Translocations and centromere positions. Translocations are especially useful in determining the location of genes in relation to the centromere and other visibly differentiated regions of the chromosome, due to the fact that their position in the chromosome can be determined cytologically and their linkage relations with known genes also can be determined. The following is a summary of available data on the relative positions of translocations and genes in the neighborhood of the centromeres in chromosomes 1 to 9 inclusive, with a few records for chromosome 10. These data were compiled chiefly from Dr. Anderson's records while in residence at the California Institute of Technology for several months in 1942 and 1944.

Chromosome 1. - Information on translocations in the short arm of chromosome 1 was summarized by Anderson in 1941. The gene P is about two-thirds of the distance out on the short arm. A minimum map distance from P to the centromere may be determined from % 1-9a which is known to be located in the short arm. On the basis of 730 plants the per cent of crossing-over between P and T 1-9a was found to be 21.2 � 2.5. Thus the location of the centromere in the linkage map is 21.2 units or more to the right of P.

A number of translocations in the long arm of chromosome 1 give less than 5 per cent of the crossing-over with brachytic. These are distributed from about L2 to about L6. The gene br is probably located in the neighborhood of L3 or L4. Only 2 of the translocations in the long arm are definitely placed to the left of br. T 1-6a was reported by Burnham and Cooper and Cooper and Burnham to be in the long arm of chromosome 1 a short distance from the spindle insertion. From their diagrams and figures a position of about L2 is indicated, which is also in accord with other data. The map position, based on 75 plants, is given as 13.4 units to the left of br. T 1-6b has been described by Burnham. The locus in chromosome 1 is given as L2.5. Very good linkage data involving 952 plants place the translocation to the left of br with 3.8 per cent of crossing over. (Data by Burnham cited by Emerson, Fraser and Beadle, 1935). Those data merely show that br is between one-quarter and one-half the distance out on the long arm. The map position of the centromere must be some where between the locus of T 1-9a, 21.2 units to the right of P and the locus of T 1-6a, 13 units to the left of br. This is a very long region. If crossing over were equally distributed over this portion of the chromosome we might expect the centromere to be about midway between P and br.

Chromosome 2. - The map location of the centromere can be rather closely delimited by a number of translocations in the interval between ts and v₄. Several of these will be considered. T 2-9b is located cytologically at 2S1 and 9L2. Linkage tests give the order definitely as B-ts-T-v₄. Crossing over between the nearest genes was

ts-T = 33/622 = 5.0 per cent
t-v₄ = 121/1528 = 7.9 per cent

Since the break in chromosome 9 is known to be in the long arm (Anderson, 1938), the wx gene is carried in the 9² chromosome. Tests of linkage relations in the homozygous translocation can be used to verify the location of the break in chromosome 2. These tests gave the following results, showing that the break is between ts and v₄.

The wx gene is carried in the 9² chromosome.

The linkage of wx with B and its independence of v₄ establishes the break in the short arm of chromosome 2 between B and centromere. The linkage of B and ts shows the break in to the right of ts and the independence of ts and v₄ locates the break between those genes. Thus the centromere is at least 5 units to the right of v₄.

T 2-5a was studied by Rhoades and described cytologically as in the long arm of chromosome 2 near the centromere. Linkage tests give the order as B-T-v₄ with 7.3 per cent of crossing over between T and v₄.

T 2-10a is located at L2, with the break in chromosome 10 well out on the long arm, 2 to 3 cross-over units to the left of g. The order on chromosome 2 is ts-T-v₄ and the data on crossing over are as follows:

Linkage data in the homozygous translocations are as follows:

Since g is distal to the break in chromosome 10 the B-ts section of chromosome 2 must include the centromere, i.e., the translocation must be in the long arm of chromosome 2.

These data may be summarized as follows:

The centromere must be 5 or more cross-over units to the right of ts and 7.3 or more units to the left of v₄. Since there is usually some suppression of crossing over in the heterozygous translocations, the total map distance of the ts-v₄ interval is uncertain. The normal value is probably about 20 units. The centromere is probably a little closer to ts than to v₄.

Chromosome 3. - The summary of translocations involving chromosome 3 published by Anderson and Brink places the centromere in the general neighborhood of ts₄. Since then additional data on T 2-3b has indicated that ts4 is in the long arm of chromosome 3. This translocation shows about 4 per cent of crossing over with v₄. The order is probably B-sk-v₄-T. Linkage tests in homozygous T 2-3b stocks give the following cross-over values.

These data all agree in placing the translocation beyond v₄, consequently in the long arm of chromosome 2. The linkage of ts₄ with B and v₄ in the homozygous translocation places the break between the centromere and ts₄, and shows that it is the long arm that is involved. From this it may be concluded that the centromere is to the left of ts₄, i.e., between d and ts₄.

