Section 7 — Cytogenetics 



large middle deficiencies (t° and t" 1 groups) of 

 terminal deficiency (t 12 ). The spatial order — 

 deficiencies on the chromosome fits well with 

 the pattern of action of t-alleles which, when 

 homozygous are embryonic lethals, and indicates 

 that genes involved in different embryonic pro- 

 cesses are grouped in different regions of the 

 chromosome of. 



Lethal t-alleles suppress recombination in the 

 region of 8-10 crossover units between T and 



tf; it is suggested that this effect is due to 

 asynapsis accompanying a deficiency and spread 

 on other chromosomal regions. The possibility 

 that in the origin of a new t-allele which is 

 connected with the presence of an old t-allele 

 and with exceptional recombination within the 

 region where recombination usually is suppressed 

 by this allele, effective pairing occurring in the 

 time of chromosome reduplication can be 

 involved, will be discussed. 



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