1. Studies on the Hm locus.

In 1948 a single cross (K61 x Pr) which was hm/hm was crossed by a number of translocation stocks, 20 x R‑20, ‑41/-42, and ‑50 (from Conn.) and 1‑5, 1‑7, 1‑7a, and 1‑9b (from Cal. Tech.). All translocations involved chromosome 1, and in all but 20 x R‑50 the point of interchange was in the long arm of this chromosome.

In 1949 the F₁ progenies were grown in rows of 45 plants and inoculated with a spore suspension of Helminthosporium carbonum Race I when about one foot tall. Plants were rated for resistance one week after inoculation, 3 weeks after inoculation and again when mature. Since susceptibility to infection by this fungus is inherited as a single recessive factor it might be expected that the F₁ progenies (HM hm) would be completely resistant. This expectation was realized in all but one case, hm/hm x 1‑7. In this progeny it was apparent at the first scoring that all plants were more susceptible than is usual in heterozygous material, and that some plants were quite susceptible. The critical factor in scoring was size of lesions, and a scale was used where 10 = complete susceptibility (of the K61 x Pr type) and 1 = complete resistance. On this basis all mature plants in the above progeny had a rating of 2 or more with some plants being as high as 6.

Since limited observations showed the translocation parent (1‑7) to be completely resistant, and the F₁ progeny to show an intermediate degree of resistance, and the break in chromosome 1 was on the long arm in the vicinity of the Hm locus, it seemed possible that a position effect of the Dubinin type might be involved. To check this hypothesis, many F₁ plants of various degrees of intermediate resistance were back‑crossed by the susceptible parent (K61 x Pr). If the position effect hypothesis were tenable some completely resistant plants should be found in the back‑cross progeny. They would arise from crossing‑over between the locus of hm and the point of translocation so as to put the Hm allele in a normal chromosome where presumably it would function as a complete dominant. The back‑cross progeny would then consist of (1) semi‑sterile plants of intermediate resistance and (2) fertile plants of complete susceptibility in the non‑crossover group; and (1) fertile plants of complete resistance and (2) semi‑sterile plants of complete suceptibility in the cross‑over group. Our findings in 1950, however, showed that the fertile plants that were not completely susceptible were of intermediate resistance, just are the majority of their semi‑sterile sibs, rather than completely resistant as a position effect hypothesis would demand. Thus this explanation is ruled out. In 1950 the back‑cross progenies (T Hm/+hm)x(+hm/+hm) were inoculated when about 1 foot high and scored three times during the growing season. The final ratings are summarized in table 1 as to numbers of plants falling into each class (where 10 = complete susceptibility and 1 = complete resistance). It is apparent for the first four progenies that the expectation of approximately fifty per cent homozygous hm/hm (completely susceptible) plants is realized. As was found in the previous year the majority of the heterozyotes were not of complete resistance but of the intermediate type. In 1950, 59 plants of the translocation (1‑7) were inoculated and all were found to be completely resistant. The rating of plants of intermediate reaction was independent of the location of Hm in a normal or translocation (as has been mentioned above). This can be inferred from the fact that the 11 plants that had normal pollen and were not completely susceptible (cross‑over types) were intermediate in resistance. The ratings of these 11 plants were 6, 5, 7, 3, 5, 7, 1, 4, 4, 7, and 1. Had the intermediate ratings of the F₁ plants been due to the proximity of Hm to the point of translocation the plants above resulting from cross‑overs should all have had a rating of 1. The intermediate phenotype of the heterozygous plants (both in F₁ and back‑cross progenies) may be ascribed to one of two alternative explanations which cannot be differentiated. The translocation parent (1‑7) may be carrying an Hm iso‑allele -- a different allele but distinguishable only by special tests -- such that Hm'/Hm' is resistant but Hm'/hm is intermediate in reaction. The other explanation is that 1‑7 is carrying the allele for resistance Hm with closely linked modifiers working towards susceptibility so that Hm mod./hm mod. is resistant while Hm mod./hm is intermediate. This second hypothesis while explaining the observed data as well as the first does not seem to be as sound.

Table I

Back‑cross Progenies (T Hm/+hm) x +hm/+hm. 1950

Rows	Rating of F1 in 1949	Distribution of plants in different classes Classes of Resistance										Total
		1	2	3	4	5	6	7	8	9	10
2245‑48	6	3	2		2	3	6	2	6	0	29	56
2249‑51	6	0	2		2	3	2	1	0	0	27	40
2252‑64	5	7	21		19	9	9	5	2	0	94	199
2265‑79	5	12	27		8	6	6	0	0	0	97	176
2280‑94	2	37	22		15	13	14	13	24	0	62	214

An interesting situation is found in regard to the fifth back‑cross

progeny in Table 1. Although the F₁ parent had a low disease rating of 2, it

showed a greater percentage of intermediate plants than any other progeny and

a significant deficiency of completely susceptible plants. Quite a different

picture is obtained if the results of the first and second scoring, are tabulated with the third scoring as in Table 2. It is obvious that the 51 plants (113-62) first scored as susceptible later had a lower disease rating, some plants changing in score as much as 9 points on the scale.

Table 2

Backcross Progeny (THm/+hm) x (+hm/+hm) 1950

Rows	Rating of F1 in 1949	Distribution of plants in different classes Classes of Resistance										Total
		1	2	3	4	5	6	7	8	9	10
2280-94^1	2	72	10	9	5	2	6	3	0	0	113	214
2280-9A^2	2	59	18	9	9	7	12	8	27	0	65	214
2280-9A^3	2	37	22	14	15	13	14	13	24	0	62	214

¹As classified at first scoring.

²As classified at second scoring.

³As classified at third scoring.

With a further grouping of the intermediate from line 3 of Table 2 into two groups -- 1‑2 and 3‑8, the following situation is found at the third scoring:

Completely susceptible	62
Susceptible changing to intermediate	51
Intermediate	48
Resistant or nearly so	53

In the light of the above data it is suggested that the F₁ parent of Rows 2280‑9A was heterozygous for a single dominant factor not linked to the locus hm and which is rather effective in modifying the phenotype towards resistance as the plant grows older.

A composite of all back‑cross progenies gives 642 plants of which 34 were cross‑over types giving an estimate of 5.30% recombination between the hm locus and the point of translocation in chromosome 1 in 1‑7.

O.E. Nelson and A.J. Ullstrup