eile were grown from these kernels, all should possess two normal chromosomes 9, The selection of the C Bz «= C bz, Sh = sh, 4x = wx kernels, on the other hand, should sive rise to plants with a normal ehromosome 9 carrying C sh bz wx ds and a Duplication chromosome 9 with the various constitutions indicated in the supplement to table 8, Because the region between Bz and ix is the longest, the most frequent of the cross-over classes should be;:normal chromosomes 9 with I De? Sh Bz wx and Duplication chromosomes $ with C ds sh bz dx Wx Sh De® (crossover region 4, supplement to table 8), Crossing over ratios may be determined in regions 2, 3 and 4 by comparing the frequencies of the various classes of I kernels showing variegation, The numbers in the I bz = C ba, sh wx class, (region 2), the I bz - C ba, Shesh, wx class (region 3) and the I + C Bz -C bz, Shesh, wx class (region 4) are 12 : 3: 76, respectively. The crossover units for regions I to sh, 3h to Bz and Be to /x in normal chromosomes 9 are approximately 3: 2: 21, respectively. The agreement in the two cases in relative frecuencies in the several orossover recions is close, No serious disturbance in the relative frecuencies of crossing-over in these regions 1s occurring in the Plants that are heterozygous for the duplication, Moreover to test the projected constitutions of the zametes produced by plants 4628D-10 and 11 (table 8) 4t would be necessary to grow plants from the various classes of kernels in this table and test the chromosomal and genic constitutions of the chromosome 9 contributed by the male parent, f#ecause the constitutions of the ciromosome 9 in the gametes of the mother plant (4306) were probably the same as those produced by the two tested plants of sub-culture D, the probable constitutions of the plants in subecultures * to L can be anticipated, (tate 3) On the self-pollinated ear of plant 4306, the various crossover Classes of kernels of the tyves indicated in table 8 were likewise present. The chromosome 9 constitutions of the examined plants ar'sing from a selected number of such kernels has been given in table 5, The correspondence of chromosome constitution with expectancy on the basis of the selection was confirmed by the cytolozical analyses, Plants in sub-cultures G and H of culture 4628 should carry two normal ehromosomes 9, one with I Ds 3h Bz wx and one with ¢ ds sh ba wx, these plants should be Ac ac, as the type of variegation observed in the kernels from which they arose would suggest, The plants in sub- culture F should have the same two chromosomes as plants in sub cultures Gand H but these plants could be either ac Ac or ac ac, The plants in culture I, not examined cytologically, could be expected to heave two normal chromosomes 9, one with I Ds sh bz wx (an I ds ah bz wx chromatid is infrequently produced) and one with C ds sh bz wx, These plants could be ac Ac or ac ac. The exact senic constitutions of chromosomes 9 in the plants in subd-cultures K and L could not be projected in advance other than to anticivate the presence of the duplication chromosome 9 resulting from a crossover in recions z to 5, and chromosomal constitutions of these plants will now be civen, {c) Sub-culture F Sy aopropriate crosses of plants in sub-cultures F to L, the genic constitutions of the two chromosomes 9 in each tested plant Was determined, Plant 4628F-1, known to have two morpholosically normal chromosomes 9 because it was examined cytologically, was crossed to two C ds sh bez wx ds ac female plants. The tyves of kernels ~13= appearing on the two resulting ears are given in table 9. This plant was obviously Ae Ac (allelic positions) in constitution, The supplement to table 9 indicates the types of chromatids that plant 4626F-1 would produce on the basis of the given constitution, The observed ratios of the various classes of kernels on these ears confirms the projected conatitution of this plant. The plant had a normal chromosome 9 with I pet Sh Bz wx and a normal chromosome 9 with C ds sh bz wx. The Ds locus is present just to the right of I, It should be noted that crossing-over between IJ and Sh is not affected by the presence of this Da locus (4.9% crossing-over). d#ith the same aleurone genes in chromosome 9,the variegation pattern produced by a Ds locus in this new position is strikinsly different from that produced by a Ds locus in its standard location, In this new position,a 9s mutation will give an acentric fragment carrying I and a dicentric chromatid with Sh Bz wx, as shown in the following diagram: I Sh 82 wx ) ( ; I Sh Rg wx The dicentric chromatid produced by a Da mutation at this locus will undergo the breakage-fusion-bridge cycle beginning in the ananhase followins the Ds mutation. In this first anaphase, the genes 3h and 32 will be located alose to the middle of the bridge: o . me Bz 3h sh Bz NX Because the bridge may be broken at any position between the two centromeres, it can be anticinated that some of the breaks in this sh" “Sas ah be wx Cc oh bz ds ac? x —1_De 3h Be ac Aca] © %S Sh bz wx Table 9 { dL 4 Cross of 4628Fe] t I De $h Be wx “€ ds eh bz 4628F=1 Kernel type Crossover 4368Ca4 4563-14 Totals region x x 4628F] 4628F=] IeO Ba-C bz, Shesh nonec,o, 181 89 &70 C ah bs nono ,6, 153 83 236 Regions 1&2 I sh Region 1 1 1 2 Regions 1&8 C BaeC ba Sheesh Region 1 0 1 1 I beeC bz, sh Region 2 8 4 12 ¢ 3h Bz Region 2 6 6 12 I bz-C bzy Shesh Region 3 10 6 16 C ah Bz Region 3 2 3 5 I sh Regions 182 9 0 9* C bay Shesh Regions 1&3 i 0 1 I Bz-C bzy sh Regions 2&3 0 0 0 C Sh bz Regions 2&3 0 0 0 Totals 371 193 564 * This class ia not accurate for showing cross-over regions, It includes (1) losses of Ac and (2) loss, transposition oy changes in state of Dea t l s } Supplement to Table 9 yy 4 I Da 3h Bz Chromatids produced by plant 4628Fr-] DAAC DEN TNE TS C ds ah bz Chromatid constitutions Appearance of kernel in table 9 Non-crossovers I Da 3h Be IeG Bz-C ba, Shesh CG ds sh bz C ah bz Cross-overs I ds sh bz I sh Region 1 C De Sh Da C BaeC bz, Shesh Cross