INTERCHROMOSOMAL TRANSFER 25-4 



that some non-specific local factor may be involved which either 

 facilitates transfer or insures the adhesion of the transferred ma- 

 terial to its new position. 



Conversion is obviously controlled by heredity since it occurs 

 with high frequency in some lines (e.g. CIA) and rarely in others. 

 Conversion may result from the nature of the association of the 

 heterozygous alleles at synapsis, either due to the duration of 

 synapsis, the condition (stickiness) of the chromosome or some 

 other general character. I say some general character, for sev- 

 eral alleles are usually converted simultaneously. Since it is 

 genetically controlled, a gene may influence the stickiness of 

 chromosomes or duration of synapsis. Whether or not a convert- 

 ing mechanism is selected in higher organisms may depend upon 

 the advantages accruing to the occasional appearance of recessives. 

 The failure to demonstrate conversions in Drosophila may suggest 

 that recessive alleles have been selected which serve specific func- 

 tions, as Dobzhansky's evidence on seasonal selection of chromo- 

 somal aberrations indicates. The failure to discover recessive 

 Mendelian genes in Sciara may mean that a converting mechanism 

 is operating here which prevents the appearance of recessives. 



