CHROMOSOMES IN GENES 



63 



exactly the same as that which we must 

 draw now for all actions of chromatin re- 

 arrangements (position effects). For all 

 these the normal allele is not a gene, but 

 the normal pattern of the chromosomal sec- 

 tion in question or of the whole chromosome 

 as the case may be (see the following exam- 

 ples). The normal allele, which is an ex- 

 trapolation from the existence of the mutant 

 locus, is, then, not existing in cases of 

 position effect, and not existing at all if 

 mutation should turn out to be position- 

 effects altogether, as I am expecting. 



Let us take now another case elaborated 

 by Muller and others involving a standard 

 recessive "gene" scute, which changes the 

 bristles of the fly in definite ways. Here 

 again a number of alleles exist, quite a few 

 of which have been shown to be inversions, 

 very small and very large ones, with one of 

 the breaks in the neighborhood of the scute 

 locus at different points, and one also a 

 translocation. There is a certain amount 

 of specificity of action for this group of 

 similar inversions, but, generally speaking, 

 one might say that whenever a break oc- 

 curs in this chromosome segment, the 

 effect is a reduction of bristles. There 

 is now a nearby locus, called achaete, 

 which also reduces bristles, and another one. 

 Hairy-wing, which increases bristles. The 

 latter has now been found to be a small du- 

 plication, comparable to Bar (Alekhanian, 

 Demerec and Hoover). In other words, 

 there is a rather small segment of the X- 

 chromosome which influences adversely nor- 

 mal bristle development, whatever happens 

 to it in the form of pattern changes. 

 (* 'Pattern" in this discussion always means 

 linear pattern along the chromosome, which 

 is changed by breaks and their conse- 

 quences, in form of inversion, translocation, 

 or duplication of parts.) I may add at 

 this point that it is typical for the genetic 

 structure of the chromosomes in Drosophila 

 that at many points (Jones has counted 

 over 50) clusters of a number of mutant 

 loci are found which have a similar effect 

 upon the phenotype, just as in the case 

 under discussion. But only a few of such 

 chromosomal sections of different size have 



thus far been studied in regard to the action 

 of breaks, as was done in the example just 

 reported. (Another such section is near 

 yellow. ) 



A frequent t5^pe of genie action is the 

 enhancing or inhibiting effect of so-called 

 modifiers. Here is an example for a par- 

 allel position effect. The so-called domi- 

 nant mutant Beaded in Drosophila produces 

 a slight nick on the wing-margin in only a 

 few per cent of the individuals. If an in- 

 version is present in the sister chromosome, 

 whether long or short, near or away from 

 the Beaded locus, the Beaded action is en- 

 hanced to high grade scalloping in up to 

 100 per cent of the flies (Goldschmidt and 

 collaborators). Here the neighborhood of 

 the genes is not involved at all, but a gen- 

 eralized effect upon development which is 

 caused by different changes of pattern in 

 another chromosome. In this case the 

 sister chromosome is involved. But there 

 are also cases where the effect is produced 

 by rearrangements in non-homologous chro- 

 mosomes. There are, for example, in the 

 second chromosome of Drosophila a number 

 of alleles producing a definite effect upon 

 wing venation, some of which are known 

 deficiencies. The effect of each of these 

 separately, as well as of their compounds, 

 is considerably enhanced if a translocation 

 from the third to the first chromosome is 

 present (Goldschmidt and collaborators). 



Another frequent type of modifying 

 "genes" are those which affect the domi- 

 nance of a definite locus. Dubinin found 

 that the recessive action of cubitus inter- 

 ruptus in the fourth chromosome of Dro- 

 sophila becomes dominant in the presence of 

 any translocation between this and the 

 Y-chromosome. We found that the reces- 

 sive locus vestigial becomes more or less 

 dominant in the presence of many defi- 

 ciencies, inversions and translocations in 

 other chromosomes under certain conditions 

 (Goldschmidt and Gardner). 



Only one more rather strange position- 

 effect may be mentioned here. A mutant, 

 mottled (eyes and pigmented membranes) 

 was found by Muller in Drosophila. It has 

 been shown since that this mottling is 



