Genetics 41 



gether in the first meiotic prophase, the chromatids break and the 

 broken ends subsequently join. If nonsister chromatids are joined, 

 then crossing-over has occurred. Crossing-over does not occur with' 

 equal frequency along the length of the chromosome. It is rare or 

 absent near the centromere and at the very ends of the chromosomes. 

 Near large blocks of heterochromatic material, crossing-over also is 

 reduced. In some organisms, crossing-over is effectively restricted 

 to certain parts of the chromosomes; in others, it seems to occur 

 rather evenly throughout the length of the chromosome arm. Per- 

 haps the diflFerence lies, in part at least, in the amount and distribu- 

 tion of heterochromatin. Occasionally crossing-over is suppressed 

 entirely, as in male Drosophila and female silkworms (Bomhyx 

 mori). The occurrence of a crossover also interferes with the forma- 

 tion of a crossover immediately adjacent. Interference may be meas- 

 ured by studying linkage, and it can be shown to vary along the 

 chromosome and between diflFerent chromosomes. Probably inter- 

 ference is also a structural phenomenon. 



If two factors are located some distance apart on the chromosome, 

 it is possible that more than one crossover will occur between them. 

 If double crossing-over takes place, the effects on recombination 

 depend upon which chromatids are involved (Fig. 3.1). If the same 

 two chromatids of the four associated in the bivalent are involved 

 in both crossovers, the occurrence will not be detected unless a third 

 factor located between the original two is also observed. Should the 

 other two chromatids experience the second crossing-over, each 

 chromatid will have one crossover. This crossing-over may be re- 

 ferred to as two-strand and four-strand exchange, respectively. Three- 

 strand exchange results in the formation of a noncrossover chromatid 

 and three chromatids with a single crossover. In general with mul- 

 tiple crossing-over between two factors, the resulting chromosomes 

 with an even number of crossovers and those with no crossovers 

 will appear as parental types. Chromosomes with an odd number of 

 crossovers between the factors in question will be recombinant types. 

 Since the number of chromosomes with recombinations equals the 

 number with no or an even number of crossovers, recombination 

 cannot exceed 50 percent. 



Crossing-over is influenced by environmental factors, such as 

 temperature, and is also under genetic control. This genetic-control 

 mechanism is rather complex and not well understood. Apparentlv 

 genes determine the length of time available for synapsis (which 

 affects crossing-over) and the localization of crossovers; there are 

 genes that have the effect of preventing synapsis altogether. Genetic 

 disturbances of other components of meiosis would be expected also 



