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CHAPTER 24 



sive lethals by means of a thousand separate 

 matings with Fi females, as described, ap- 

 proximately two of these matings are found 

 to yield no + sons, so that the recessive 

 lethal mutation rate of 0.2% is a fairly typical 

 one. For each dose of 1000 r of X rays to 

 which the adult male is exposed, approxi- 

 mately 3.1% more sperm are found to carry 

 X-linked recessive lethals. 



The Base technique has certain disadvan- 

 tages. When used as described, it detects 

 only those recessive lethals which cause 100 

 per cent mortality before adulthood. Other 

 recessive lethals that produce sterility or 

 cause death of the adult before it can mate are 

 not detected. No recessive lethals are de- 

 tected unless they are hemizygous in the 

 Fz male, as mentioned, and a considerable 

 number are known to occur whose lethality 

 is reduced by genes normally present in the 

 Y chromosome. 



On the other hand, the advantages and ap- 

 plications of the Base technique are numer- 

 ous. A few can be mentioned here. The 

 presence or absence of + males in Fo is easily 

 and objectively determined. Since the reces- 

 sive lethal which is detected in Fo is also car- 

 ried by the heterozygous-Bar F2 females, this 

 permits further study of the recessive lethal 

 in that generation and subsequent ones. In 

 such studies it can be discovered whether 

 the lethals are associated with intergenic 

 changes; those that are not are designated as 

 recessive lethal point mutants. The Base 

 technique can also be used to detect recessive 

 lethals that occur in a Pi Base chromosome, 

 the absence of Base males among the F2 prog- 

 eny indicating such a mutation. Moreover, us- 

 ing standardized environmental conditions, it 

 becomes possible to detect hemizygous muta- 

 tions which either lower the viability of the 

 males without being lethal or raise their 

 viability above normal. Most important is 

 the additional possibility of studying the 

 viability effects of recessive lethals when in 

 heterozygous condition. 



Although the Base technique may be used 

 also to detect X-linked mutants producing a 

 visible morphological change when hemizy- 

 gous, all those "visibles" which are also hemi- 

 zygous lethals are missed. The ''Maxy"' 

 technique (refer to Figure 24-2) overcomes 

 this difficulty. The males used carry an X 

 chromosome containing two medium sized 

 paracentric inversions, In49 (having no ob- 

 vious phenotypic effect) and B^'^ (having a 

 dominant effect like Bar). The X also carries 

 a minute paracentric inversion (of negligible 

 phenotypic effect) associated with the reces- 

 sive lethal IJl (located to the left of yellow 

 body eolor, y), which is also present. This X 

 also carries the recessive mutant oeelliless (oe) 

 which removes the ocelli (simple eyes). The 

 males are able to survive IJl because they 

 carry the normal allele, IJ1+, as a mutant 

 duplication in the longer arm of the Y chro- 

 mosome. The genotype of the male may be 

 written as: IJl In49 oc B"'/IJ1+.Y. 



One of the female's X's is the same as the 

 one in the male. The second X carries IJl^ 

 and a recessive lethal mutant / (located be- 

 tween seiite bristles, se, and white eye, vv), for 

 which the other X carries the normal allele, 

 /+. This second X also carries the long in- 

 version In 5c-^ sc^ (as in Base), plus reces- 

 sive genes for yellow body color (j'), out- 

 stretched wings and small eye (odsy), echinus, 

 rough eyes (ec), forked bristles (/), singed 

 bristles (sn), dusky wings (dy), cut wings (et), 

 and recessive genes for the following eye 

 colors: carnation (ear), garnet (g), vermilion 

 (v), raspberry (ras), carmine {cm), ruby (rb), 

 white (vv), and prune (pn). The arrangement 

 of all the markers in this X is as follows: 

 y I 5-c- ' car odsy f g dy v ras sn ct cm rb ec w 

 pn sc^. 



Since, by convention, only mutants are in- 

 dicated in the genetic formulae for these X's, 

 you must realize that the 5'^^-containing X 

 carries /+ as well as the normal alleles of all 

 the other recessives in the second kind of X. 

 Similarly, the X carrying the multiple reces- 



