The controlling element Dt2 was located on the long arm of chromosome 6 near Pl. The somewhat meager data from the cross of Y Dt Pl/y dt pl x y dt pl were as follows:
 

Parental	co reg 1	co reg 2	doubles	total
103	38	7	2	150

Therefore the tentative map distances were: Y 26 Dt 6 Pl. We would like to have a good kernel marker for studying Dt2 but Y1 is too far away. Therefore we attempted to produce some chromosome aberration (e.g., an inversion) that would shorten the map distance between Y and Dt2. To do this homozygous a-m, Y Dt immature pollen was treated with x-rays and crossed on a-m, y dt silks. The stage was around the 1st division of the microspore and the dose was 1200r. A total of 912 yellow dotted kernels were produced. These were planted and the resulting plants backcrossed again to a-m, y dt. From this cross 903 ears were produced and were examined superficially for cases where Y and Dt appeared to be closely linked. 44 ears showed a possible reduction in crossing over between Y and Dt. One ear, which had 62 Y Dt:41 y dt:36 Dt:0 y Dt, was selected as a good case for further study.

Yellow dotted and white nondotted kernels from a subsequent backcross were planted for material to test for gamete abortion, for female and male transmission and for material on which to do cytological tests. The results were as follows:

Gamete abortion--Plants in both classes had normal pollen and subsequent ears had full seed set indicating that there was no gamete abortion.

Female & male transmission--Reciprocal crosses of Y Dt/y dt x y dt produced the following data:
 

	Y Dt	y dt	Y dt	y Dt	Total
heterozygous female	446	435	0	0	891
heterozygous male	154	816	1	0	971

These data indicate that Y Dt linkage is very tight (the one Y dt kernel could be a contamination). The transmission of this low crossover segment is normal through the female but the male transmission is reduced to 32% of normal.

Test for linkage with Y1--The low crossover (lco) Y Dt stock was crossed to a yellow dent stock (Alexander High Oil), which is presumed to have Y1 on chromosome 6. The cross was a-m, Y Dt (lco), R x A, Y dt, r (Aho). The F1 was then backcrossed to a-m, Y dt, R and the colorless kernels were separated for Y and Dt. The results were as follows:
 

	Y Dt	Y dt	y dt	Total
colorless kernels	82	53	48	183

These results indicate that the Y linked to Dt2 is not allelic to the Y from Aho and suggest that Y in the lco segment has been moved to a new position.

Rodney Higgins and M. G. Neuffer

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