148 BOTANY: CREIGHTON AND MCCLINTOCK Proc.N.A.5. THE CORRELATION OF CYTOLOGICAL AND GENETICAL CROSSING-OVER IN ZEA MAYS. A CORROBORATION By Harriet B. CREIGHTON AND BarBaRA McCLINTOCcK Botany DEPARTMENT, CORNELL UNIVERSITY Communicated February 9, 1935 There has recently been some skepticism expressed (Brink and Cooper, 1935)! as to the value of the studies on the correlatién of cytological and genetical crossing-over in maize published by Creighton and McClintock (1931)® because of the fewness of the data. Since the paper by Stern (1931)? dealing with Drosophila and having much more extensive data appeared at practically the same time and yielded the same conclusions, the authors felt it unnecessary to add to the ever-increasing amount of pub- lished work merely to record more evidence of the same nature without supplying anything essentially new or advancing. Therefore, confirmatory data which have accumulated since the time the joint paper mentioned above was published have not been considered for a separate publication. However, we now feel forced to add more data merely to counteract any suspicion that the evidence previously presented constituted insufficient proof. This will be done in as brief a form as possible, since a discussion of the method has been given in the paper mentioned above. Chromosome 9 in maize is characterized by its relative size in the chromo- some complement and by the 1:2 ratio in lengths of its two arms. The end of the short arm in some strains possesses a large knob while other strains have a very small knob or no knob. Evidence that the knob or knobless condition of a particular chromosome 9 is inherited with the same precision as a gene has been given in the previous paper and has been confirmed in many additional crosses. The knob, therefore, could be used as one cytological marker for this chromosome. The presence of an inter- change between chromosomes 8 and 9 (Burnham, 1930,? 1934°; McClin- tock, 19307) which broke chromosome 9 at a position on the long arm a short distance away from the spindle fibre attachment region provided the second cytological marker. That the genes yg, ¢, sh, wa™* lie in the inter- changed chromosome which possesses the short arm of chromosome 9 has been shown by McClintock, 1931,* Creighton, 1934, and Burnham 1934.34 With reference to the knob and the interchange point, the order of the genes is knob-yg-c-sh-wx-interchange with yg very close to the knob (Creighton, 1934)§ and wa close to the spindle fibre attachment region (Burnham, 19344 and unpublished). The standard crossover values for these genes alone are yg-c 21%, c-sh 3.8%, sh-wx 21%. The crossover value of wx to the interchange is 13.7% (Burnham, 19345). That there is very little crossing-over between the knob and yg can be seen from the data given below. VoL. 21, 1935 BOTANY: CREIGHTON AND MCCLINTOCK 149 A plant with the constitution knob- Yg-C-Sh-Wx-interchange was crossed. to a plant with the constitution knobless-yg-c-sh-wx-normal. The F, was backcrossed to knobless-yg-c-sh-wx-normal. Two hundred and sixty-one individuals resulting from this backcross were examined cytologically to determine the presence or absence of the knob (knob or knobless in table below) and the presence or absence of the interchange (interchange or normal in table below) in the chromosome carrying these genes contributed by the F, parent. Since there are five regions in which a crossover can be detected, the results have been tabulated according to crossovers which occurred in each of these regions. The tabulated results do not represent the total backcross progeny. A higher percentage of Yg-C-Sh-Wx and yg-c-sh-wx plants were examined cytologically in an effort to obtain cross- overs between the knob and yg. Likewise, more Yg plants were examined cytologically than yg, since plants homozygous for yg are reduced in vigor and often do not afford sufficient material for cytological examination. TABLE 1 Kwnos- Yg-C-Sh- Wx-INTERCHANGE >< KNOBLESS-yg-¢-sh-Wx-NORMAL K.NOBLESS-Yyg-¢-Sh-wx-NORMAL NUMBER oF INDIVIDUALS Non-crossovers 1. Knob-¥g-C-Sh-Wx-interchange ' 84 2. Knobless-yg-c-sh-wx-normal 45 Crossovers in region 1 3. Knob-yg-c-sh-wx-normal 3 4. Knobless- ¥g-C-Sh-Wx-interchange 1 Crossovers in region 2 5. Knob-Yg-c-sh-wx-normal 13 6. Knobless-yg-C-Sh-Wx-interchange 11 Crossovers in region 3 7. Knob-Yg-C-sh-wx-normal 3 8. Knobless-yg-c-Sh-Wx-interchange 3 Crossovers in region 4 9. Knob-¥g-C-Sh-wx-normal 53 10. Knobless-yg-c-sh-Wx-interchange 18 Crossovers in region 5 11. Knob-Yg-C-Sh-Wx-normal 16 12. Knobless-yg-c-sh-wx-interchange 3 Double crossover involving regions 2 and 4 13. Knobless-yg-C-Sh-wx-normal 1 Double crossovers involving regions 4 and 5 14, Knob-Yg-C-Sh-wx-interchange 5 15. Knobless-yg-c-sh-Wx-normal 2 It is obvious from the data given above that a genetic crossing-over be- tween the genes Yg-C-Sh-Wx involves a cytological crossover between the 150 GENETICS: F. H. CLARK Proc. N. A. S. knob and the interchange point. These data, therefore, supplement those given in our previous publication and indicate the soundness of the con- clusions drawn. * The genes referred to in this paper by symbols are: yg, yellow-green plants; c, colored aleurone; sk, shrunken endosperm; wx, waxy endosperm. 1 Brink, R. A., and Cooper, D. C., Genetics, 20, 22-35 (1935). ? Burnham, C. R., these Proceepinecs, 16, 269-277 (1930). 3 Burnham, C. R., Genetics, 19, 480-447 (1934). ‘Burnham, C. R., Am. Nat., 68, 81-82 (1934). 5 Creighton, H. B., these ProcEEprncs, 20, 111-115 (1934). § Creighton, H. B., and McClintock, B., [bid., 17, 492-497 (1931). 7 McClintock, B., Zbid., 16, 791-796 (1930). ® McClintock, B., Ibid., 17, 485-491 (1931). * Stern, C., Biol. Zbl., 51, 547-587 (1931).