 (1934), Muller, and Bridges on giant chromosomes in the salivary gland cells of Droso- 

 phila. These giant chromosomes are more than 100 times as large as the usual ones and 

 are marked by cross-bands which correspond in position to certain known genes and thus 

 they constitute visible chromosome maps. Some mutations of Drosophila are evidently 

 caused by changes in the positions of genes in relation to other genes (Muller, 1934), 

 whereas others are caused by changes in the genes themselves. 



