HOW DO THE MODIFIERS OF LINKAGE VALUES AFFECT DETACHED PORTIONS OF THE CHROMOSOME? BY WELLINGTON SHANG HSU THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE IN AGRICULTURE COLLEGE OF AGRICULTURE UNIVERSITY OF ILLINOIS 1922 Digitized by the Internet Archive in 2015 https://archive.org/details/howdomodifiersofOOhsuw UNIVERSITY OF ILLINOIS - ^0- - J V — V* f! \ mi iTlwiTtK'liiirn feiovfUJT HQ ym^iavmu \,i ^ \ IP.H 1 ,;i < I.' H'CJ' M I Vi •! ; •,>• I -A : iH 1, 'IHT TAHT V^J.T«:^0^OT EiaiHT ' <3:. 'V : if •-,.. f .Jr~*.i.'? j> 3 'S v 3 t„..L »i:^r^,, 4 ,j^tiaJrn,TM|, , 4 f ^ ih ■ K ■ t ■ - •j: il ji 3 ‘, , ', \ ;• •^\ , . I . .. " ■■ ■ • V t» ...J * W a. ^ ;rj. ) < -i '1^.1 •! ,« 1 A>‘ M :im Hh% Yii aHVOUH^iA tt •-. J jJ-y ,.V'i ^ m •.■■. i*#v' :v{ ,(l ■ ■• v'Hfj * .vv.i.'-. ,^ . ■• \' • r» *r^/ If a ‘ ^ l' > V.WA'IHO aA-)H I A' 0 CONTENTS PAGE I IJaTRODUCTION 1 II MATERIALS AITO METHODS 4 III THB DATA AND DISCUSSION A- REGION l<5b2 8 B- REGION 4 13 IV CONCLUSIONS 13 V ACKNO’IPLEDGIMENT 17 VI literature cited 18 VII TABLES A- TABLE I 3 B- TABLE II 9 C- TABLE III AND IV 14 D - TABLE V 19 E- TABLE VI 21 F- TABLE VII 23 iV I HOW DO THE MODIFIERS OF LIHKAGE VALUES AFFECT DETACHED PORTIONS OF THE CHROMOSOME? I nJTBODUCTION The axnooint of llteratixre that has been published recently bearing on the phenomenon of linkage of hereditary factors makes it unnecessary to give any description of the same at tills time. To eiQ)lain the vest accumulation of data obtained from intelligently planned, and carefully controlled experiments, several theories have been advanced; but as time goes on it is becoming more and more obvious that the chiasmatype theory is the most plausible. According to the chiasmtype theory, Mendelian factors, or genes, reside in a linear series on the chromosomes, and each has a definite locus with its allelomorph occupying the corresponding locus on the other member of the homologous pair of chromosomes. The immediate cause of crossing over is attributed to the breaking of the homologous pair of chromosomes, which twist arour.d each other during synapsis, and the subsequent reunion of them, each taking a piece that formerly belonged to the other. Taking this theory as a working basis, geneticists had accismulated sufficient data to convince themselves, until quite recently, that the crossover value between two Mendelian factors, is constant and invariable* It is the amount of such data and the accepted concept of the linear arrangement of genes on the chromosome that led Morgan to believe that crossover valtie between two linked factors is a fhncticn of distance. Attractive and suggestive as Morgan's postulate may be, it has not been wholly successfhl, how'ever, in piecing itself beyond question. New searches were conducted by different investigators and I i 'l, . V « •J. ' • . L ' ' " . ■ - 2 - th«y have showi that cxx»saover values are variable, being Influenced by age (Bridges *16), t^s^ierature (Plou^ *17), and also certain specific genetic factors (Sturtevant *17), The array of evidence against Morgan's view was amplified by the work of Detlefsen and Boberts (Datlefsen and Hoberts *21) tnho worked on Drosophila, and through selection have been able to reduce a crossover value of 32^ (from white to miniature), to practically zero, in one of tl^e series at the end of the 12th generation. Instead of accepting Morgan's hypothesis of distance, Detlefsen C 2 I) propounds that the linkage value is at least to a large extent "determined by the different possible combinations of