284 CARNEGIE INSTITUTION OF WASHINGTON. 



hybrid. Hence the mutant may be used as a dominant with reference to 

 one character, and as a recessive with reference to another. 



Among the new recessives calHng for special mention is rose eye-color, 

 to the left of dichsete. This mutant, along with minute-i, will make it possi- 

 ble to explore the region between hairy and dichsete (15 units long). The 

 males are sterile, but the females are fertile. Another mutant, called lance-b, 

 having narrow wings, is a recessive in chromosome II. It also has sterile 

 males. A third recessive mutant, called parted (because the thoracic hairs 

 diverge in the mid-line) has sterile males. The genitalia are rotated through 

 180° from normal. A fourth mutant called deltex (delta-in-X) looks like 

 the dominant delta in chromosome III, but is a recessive in the X-chromo- 

 some, lying between crossveinless and cut. 



These and other mutants have been used in improving the maps. Some of 

 the old loci have been more accurately placed. Thus, warped, instead of 

 being to the right, is to the left of pink; and curled, instead of to the left is 

 found to be two units to the right of pink. This region to the right of pink is 

 peculiar with respect to crossing-over, and the position of curled will make it 

 useful for further study of this region. 



Several new multiple stocks, including some of these new mutants, will 

 make it easier to work with almost any new problem that arises. This is 

 especially true for chromosomes III and I. In chromosome I a set of stocks 

 has been made with alternating loci that will permit a study of this entire 

 chromosome in almost any degree of detail. Other stocks have been freed 

 from characters that in the past have limited their usefulness. 



Balanced stocks have been found advantageous in keeping certain mutants, 

 which, because of sterility or low viability or the lethal nature of the homozy- 

 gote, could not be maintained without selection of particular types of parents 

 in each generation. 



The mutant used in balancing must be itself a type that is lethal when 

 homozygous, or poorly viable, or sterile. The absence of crossing-over, 

 that is also essential for balancing, may be due either to the closeness in the 

 chromosome of the genes concerned, or to the presence in one or the other of 

 the homologous chromosomes of a dominant crossing-over suppressor. A few 

 examples of the way in which balanced stocks are prepared will serve to 

 illustrate some of the advantages of the Drosophila material. Minute-h is 

 lethal when homozygous. Its locus is within two-tenths of a unit from that 

 of dichaete, which is also dominant and lethal when homozygous. The latter 

 can be used to balance minute-h when the two genes are in opposite chromo- 

 somes. Practically all the flies in such a balanced stock will continue to be 

 both minute and dichsete without selection. Half the offspring from an 

 outcross will be minute-h, the other half dichaete; and since the balancer 

 (dichsete) has been separated from the balanced (minute-h) either form can be 

 used as if it had come from homozygous stock. 



The new mutant rose, whose males are sterile, has been maintained in 

 balanced stock by the presence in the opposite chromosome of a new crossing- 

 over suppressor with an associated lethal. 



It may be also worth while to show by way of illustration how the possession 

 of balanced stocks makes it possible to work out the inheritance of certain 

 characters which would otherwise present unusual difficulties. The new 

 mutant, no-wing, has sterile females and rarely fertile males. Only a small 



