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Since 2D F1 monosomic plants carried no chromosome 2D from Chinese Spring, there was no Nr in their genomes. In addition, 2D F1 monosomic plant was heterozygous for genes promoting development of SS. Therefore, all or part of 2D F1 monosomic plants should exhibit SS phenotype, if the genes promoting development of SS are dominant or incompletely dominant. However, the F1 monosomic analysis indicated that all 2D F1 monosomic plants exhibit only normal spikes, as the other F1 plants did. So, the genes on chromosomes 2A and 4A which promote the development of SS are recessive, and the gene symbol ss was used to represent them in this paper. The conclusion of Koric (1973) and Dencic (1988) that dominant or incompletely dominant genes promoted the development of SS is not valid for present experiment. In this aspect, the ss gene is synonym of the bh (branching head) gene which was first used by Sharman (1944) in emmer wheat.

Based on the result of F2 monosomic analysis, we conclude that the SS character in this cross is controlled by three genes, i.e., two recessive ss genes on chromosomes 2A and 4A which promoted development of SS character, and a strong dominant inhibitor of SS (Nr) on chromosome 2D which prevented the expression of this character. In the light of this conclusion, the segregation ratio of spike type in progenies which derived from backcrossing F1 disomic or monosomic plants to LYB could be predicted. For convenience, we adopt the symbol ss l and ss 2 to represent the genes on chromosomes 2A and 4A respectively. Clearly, the genotype of LYB is ss1ss1ss2ss2nrnr, and that of Chinese Spring is SS1SS1SS2SS2NrNr. According to Mendel's laws, F1 disomic plant (SS1ss1SS2ss2Nrnr), or F1 plant monosomic for chromosome other than 2A, 2D and 4A (the genotype is SS1ss1SS2ss2Nrnr too), is expected to produce 8 different genetic constitutions of gametes, among them, 1/8 gametes is ss1ss2nr, 1/8 gametes is ss1SS2nr and 1/8 gametes is SS1ss2nr. These three types of gametes (total proportion is equal to 3/8) combine with male gamete (ss1ss2nr) of LYB to form the genotypes ss1ss1ss2ss2nrnr, ss1ss1SS2ss2nrnr and SS1ss1ss2ss2nrnr for SS phenotype. So, the expected segregation ratio of spike type in the BC1 population which derived from backcrossing F1 disomic plants, or F1 plants monosomic for chromosome other than 2A, 2D and 4A, to LYB would be 5:3 (normal :SS). Among the female gametes formed by 2A F1 monosomic plant (ss1SS2ss2Nrnr), those having the genetic constitution ss1ss2nr, ss1SS2nr, Oss2nr (O: delete chromosome 2A and thus delete SS1 locus) or Oss2nr combine with male gamete of LYB to form genotypes ss1ss1ss2ss2nrnr, ss1ss1SS2ss2nrnr, ss1ss2ss2nrnr and ss1SS2ss2nrnr for SS phenotype, and frequency of each the 4 types of female gametes is 1/8. Thus the BC1 population which derived from backcrossing 2A F1 monosomic plants to LYB should exhibit 1/2 SS spike plants. In the same way, the BC1 population which derived from backcrossing 4A F1 monosomic plants (SS1ss1ss2Nrnr) to LYB should exhibit 1/2 ss spike plants too. The 2D F1 monosomic plant (SS1ss1SS2ss2nr), produces only 1/4 female gametes (SS1SS2) which combine with male gametes (ss1ss2nr) of LYB to form genotype SS1ss1SS2ss2 for normal spike. Therefore, the expected segregation ratio of spike type in the BC1 population which derived from backcrossing 2D F1 monosomic plants to LYB is 1:3 (normal: SS). Chi-square analysis showed a good fit to the expected segregation ratio in BC1 progenies (Table 2), indicating that the result of the backcross test supported the three gene inheritance pattern based on the data of F2 monosomic analysis. Klindeworth et al. (1990) concluded that the SS character was primarily conditioned by the group 2 chromosomes. The presence of genes controlling SS character on chromosomes 2A and 2D in this cross supported this conclusion. Moreover, the finding that chromosome 4A carried ss gene indicated that the SS character is also controlled by chromosome other than group 2.


Acknowledgments

The first author wishes to express his appreciation to Prof. YL Zheng for his helpfud assistance in the preparation of this manuscript.

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