CYTOGENETIC CORRELATIONS AND CROSSING OVER 97 



tailed investigations of these processes became possible after the demon- 

 stration that X-rays could break chromosomes. One of the most vivid 

 demonstrations of chromosome fragility was provided by McClintock's 

 analysis in maize of a repeating cycle of breakage-fusion-bridge forma- 

 tion in the outermost cell layer (aleurone) of the endosperm (that 

 part of the corn kernel in which starch is stored). 



Many of the studies were carried out with chromosome 9 because a 

 number of linked genes are located there which influence kernel devel- 

 opment in an easily observed manner. In the short arm of chromosome 

 9, the following order was established: terminal knoh-Yg-C-Sh-Wx- 

 centromere. Yg is a plant color gene; C affects pigment formation in 

 the aleurone layer, c giving a colorless phenotype; Sh determines normal 

 endosperm development, sh giving shrunken endosperm; Wx determines 

 formation of normal starch which stains blue with iodine, and wx deter- 

 mines a mutant waxy form of starch which stains red with iodine. 



Figure 4.2a shows a normal chromosome 9 and a rearranged chromo- 

 some obtained after X-irradiation. When these chromosomes pair at 

 meiosis, one finds the kind of configuration shown diagrammatically in 

 Figure 4.2/?. The complicated configurations of rearranged chromosomes 

 seen at meiotic prophase have provided some of the best cytological 

 evidence that forces exist which determine a point-to-point localized 

 pairing between homologous chromosomes in meiosis. When crossing 

 over occurs between the paired chromosomes of Figure 4.2^, various 

 types of aberrant products may result, depending upon the location of 

 the exchange. In Figure 4.2c a dicentric (two-centromere) chromosome 

 is shown, which was obtained as a result of a crossover somewhere in 

 regions 1 to 5. When such a dicentric chromosome divides at the next 

 mitosis, the two centromeres move to opposite poles of the mitotic 

 spindle; the connecting bridge of chromosomal material is stretched and 

 breaks, so that at each pole there is now a chromosome with a broken 

 end. This broken end then initiates a breakage-fusion-bridge cycle, of 

 the type shown in Figure 4.3. With the use of genes that are expressed 

 in the endosperm, it is possible to follow the succession of breaks and 

 consequent losses of particular dominant genes. 



In further studies of the breakage-fusion-bridge cycle, it was shown 

 that broken ends fuse 2-by-2 in endosperm tissue, but not in developing 

 plant tissue. There they heal, and if the chromosomal rearrangement 

 is not lethal, it remains stabilized as such. In general, in the endosperm, 

 fusion of broken ends occurs even if one broken end is introduced by 

 one parental nucleus and the second end comes from the other parent. 

 The mechanisms of healing and of fusion have not been established, but 

 the thorough analysis of the behavior of broken ends carried out by 