uultiple modifying factors." In order to test out this view, he actually performed a hybridization experiment between the low crossover stock and normal flies, A portion of his data is reproduced in Table I. In the table, there ere to be found in the first and second lines the distributions of crossover values of normal populations and generation of series B, In ^42 generation of series B low stock, he mated fifty red long females to normal white miniature males, and the crossover val'ues of these females are shown in the third line. To secure a representative distribution of crossover values of the Fp hybrid females thus derived, he chose four such females fTom each of the forty- four pairs and crossed them to their brothers. The distribution of such a population is given in the fourth line. It is stated that since it is impossible to test out the crossover capacity of the male parents, we mist accept for them the value of series B Fp generation, since they came from a stock which had been used for class work for many years and always found to give a value approximating 33^, The Fg distribution shown here is the total of three distinct and separate F 2 distributions which came from P^^ pairs No. 2,5 and 6. It is almost needless to point out that the F^ distribution lies in between those of the two parents and so do both the value of the mean variate and mean total ^ I ~-Vi' ■ ■ A' ' . '""■ lU"- ' ■ ''' C./« )- '?(PM ■ ‘fft •* -I - /.■ .V*” ■""■ •• 1 1 ■'' ■ V '■ •isfc\!ijr » V ' A. 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'aA4 ,, -■ ! ji ■ ,-..*. li-a; *^*i»-.V a ► o a a o »4 o o C o •H ■*3 a a Eh EJ a ;o Vi a rH +» a R El a *» Vi a a a EH § *> a n a eiO Vi a o El a . ► p << C >» V (3 a a > bO O El El a a Pi Pi a a Vi a El o o a o o a *» o EH a a a El o a El > o o S iH a a a a a > d •n CO m CO in. rH CO in 00 CO in irt CO ft a * u a o %H a 4> ^ o > a a o u a c a o O' O' CO pi CO • 0 a • a Cr» rH CO ip 0 CO rH «0 lO CO CO rH 00 to 00 0 CO 0 vO C- iH 0 rH P» 0 m rH CO' 0 CO a I a in CO rO 0 p»- 1 El rH 0 in «H ^O' 1 0 rH a 0 •fl. CO s ft rH ■*» rH Oe P in •H C 5 rH - u +> in in c a rH r“ ft *d 0 CO a in t- EH rH in tn o» CO ■O' Tf CO CO CO 00 <0* o» tc; CO 00 CO O' -o O' n a a fH •iH cu El C a ,d 0 CO 2 iH a +» d a a 0 Vi rH •rH d d a 0 A a H 0 El a a Oi apL, rH “4r crossover value. The T 2 has a wider range of distribution than that of the but the values of the mean variates and the mean total crossover values is about the same as the Fx, Such features characterising a hybridization eij>eriment vdiere multiple factors with incoB^lete dominance are involved are further confirmed by the increased standard deviation of the Fg distribution. It is this and other genetic analyses that led Detlef sen ( *21) to wonder "Just sdiat part distance between two genes on a chromosome map may play in determining linkage values. " The present work, vdiich is really a minor part of his, was ^undertaken with the hope of throwing further light on the whole problem by attempting to find out how the modifiers of linkage values affect detached portions of the chromosome. Would the separated pieces still give low values, or would they be- have in some other fashion? II MATERIimS mB METHODS. To cope with the coiqplexity of such an experiment, Professor Detlef sen developed a special system of designation Which I shall use in the following description. According to this system, the piece of chromosome between vdaite and miniature is divided into three different regions as shown In Figure 1. Individu- als