INTERGENIC AND INTRAGENIC CHANGE 



Changes of balance likewise mimic gene differences in Dwsophila. 

 Notched wings and Minute bristles are produced by deficiencies. 

 They can usually be recognized in two ways : by the absence of from 

 one to ten bands from the polytene chromosomes, and by the 

 absence of the allelomorphs of particular genes as shown by the 

 failure to "cover" a recessive in breeding experiments. Some 70 

 varieties of Notch and 80 of Minute, both spontaneous and induced, 

 have been described; all the Notches are at the end of the X chromo- 

 some while the Minutes may lie in any of the different chromosomes. 

 Minutes seem to be lethal when homozygous; Notches are lethal 

 to the male (which has no allelomorph to them in the Y) and they 

 camiot, therefore, be obtained homozygous in the female. All these 

 deficiencies are dominant (for which reason they are given capital 

 letters in the Drosophila usage*) and are known by the phenotype 

 they give in the heterozygotc. 



Thus a deficiency is recognizable in breeding by its drastic effect 

 and its segregation as an allelomorph of a group of genes. In these 

 two respects duplication is similar, but of course its effect is less 

 drastic and its relation to other genes is the reverse. Deficiency 

 "uncovers" a recessive allelomorph. Duplication "covers" even a 

 homozygous recessive. Duplications also resemble deficiencies in 

 usually affecting the bristles and the wings. The most interesting 

 of them, however, is the narrow-eye or Bar factor in Drosophila 

 (Fig. 21). 



For 25 years Bar was regarded as a simple gene-difference in 

 the X chromosome. It was odd merely in its property of unequal 

 crossing-over. Homozygous Bar females sometimes gave progeny 

 in which the Bar gene was replaced either by the normal allelomorph, 

 or by a new one of greater strength (giving still smaller eyes) called 

 Double-Bar. Finally the polytene chromosomes showed Bar to 

 have two like pieces side by side instead of one, as it were ABBC 

 instead of ABC. Then unequal crossing-over in the homozygote 

 ABBC/ ABBC reconstructs the chromosomes, so as to give the types 

 ABC and ABBBC detectable genetically and cytologically in the 

 next generation. 



A remarkable incidental observation in these experiments was that 

 the heterozygous combination, ABCjABBBC, although the same 



* Sec Appendix 2, on Symbols and Symbolism. 

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