GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 159 



exchanges of substance between maternal-derived and paternal-de- 

 rived chromosomes, and thus establishing the cytological basis of 

 the phenomenon of linkage. The four gametes formed by the 

 extra (equational) division following upon reduction division, are 

 thus usually different. 



Variations in Chromosomal Behavior 



The commonest of these variations is crossing-over, illustrated 

 in Figure 19, which occurs when two chromosomes of a pair twist 

 around each other in some stage of germ cell formation (meiosis). 

 Less common forms are illustrated in Figure 20: inversion, when 

 a block of genes breaks loose from a chromosome and returns 

 to that same chromosome "upside down"; translocation, in which 

 a loose gene block joins another chromosome; deletion or defi- 

 ciency, in which the loose gene-block is lost. Heritable variations 

 produced by x-rays often involve chromosomal variations. 



A great impetus was given genetics in 1910, when Professor T. H. 

 Morgan and his school began work with a tiny yeast-eating "fruit 

 fly," which matures in about 10 days and lives only about a month 

 (Drosophila melanogaster). About thirty or more prolific generations 

 can be cheaply raised in a year, in quart milk bottles in the laboratory, 

 quite independently of weather conditions. The breeding and cross- 

 ing of strains is readily controlled. There are many recognizable 

 variations, and a biologist can live long enough to make large numbers 

 of "Mendelian" experiments, not possible where the life cycle is long 

 or the progeny few. With elephants the gestation period is 22 months, 

 and elephants do not produce litters used to count Mendelian 

 averages. 



From the strength of linkage between genes, it became possible to 

 construct a map of the chromosomes indicating the relative position 

 of many genes. Figure 21 is a "gene map" prepared by C. B. Bridges; 

 the practical meaning of a few of the characteristics identified in the 

 "shorthand" of geneticists, is shown in Figure 22. 



A large variety of plants and animals, including microorganisms, 

 are now being used in genetic experiments, and besides the visible 

 characters or differences which so readily meet the experienced eye, 

 many latent or hidden chemical differences are being explored. Thus 

 by raising microorganisms in media with carefully controlled chemical 

 content, it has been possible to learn something of the synthesizing 

 properties of their genes, as shown in the work of Prof. George W. 

 Beadle and collaborators of Stanford University with the red bread 

 mold Neurospora crassa. 



