50 



AN INTRODUCTION TO MODERN GENETICS 



loci in question. If the chance of a breakage is the same at all points 

 in the chromosome, then the chance of a breakage between loci 

 A and B simply depends on the distance between A and B, and 

 the additive theorem of linkage values is explained by the additive 

 theorem of intervals of length. We shall see later how far the chance 



L 



NON CROSS OVERS 



CROSS OVERS 



cr 



IL IL 



Ti 



carnation 

 bar 



non-carnation 

 non-bar 



Fl 



non-carnation 

 bar 



cr 



1 



L 



carnation 

 non-bar 



Fig. 17. Stern's Proof that Recombination involves Chromosome Breakage. 



— A fly (D.melanogaster) was obtained with one X chromosome (carrying carna- 

 tion and Bar) fragmented into two pieces and the other X (carrying the normal 

 allelomorphs) with a piece of the /attached to it. This fly was crossed to a carnation 

 non-Bar male. The diagram shows the males obtained in the F^ ; where crossing- 

 over has occurred, and carnation becomes separated from Bar, the attached piece 

 of Y has been broken off from the whole X and attached to the fragmented X. 

 The same conclusion could be drawn from the F^ females, which are not shown. 

 The y chromosome is drawn L-shaped. 



of a break is really the same at all points (p. 97). Here we may note 

 that Stern^ has conclusively demonstrated that the formation of a new 

 combination does actually involve a breakage and rejoining, which is 

 called a crossing-over; his proof depends on being able to label both 

 ends of two chromosomes both cytologically and genetically, and 

 showing that when a cross-over occurs the chromosomes exchange 

 ends. 



The additive theorem of linkage values is thus explained on the 

 hypothesis that the genes are arranged in a linear order along the 



^ Stern 193 1. Similar evidence was found in maize by Creighton and 

 McClintock 193 1. 