carrying the different pieces of chromosomes are named accordingly. For instance, individuals carrying the piece of chromosome from red to cross-veined end the rest of the chromosome, comes from the other member of the homologous pair is considered as Region 1. Those carrying the pieces from cross-veined to not-cut is called Region 2, etc. The shaded portion in the figure represents the region outside of selection. With reference to this region a more detailed llscusslon will appear in the latter part of this paper. n lw-f laSSl'u Sirfi m/j’tHtiwt^tlXI: r** „ ^ ' .' „.. ,= ,.K.u».W?e.-.i-.- I - t '’■>«-"<*r' •f- «j .. - .'•ut'.fKa U ("X‘^/.-'V *i.' fiM-1'’' -■• . -■ H? "** ***" s,-' ' V..’ ■.?:V*Vi v V i* %>l^ ■>' i<*h ■ ' ■■•■■“ V‘j. •'•^' A. I J--*. ^ U-v . ^ .‘ij t > ky j :if (:,: ' , -.^.^I'rlctd ^4!) ^ >’ - ' ’■ . . • ' iV :f [•..>.- fe -,i* I ( [•■V* •,*-«' ^ *. ' - . ^ - , . f,.t;--' % 1 n4*'i r7' ', .1 i,- ...... '*r!r**'’^i_ If • ., t ‘ \>\. ' • V . ' li ♦ Vf\-i>^ 4 sa*r -wk;? i .'• ' v'"*^,'V iaMHMrrw t '• * * ^ I . .!• - > ^ jji' ;'■ 'i:iV ,?Vi,:'i c ri- ■..;!Sf.f ;«cr I w-‘-» ,g(iio^^-«^ ^l^ i' (A/ r .*j --i .5. .-.frCMH i^ ■’ ■>■■ •• ■i vtv ;. ;’!!J -.r t,t»^’'‘ ,. .■ ..; •■, .Wit ..»:•• .»?» .*’««to|^r‘'''''' “ ' ■ ' »i '' /. ■':■ . . 'm*gn TB i m . ^ / ■'.£-/ ••'»■' ■ ‘il-J <4, -*’ f ^ , ■. '> , 1, '-. ; • ' !l ' • ^ ‘W i'*.' Vtfiik* ' 'VVWffiJI i: <* Baglon 1 £ 2 18.5 < Region 1 12.2 < Region 2 6.3 < Region 3 16.1 < Region 4 20.4 ./ \ wtoite w croseveinlees:- c cut;« c-^ miniature:- m forked:- f Fig. I. Showing the relative positions of the different factors involved in the experiment, and Uia meaning of "Region". The shaded portion represents the region that is outside of selection influence. ' ilMiyfeiiii ** ' I .f> P L t k-. if!?- f ' V ii ^:: f' L % - 6 - The origin of our experiiTiental materials was as follows: Mr. L. S, CleiQentd crossed normal homozygous recessive females of the zygotic constitution, WjCjChiinif fdiite (eye) cross veinless (wing), cut (wing), and forked (bristle) to a dominant male of the genetic formula, W,C,C'b)^*^ (red, cross-veined, not cut, long, and not forked) which he obtained from Prof. Detlefsen’s low crossover stock F 7 g, As usual he secured homozygous recessive males and dominant females heterozygous for the factors involved:- wcchmf 9 wccbmf i»' -, . 5,,> *‘")^P^ :i:^ ■'- ■ •. . 4- : ,,'V .' , ' ! yi, ,,^<,7^ •1'' /-€ ' ■ .- ■» ■* fi •I*'. ',- ’ ■■•■ . 'll:*" , ,#.rv>, /.'.'• :’" ' ".'I-.' '■': . ‘>‘- '' ^ '■' ’ , V- '■? - '< V- ■ ' ^ ■' ^ \ ,” <. 'ji ti 7' V."^V'. J''i ./«■*. .•>! ■ ■»>. , «gg'j*.~t.*-‘^ A list of the classes of offspring obtained and Heglon 4 females. Parental — Crossover — — Reglo w C Cb^^ w c °b w c Cb w c Cb W c Cb w c Cb w C Cb w c Cb W c Cb w c Cb w C ®b vr c °b w c Cb w c Cb W C Cb w c Cb 1 A 2 W C Cb w c ®b W c «b w C 1 ! o W c T^b w c ®b w c Cb W C _fb Parental Crossovei^-- M F m f f m f M f m f m F m f M F f M f m F from Region 1 A 2 CAT Region 1 Double Crossovers Region 2 -is- Theee eroseovers made up the material of Experiment 1. Experiment 2 had a somvvdiet elmllar origin. In Experiment 2, the male that originated the population was from Eg^ generation of Dr, Detlefsen's low crossover stock and the females to which it was mated came from a quadruple recessive stock v^lch had been subjected to six generations of inbreeding. This quadruple recessive stock was a white, cut, cross veinless, miniature. Our methods of procedure with the Eg segregates of the various regions are the same, for females carrying any given