Chromosome 4. - A number of translocations in the proximal regions of both arms of chromosome 4 adjacent to the centromere all show close linkage with su, usually accompanied by much suppression of crossing over. These data indicate that the centromere is in the general region of the su locus. Data on T 2-4c place su in the short arm. This translocation is very near the centromere in the short arm of chromosome 4, and is far out on the long arm of chromosome 2 between v₄ and ch. Linkage date from homozygous T 2-4c show ts₅ and su to be linked and su to be independent of Tu. Thus the break is to the right of su. Further data on this homozygous translocation areas follows:

From heterozygous stocks of this translocation chromosome 2 linkage relationships and adjacent to the break were:

for chromosome 4:

The linkage of su with v₄ in the homozygous translocation demonstrates that the translocation must be between su and the centromere of chromosome 4. This places the centromere at least 9 units to the right of su on the linkage map.

Chromosome 5. - The position of the centromere in relation to the known genes of chromosome 5 was determined very accurately by Rhoades in 1936, with the aid of a fragment of chromosome 5, which apparently consisted of the centromere and the entire short arm of the chromosome. In the metaphase of the first meiotic division in the microsporocytes the fragment formed a trivalent with the two normal number 5 chromosomes in approximately half of the cells; in the remainder of the cells it was present as an univalent that was rarely included in either daughter nucleus. From the known cytological behavior of the fragment the expected back cross ratio from fragment plants of the constitution AAa with a in one of the normal chromosomes was calculated to be 5A:3a or 37.5 per cent of recessives. This ratio differs sufficiently from the ordinary 1:1 back cross ratio of disomic inheritance so that with the aid of the fragment chromosome genes located in the short arm could be distinguished from those located in the long arm of chromosome 5.

Another test employed by Rhoades to identify the genes in the short arm was the occurrence of fragment-carrying plants homozygous for the recessive gene in the back cross progenies of fragment plants carrying a recessive allele in one of the normal number 5 chromosomes. If the locus under consideration was in the short arm none of the fragment-carrying plants would be homozygous for the recessive allele, barring rare exceptions resulting from chromatic crossing over.

Utilizing these tests it was found that the A₂ and bm loci were in the short arm and bt, pr, ys, v₂ and v₁₂ were in the long arm of chromosome 5. The available cytological and genetical data from translocations involving chromosome 5 confirm the findings of Rhoades relative to the position of the centromere between the bm and bt loci.

Chromosome 6. - There are available six translocations recorded cytologically at about 6L2 or 6L2.5. These are T 1-6c, 2-6c, 4-6a, 4-6b, 4-6c and 6-9b. All are closely linked with Y and are definitely to the left of Pl. All show a reduction of crossing over between Y and Pl to 5% or less, in the heterozygous condition. Proven cross-overs with Y have not as yet been obtained for study. With so much suppression of crossing over, little can be inferred as to the location of the Y locus with reference to the centromere. Translocations in the satellite or nucleolar region are located well to the left of Y. Data on 3 translocations between the centromere and the nucleolar region are too meagre to give any satisfactory evidence as to the position of the centromere.

Chromosome 7. - Translocation 2-7b is located about one-fourth of the way out on the long arm of chromosome 7 and at about the same relative position on the long arm of chromosome 2. Linkage tests place it near ra, with slightly less than one per cent of crossing over. Linkage tests in the homozygous translocation show linkage of ra and gl, which places the translocation to the left of ra. This is also confirmed by the linkage of B and ra (B-ra=167/462=36.1%). Since B is in the short arm of chromosome 2 and is thus in the 2⁷ chromosome ra must be in the translocated portion of chromosome 7. Several translocations in the short arm of chromosome 7 have been tested for linkage with ra as follows:

Chromosome 8. - The only genes known to be located in chromosome 8 are in the distal region of the long arm. From the data of Anderson (1939) the location of the centromere must be 30 units or more to the left of ms₈.

Chromosome 9. - Translocation 5-9a is located in the short am of chromosome 9 near the centromere and is about 2 cross-over units to the right of wx. This places the centromere at least two units to the right of wx. T 3-9a in the long arm of the chromosome gave 3.6% of crossing over with wx, indicating that the centromere is probably not far beyond the minimum of 2 units. The gene v has not been located definitely but is believed to be in the long arm not far from the centromere (Beadle 1932, Burnham 1934b). Its map position is 12 units from wx.

Chromosome 10. - The only chromosome 10 genes which have been tested with translocations are g and R. Both are located far out on the long arm, apparently beyond L.6. Translocations to the left of L.3 have given from 9 to 23 per cent of crossing over with g. Probably there are different amounts of suppression involved. The centromere must lie at least 15 units to the left of g.

E. G. Anderson and L. F. Randolph