SPECIAL PROBLEMS OF MEIOSIS 



71 



been possible to show that this is a ' gcnetical 

 illusion ' — in the actual physical chromosome the 

 genes are approximately evenly spaced. The central 

 region is shortened on the genetic map while the 

 part between this and the ends is artificially magni- 

 fied. This means that crossing-over takes place 

 more often in this region than in a region of the same 

 length near the spindle attachment ; there is a 

 partial localization of chiasmata. 



In a number of organisms — Fritillaria (Liliaceae), 

 Mecostethus (Orthoptera, Acridiidae) and many Tetti- 

 goniidae — another type of localization is found in 

 which chiasmata are entirely restricted to certain 

 regions of the chromosomes (usually the region next 

 to the spindle attachment). Thus in the grass- 

 hoppers of the genus Mecostethus there is usually a 

 single chiasma in each bivalent which is situated 

 very close to the spindle attachment. ^^^ In this, 

 which is an extreme case, only those genes which lie 



TABLE VI 



Chiasma-frequencies and Genetic Length of the Ten 

 Chromosomes in Maize {Zea Mays)^^ 