region were mated to the ultimate recessive males. The different classes of offspring resulting from these two crosses are shown in the accompanying chart. Each female was kept with three or four males of the right kind in 8-drachm homeopathic vials. The common method of culture for Drosophila was followed. Elies were r^sooved to new vials every other day and a new supply of males was introduced to take the place of the dead ones, should deaths occur. Nine days, from the day when the first offspring emerged, were allowed for the last one to be hatched out in each vial, and three countings were made during the nine days. All calculations were done twice on one of the best calculating machines on the market, end it is hoped that arithmetical errors have been reduced to a minimum. Ill DATA AND DISCUSSION. A - Region 1 and 2. The distributions of cross over values for Region 1 and 2 in each experiment separately and combined into a total are given. They are plotted in class intervals of 1.5^. The distribution for Experiment 1 ranges from 0 to 18.5^ and for Experiment 2 from 0 to 14.25^. The total of the two combined is shown in the last line in the table, and the curves plotted from them are shown in t ‘ x 5J^ A A- r^r v: r m ii »'•*■ til'- (,-«:»*. ;. ^ w *-• - ■ II-, . J ' i ■ ,« ^ //•‘i -j W% 1^.. fr, ^ * ;•», • 'r' ».' >Y ', -I , „Y . . •;. '■ .»■•« j»-‘*..3rv ^ ' •;.' l>\ 'Tfi ,p|[^i%' s-|;'"'a||W “■;.L '.> ^Sr V ,»x.« L '• *.♦ > lUd. 15!>‘»9 •>: ; «ikttf f :.»*i'ltl<>: 4 k' ». * t ji ' ,‘r t/i ♦ i*' •'' '-A ^ ■' V ''»v- - 'A. 'if .’v , ' _'®'. •** A. I ' . ‘ ■ ':■£ .* .IL ,-rL- ,1 a '■Xf™') £ •'1; £ -'• -s- .■-•• " - -5 .. ■ ' ^ ^ •.,.r,<-r; ,J4J,, .-Vt Kift -j;? .., ipV. . ' * ' f^7; ••t-'ji--;* 4' *f t T 1/ iVl ' , ® *f : ( ' ■■ h _4 * -'_- • . • '/ «V> V , L ^ » *7 f ^ V . k Iji ' w ;• jc-: -. i s. 4 ‘i^.«7i'** ' * xCii V •• **’* - i ;:. iv-'i tis.; * ’•■ ■ - -- .,.n .,, j.3*j4Hi»ji ^ U' ••• ‘ ; . (O'!*. i/W sf r »i;j^ - ^ - ; .{ r ifU r-=*sfc .rrrtuT'^^amsrStss*..^^ TABLE 2 CM «« a O •ri O PEI % ® CD (Sl s > lol lO CM lO (0 m N. a > s >• o a « o »4 o «H o a o ft *> •H «4 4> .9 « *M • o ® 55 ® $3 ! o j*^ !■** o o (3 o •H 4* d >4 O a o C9 Si ^ 9 P ^ «H +> P V •H d o t4 ► o « o d o t* 9 I ® ^ q> FI to O r-« -»> o rO U 9 > CM O e • « to ® P3 u o I > •H O 9 • a to © >« « o to »< © 0< © rH i “•! Vi © O V» © > o © « n « (H o © ► o « h © ► O • tt o M O *3 4» o E 4 o» O • 00 'I* o CO vO CM a* CM e- « • • CM CM CM 00 tn • • • CO CO CO in g g CO 00 r-l e- O m 00 CO in o» vo LO vo m U3 cn o fH CM CM CM CM e- CO CO O O CM g CO vO C- CM CO 00 MS M3 in o 00 CO g CM in in g ►1 •4 (0 IP Experiment II Experiment I ft II - 11 - Figare 2, The most striking feature of the curves is the large numher of individuals in the first class, making them look rather unique for the experi- ment. However, if we turn to the hack and look thrcu^ Tables 5 and 6, we will find that there are quite a few of the F£ segregates giving such a small number of offspring that their crossover values reached zero. It is true that zero percent crossover value should be expected to occur in this case, if we ejq)ect modifiers of lixkage values to segregate at all; but still we should not over- look the fact that over half of the individuals that give zero percent are those that have exceptionally small number of offspring. Therefore due allowance must be made on the first class in Table 2, and thus the medal classes of the curves could be shifted to 5.25^ for Es^eriment 2 and 3.75^ for E:q>eriment 1 and Experiment 1 and 2 combined. From