Chromosome 2. Tetraploids

In the course of his intensive work on tetraploids, L. F. Randolph created a stock containing the genes lg, gl, b, v4 and a corresponding stock containing the dominants. Both stocks were homozygous A1, A2, A3 and also Rg which is necessary for definite classification of the genes B-b in the seedling stage. The stocks were multiplied and then selected for distinct expression of the four marker genes. Following this, J. E. Welch studied the linkage relations of plants duplex for each of the four genes when backcrossed to the multiple recessive. Beginning at this advanced point, I can contribute some additional information.

 

The cross of a plant duplex for all four markers
+  +  + +
+  +  + +
lg gl b v
lg gl b v
 
by a multiple recessive one
lg gl b v
lg gl b v
lg gl b v
lg gl b v
 
should give as a parental class ratio four plants
simplex for all genes
+  +  + +
lg gl b v
lg gl b v
lg gl b v
 
to one plant duplex for all genes
+  +  + +
+  +  + +
lg gl b v
lg gl b v
 
to one multiple recessive plant.
lg gl b v
lg gl b v
lg gl b v
lg gl b v

 

Numerous other arrangements are possible in plants derived from crossover gametes; but for any one gene, the individual plant should have the recessive allele represented either two, three or four times. The last type is obvious phenotypically since it is homozygous for a recessive marker. Further, a cross of this nature should and did segregate in the ratio of 3.6 - 5 dominants : 1 recessive for each of the four genes.

If several individuals with dominant phenotype are selected from such a backcross progeny, and again backcrossed to the multiple recessive, one should find that certain of their progenies give simplex ratios for all four gene members.

Twenty individuals were tested; their distribution is as follows:

2    duplex ratios for all four genes

1    duplex ratios for gl, b, and v; simplex ratio for lg

1    duplex ratios for lg and v; simplex ratio for gl and b

4    duplex ratio for v; simplex ratios for lg, gl and b

2    duplex ratio for lg; simplex ratios for gl, b and v

9    simplex ratios for all four genes

The study of progenies, derived from backcrossing plants simplex for each gene to the multiple recessive stock, should give the most direct measure of recombination frequency in a tetraploid for comparison with those in similar diploid stocks.

While 4,315 mature plants were studied, obviously only part of these may be used in the calculation of recombination frequencies from simplex ratios for any given region. The data are tabulated as a three-point test for lg, gl and b and as a two-point test for b and v. This enables one to utilize larger numbers than would be possible in a 4-point tabulation. No records were used unless the ratio of dominant to recessive allele was a good fit for a 1:1 ratio. In this manner, any possible effects of either differential viability or poor expression are kept at a minimum. Note that the total is smaller for the 2-point test as a number of cultures were not usable since the v4 class was deficient.

 

0 (+  +  + 1033   0 (+ + 484  
  (lg gl b 1036   (b v4 427  
 
1 (+  gl b 196   (+ v4 352  
  (lg +  + 192   1 (b + 359  
  1622  
2 (+  +  b 181  
  (lg gl + 219   786 b : 836 + D/P.E. = 1.9
  779 v4 : 843 + D/P.E. = 2.4
 
1&2 (+  gl + 55   1500 lg : 1466 + D/P.E. = 0.9
  (lg +  + 54   1506 gl : 1460 + D/P.E. = 1.3
  2966   1467 b : 1499 + D/P.E. = 1.0

 

The diploid recombination values used in the following table are taken from Fraser, Jour. Hered. 30: 375-378, 1939.

 

  4n 2n Difference
 
lg - gl 16.80.46 19.510.40 2.70.61
gl - b 17.20.47 21.60.41 4.40.62
b  - v4 43.80.83 33.20.47 10.60.95

 

The observed differences between 2n and 4n are significant, but a discussion of the possible causes is too lengthy for this preliminary report.