THE LINEAR DIFFERENTIATION OF THE CHROMOSOMES 93 



pairing, since they reduce the fertility of the flies; in some cases the 

 effect is confined to females, in others the fertility of males is also 

 affeaed. 



Many factors are known which have a local effect in reducing crossing- 

 over (so-called C factors).^ These crossing-over suppressors are ex- 

 tremely useful in preparing flies of a required constitution. They are 

 also used to carry on stocks of mutations which are lethal when homo- 

 zygous by means of the balanced lethal technique; if one has a different 

 recessive lethal in each of a pair of chromosomes, and suppresses all 

 crossing-over, the only viable type is the heterozygote which therefore 

 breeds true.^ 



The C factors are probably all inversions and probably obtain their 

 effect by reducing zygotene pairing. The pairing in inverted loops 



Fig. 41. A Balanced Lethal System. — Beaded Bd is a gene in the 3rd chro- 

 mosome of D. melanogaster, which has a dominant effect on the wing margins 

 and is lethal when homozygous. A stock was found in which one 3rd carried Bd 

 and the other carried a recessive lethal 13a and a cross-over suppressor 

 (? inversion) C3a. In this stock only the double heterozygote could survive 

 and it therefore bred true, thus: 



6d//3o, C3a (selfed) 



Offspring 1 BdBd 2 6d//3fl C3a 1 13a C3all3a C3a 



(dies) (dies) 



(p. 100) which inversions show in salivary gland chromosomes is 

 probably very incompletely carried out in the short time available 

 during prophase, though pairing may be more nearly complete in flies 

 heterozygous for very long inversions and crossing-over may then be 

 more nearly normal. Some of the cross-over gametes, however, will 

 probably be inviable (p. 131). 



Crossing-over is also affected by environmental conditions. It varies 

 with the temperature^ and in Drosophila with the age of the fly.* In 

 neither case is the curve of variation simple, and the effect is also not 

 uniform in different parts of the chromosome, the greatest effect always 

 being found in the region of the centromere. The effect of temperature 

 is paralleled by its effect on chiasma frequency.^ X-irradiation also has 

 an effect on crossing-over, again most markedly in the centromere 

 region.^ Crossing-over can indeed be induced in Drosophila males by 

 irradiation, at least near the centromere in the autosomes. "^ It is not easy 



^ Sturtevant 1926a. - Muller 1918. ' Plough 1917. 



* Bridges 1927, 1929. ^ White 1934. 



• Mayor 1923, Muller 1925a. ' Friesen 1936a, 6. 