the description given in the early part of this paper, it is very obvious that we have also performed a hybridization es^eriment on two strain: of Drosophila differing from each other in their capacity for crossing over. In fact, as has been shown, this has been admirably done by Prof. Detlefsen him> self. The only point in which his work differs from mine lies in the fact that he \ised the Tdiole region between white and miniature; while I dealt with se^ents of it. Due to lack of experience in handling the material, and in the method of culture, the results obtained could be much better. There are, however, reflected in the data a few features that are characteristic of a hybridizing experiment vhere multiple factor Inheritance with Incomplete dominance is involved. These features stand out so strikingly that it is but Justifiable to arrange the material and present it in its logical shape. Owing to the fact that Prof. Detlefsen is dealing with the whole piece of chromosome, it is impossible for me to find out the crossover values of Beglon 1A2 for the respective generations to which the two originel males belong. However, we know the value for the whole region, and in order to present the data in the shape it is now. h-AMM t. ^ J.*? - - i B.. ' ' . . ^ -. '^" ■■ i-W: ^i‘ I'iitti*. f'. .V*>.v'.' ':-Hi’t»i 'i *!#tv -•’ ' 1^ ■' |, . ' ' Ja ' •• >■'"*“ ' ■’ ; V’ " ^ ^ • : vt,‘/|iP' ip(^', >r^v Uf' ■ ‘ ^ ,' ■■''5 ft’tv y: ''A ■■-■•- • 'V ■''/*•'•■' .*-.!’•/ ■ / »-li t «• •• , .' - .;vj risiiC i ' ■ S - »V' ,: '.'■■xii' •/»'® 1 li! ' ■ •*•' t. 0 */ ■-r-v'i’ L ' .' ' * ^ -4 ■'■''-• 4 'f ■.. •■•"'■• i'*^' r •:: , . ■>• ■:■. J --5 .. S .. J- ! ,t ■ ■ •• . !ts ? ' '' • -w ■f . i.*r» . ,.< iiia‘ ■'■* % T|i,»«-' . 'r’’\^V ''''' .'^ v'?^LW, -Bix-r- d--^::. i?.: ■. " ■^■T<^“ ^ • ■ ,.j .,ij !y , ‘ I' r -V ' * '*‘ '**'*•: ■^ *•, ■<■“;■ •' |^,*> * «■ '.«*> • f/ "■' . ■ , f jj^ yt '13 i'' n r i. .■ i'd ,;c, -4^<. 'fe . 3 ..-..-.. i .sx*. f . .!l i'.« ,i ■' ,M^ri 4 " *Vloyed had been subjected to six generations of inbreeding, vhile those used in Experiment 1 were just nonnal individuals and the male was from Fyg^ ^3 8ho^lld expect the standard deviations of the two F 2 posalatlon, to Show soma diffaranoa. A glanoa at that oolwm In the table will '4 ■ ' ■/" ■■• ;-v ' sft 'M' '. ^ '' . 4 .;^ r y , t f-i ■ - v'V‘.\i. Vm. ; , , ■• , J ;v . ■< V ■- , . •!!' j, . ,, • ' i l ■*■ . I, ' - - :;f ' • ' r .'■ . -'V w- ."■j't 'J.J ■ J ■ ■'“ - 1 •• I ■: "I ,.v '■■ ■! t . •.• ■ A , r t-i'f •V'- i-H- C' t',C ..i' “• *'• • * ' - r * . ' ■^ 7 ’ • •■ < • i’- " ■ . ' ' . i. ■ ■ ■"'rf ■ •. , t.., ~ '■■ I t. .. ;. J I ■'; V(t >1 ••\(i. .> 1 '■ '* ’■’ ■ ' '' ‘ ' ■ •t^ -ipt -“'f^ 13- tell that they do differ, and also in such a wai' as we eaqpected. The standard deviation of Experiment 1 being bigger than that of Ej^) 0 riment 2. B - Eegion 4. As mentioned at the beginning of this paper the purpose of this work is more of the nature to try to throw farther light on the whole problem of modifiers of linkage values than merely to secure more data to expand the theme stated in the subject, so I ran a test on Region 4 vdxich is the space between miniature and forked, and it is outside the original region of selection. As it appears in Table 3, this region does not seem to have been influenced by selection. Forty-one faaales were tested and with an average of 127 offspring per female, the crossover value obtained for the region is 19,75^, vdiich is very close to the recognized valuet- 20.4^. Owing to the extremely small number of