302 INTRODUCTION TO CYTOLOGY 



It is important to note further that crossovers may not occur with 

 equal frequency in all portions of the chromosome. For instance, it has 

 been shown that crossing-over in Drosophila occurs less frequently near 

 the spindle-attachment region than elsewhere. Of interest in this con- 

 nection is the observation in Zea that the chromatids, as they open out in 

 the diplonema stage, tend to remain together and therefore to form fewer 

 chiasmata near the attachment region (McClintock). From such facts 

 it follows that the map, although it shows correctly the serial order and 

 linkage relations of the genes, may not give a true picture of their actual 

 spacing in the chromosome. We shall revert to this topic in the next 

 chapter. 



In interpreting recombination data it is often necessary to consider a 

 phenomenon known as interference. If the genes are arranged in a close 

 linear series in the chromatid, and especially if chiasmata account for 

 crossing-over, it might be expected that when crossing-over occurs between 

 any two pairs of genes, the near-by pairs would show no crossing-over, 

 since the physical conditions might prevent the formation of two chias- 

 mata or of two breaks very near each other. It has been found that the 

 data often conform with these expectations. The recombination per- 

 centage characteristic for genes in a given region of the chromosome is 

 noticeably lowered when crossing-over occurs at a near-by point. This 

 interference with crossing-over varies from a high value for closely linked 

 genes to zero for very loosely linked ones (Muller, Sturtevant, Weinstein). 

 A further interesting fact is that interference in Drosophila and Zea is 

 weak across the spindle-attachment region; each arm of the chromosome 

 behaves more or less independently in this respect. The frequency of 

 crossing-over has been show^n to be affected in certain cases by age, 

 temperature, sex, and irradiation.* 



Chromatid Exchange as the Mechanism of Recombination. — 

 Many years ago it was suggested that an exchange of some sort occurs 

 between homologous chromosomes during the synaptic period.^ Cyto- 

 logical evidence for an exchange of portions between chromosomes in 

 certain insects was brought forward by Janssens in 1909. Although this 

 evidence was not considered conclusive by many cytologists, chromosome 

 exchange was provisionally assumed to represent the physical basis of 

 recombination by Morgan and his associates in their successful develop- 

 ment of the chromosome theory of Mendelian heredity. Many bits of 

 evidence have pointed toward the correctness of the exchange hypothesis, 

 but a complete demonstration of the fact that two chromosomes actually 

 exchange corresponding portions as the genes in these regions are recom- 



^E.g., Bridges (1915, 1927, 1929) on age; Plough (1917, 1921) and Stern (1926c) 

 on temperature; Mavor (19236), Mavor and Svenson (1923), E. G. Anderson (19256), 

 and Muller (1926) on X-radiation. 



9 Correns (1902), DeVries (1903), Strasburger (1905a), C. E. Allen (1905o). 



