CYTOGENETIC CORRELATIONS AND CROSSING OVER 107 



being the daughter chromatids of each parental homologue. This as- 

 sociation, called a bivalent, results from synapsis between the homologous 

 chromosomes, which are already double. As the chromosomes thicken 

 and shorten by coiling and begin to separate, it appears that exchanges 

 have occurred between chromatids. Under the microscope one sees 

 chiasmata, so called because they confer upon the separating chromo- 

 somes a cross-shaped appearance. 



The most direct evidence that crossing over occurs at the four-strand 

 stage, when all four chromatids are together, comes from tetrad analysis. 

 (see Chapter 3, p. 73). If a crossover occurs between two linked 

 markers, then four genetically different progeny are recovered, two 

 parental and two recombinant. In the development of genetics, the oc- 

 currence of crossing over at the four-strand stage was established with 

 great difficulty in organisms in which only a single product, rather than 

 all four, is normally recovered from each meiosis. Special Drosophila 

 stocks, similar to the nondisjunctional strain discovered by Bridges 

 (p. 91), in which two X chromosomes are passed from mother to female 

 offspring, provided the basis for what has been called half-tetrad 

 analysis. With this material, some details of the crossing-over process 

 were investigated. 



The principal regularities established in cytogenetic studies of crossing 

 over were that: (1) genetic crossing over involves the physical exchange 

 between segments of homologous chromosomes; (2) the occurrence of 

 genetic crossing over correlates well with the frequency of cytologically 

 observable chiasmata; and (3) there is an apparent randomness in choice 

 of nonsister chromatids which exchange with one another in any one 

 crossover event. Whether there is also exchange occurring between sister 

 chromatids in meiosis is as yet an unresolved question. 



Correlation of Genetic Crossing Over with Physical Exchange 



This correlation was established by Stern with Drosophila, and by 

 Creighton and McClintock with maize, who utilized chromosomes 

 heterozygous for cytological as well as genetic markers. The critical 

 crosses are shown in Figure 4.8; both systems represent classical ex- 

 amples of the clarity and beauty of cytogenetic experiments. 



Correlation of Crossing Over with Chiasma Formation 



Janssen first proposed, in 1909, that each cytologically observable 

 chiasma was the result of an exchange between homologous chromo- 

 somes, with the genetic consequence of recombining linked markers. 



