Twenty-two new mutants have been isolated at the k2 (tan saddle seed coat) Mdh1-n (mitochondrial malate dehydrogenase 1 null) y20 (yellow foliage) chromosomal region in this dissertation research. Mandell, PI 567.391 and PI 567.630A are Mdh1 nulls. T261 was found to be Mdh1 null and tan saddle seed coat; T234 and CD-9 are yellow foliage and Mdh1 nulls. Sixteen new mutants have been isolated with tan saddle seed coat, Mdh1 null and yellow foliage phenotypes;All tan saddle seed coat mutants, the 21 Mdh1 null mutants, the 18 yellow foliage mutants were allelic at the k2, Mdh1-n, and y20 loci, respectively. k2, Mdhl-n and y20 alleles were inherited as a single nuclear recessive alleles, respectively. Approximately 1 ± 1.36% recombination rate between the k2 and the Mdh1-n loci was detected in coupling phase in F2 plants from the crosses of T261 (k2 Mdh1-n) with wild type plants and approximately 3.04 ± 0.48% recombination rate was estimated between the k2 and Mdh1-n y20 loci in F2:3 families with tan saddle seed coat from crossing T317 (Mdh1-n y20) with T239 (k2) and L67-3483 (k2);Approximately 55-60% recombination rates between the k2 and Mdh1-n y20 loci were detected in repulsion phase in F2 plants derived from crossing T323 (Mdh1-n y20), T324 (Mdh1-n y20) and T325 (Mdh1-n y20) with T239 (k2) and L67-3483 (k2). The excess of the recombinants in repulsion phase in F2 plants was suggested to be chromosomal breakage at the k2 Mdh1 Y20 region in T239 and L67-3483. However, no recombinants were identified in F2 plants in repulsion phase in the crosses of T317 (Mdh1-n y20) with T239 (k2) and L67-3483 (k2) and no recombinations among the k2, Mdh1-n and y20 loci were detected in coupling phase in about 13,187 F2 plants;Genetic instability were evident by introducing the w4-m and Y18-m mutable systems, and by introducing the Mdh1-n allele of PI 567.630A into T239 (k2) and T261 (k2 Mdh1-n) genetic backgrounds. Sixteen bright yellow mutants with tan saddle seed coat, malate dehydrogenase null phenotypes and 21 greenish yellow mutants with tan saddle seed coat have been isolated. Fifteen bright yellow mutants and 21 greenish yellow mutants were found to be due to the mutations at the Mdh1 Y20 loci in T239. However, no mutants were identified in 945 F2 families in the control crosses with the T239 isogenic line, Harosoy, as one cross parent, indicating that the mutations at the Mdh1 Y20 loci in T239 were due to the impact of the k2 allele in T239. Finally, the genetic instability at the k2 Mdh1-n y20 chromosomal region was suggested to be receptor element activities at this chromosomal region which may contain complex chromosomal rearrangements in T239 and T261;Finally, a heritable heterozygous female semisterile factor has been isolated in L67-3483 genetic background and four monogenically inherited nuclear recessive male-sterile mutants have been isolated in this dissertation research.