no 



CELL HEREDITY 



o o o o o o o 



• • • 



o- 



(b) 



(c) 



FIGURE 4.9. Diagram of Belting's hypothesis of chromosome duplication, plus the 

 added effects of sister-strand crossing over. 



(o). The linked series of open dots indicate one chromosome (double helix) and the 

 black dots indicate a paired homologous chromosome. According to Belling, the 

 dots represented genes,- on a more modern view, consider the dots as nucleotide 

 pairs. The parental chromosomes are twisted about each other, and new strands 

 form in association partly with one homologue and partly with the other. 



(b). Consequences of this model: only two of the four chromatids can be recombi- 

 nant. To obtain recombination among all four strands an additional step of sister- 

 strand crossing over is postulated. 



(c). An example of the effects of sister-strand crossing over imposed upon Belling's 

 model. 



(o and b after Swanson, 1957, Cytology and Cytogenetics, Englewood Cliffs, 

 N. J., Prentice-Hall, p. 288). 



markers in a functionally unchanged condition. In fact, the reliability 

 of these results provided the great predictive strength of classical genet- 

 ics. Subsequently, using half-tetrads in Drosophila, it was shown that 

 not only did single crossovers occur with a fairly constant frequency 

 between any two genes but also that two-, three- and four-strand double 

 crossovers occurred with the frequencies predicted on the basis of 

 randomness of exchange between any two of the four chromatids. The 

 latter result was referred to in the literature as the "absence of chromatid 

 interference" in crossing over. Unfortunately, the results obtained are 

 not really conclusive because the interpretation involves the assumption 

 that sister-strand crossing over does not occur. 



In 1931, Belling proposed that crossing over occurred at the time of 