offspring obtained from some of the females, the distribution table drawn up is rather puzzling. Since selection does not seem to have any effect on this region, the Fg population should show a normal curve varying arounii the recogniz- ed value. Too much emphasis, however, shoxild not be laid on the distribution we obtained, since gi^ite a number of the females give so few offspring that the degree of reliability of their crossover capacity as reflected in the data is doubtless deceiving. To eliminate this difficulty, females giving less than fifty offspring are left out of consideration, and the resulting distribution is shown in Table 4, A curve plotted from this distribution is shown in Figure 3 with 17J^ as the j modal class. IV CONCLUSIONS From the above discussions of data the following conclusions may be drawn. Plotted from Table IV Individual O ' to 1/1 O' o at 3 ef o o <0 « o < (b m Figure 3. -16- 1 Segnant# of chroaooome when separated from the vihola piece of chromosome under selection still show low crossover value - a fact showing t’nat modifiers of linkage values affect detached portions of chromosome in the seme fashion as they do the vriiola piece. 2 In a hybridization experiment performed on "low" and "normal" flies with only a segment of the idiole piece of chromosome under selection, the mean value of the generation did not come out quite as hi^ as would he expected on the basis of incooplete dominance. The great portion of the population was grouped in the lower one third of the curve. This phenomenon suggests the existence of partial dominance of the "low modifiers. " 3 Selection exercised on regions 1,2 and 3 in the sex chromosome does not affect the crossover value of the region outside of this, as far ae we can see from our data. KPOK *¥.' ■ awr'TV :s^-t ‘’^ir ^ • . •.••■•>• *»;■!■»■■ ... • «T -111 . ' w ' ■i'iikii;c*"ji»' • S'.'"' * '» .» . 'ledll . ^^' l(l 4wffl .tt*»fr.M\,;, »•••»•' «•«« ■ «w* ■*■*> ■'"■■■• ’ ■ • "■ '^-v ■ ■'^ , ” s^.' .. . J ' i inr* ^. ■ ■■ '-•’♦•■».;■ . ^‘ ■ ■ Vii'A'.ftKr*..*^ ' ' .'■• '.. o '?ie^ , , 1*. .(«>). •’1.*'' ' f ., ! H’t ''. V ' ^"*** ^*Eba |p' ^ *' ■ I ' '‘ , V' ', ' '■ i ' ■ •■ ( I ^ '' ^ *' ' ' 1 ■ /' T (■' , . i ' ' 1 ^1.. i. ... r' » ^ :». .. r' ~ * ' • • \ ' ■ '■ . . • ■ ■•■•>..■“ Vi' t:^-M;} ®' ■•-.■'I ^wa6^»^7 '<^■''**'411!^!^'?^ ■ “ i I ■ ) 7 <.'('! \ , ■ '.i. . ,' X^fK ’■ ..I . . I. 4 , ::i 'St>'T\ f< X iJ^' 'v f, . ‘J ,1..^ >■■■ rr^i, ...W ^ \U'- ,. •.^■i *■: ..* ■t ...» ,'Vfi ^■. - y Mi. ■ ■w tfl ' % ,•»'. i 'jv u ^ - jjj-. .ll^ .. - 17 - V ACK'NDWLEDGEdENT . The writer is very grateful to Prof. J. A. Detlefsen for the opportvinity to work on this prohlera. It ia with a deep sense of appreciation to say that hut for his helpful suggestions and his generosity in vc^ulping the writer with necessary laboratory conveniences this work would not have been possible. -18- VI LITEUfiTUHE CITED. Bridges, C. B. 1915 A linkage variation in Drosophila. Jour. Expt. Zool. , vol. 19, pp. 1-21 Detlefsen, J. A. and Robert s £. 1921 Studies on Crossing Over. Jour. £3cp. Zool. vol. 32, no. 2, pp. 333-354. Detlefsen, J. A. 1920 Is crossing over a function of distance? Proc. Nat'l. Acad, of Sci. vol. 6, No. 11, pp. 663-670. Plou^, H. H, 1917 The effect of temperature on cfossing over in Drosophila. Jour. Ej^t. Zool., vol. 24, pp. 147-210. Sturtevant, A. H. 1919 Inherited linkage variations in the sedond chromosome. Puh. Carnegie Inst. Wash. D. C, , no, 278, pp. 305-341. il *> Begion 1 ft 2. 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