By cloning and sequencing the mtDNA of cows from different maternal lineages, it was found that multiple heteroplasmic mtDNA states within the D-loop and rRNA genes of mtDNA existed extensively in Holstein cattle. Meanwhile, the nature and transmission of mtDNA were investigated. The mtDNA population in a specific animal was found to be a mixture of different genotypes. Because heteroplasmy was observed frequently and seemingly is persistent, selected amplification of specific mtDNA molecules may not be a part of mtDNA inheritance. At same time, four hypervariable sites were located at nt 169, 216, and 1594 between nt 352 to 364. The data also suggest that replication slippage might be a common cause of the transversion, insertion, and deletion;In another study, a 3.6 kb bovine mtDNA containing several tRNA and protein coding regions was cloned and sequenced from 13 cows. Four different nucleotide substitutions were detected in these regions. The frequency of nucleotide substitutions in the tRNA and protein coding regions was much less than that in the D-loop and rRNA gene regions. Transition substitution was the major type of variations in the tRNA and protein coding regions. Transitions occurred at the third nucleotide of the codon or in a loop region of tRNA but did not change the amino acid encoded by the codon or the general structure of tRNA. The transversion at nt 9682 from guanine to cytosine was considered to be exception because the conservative amino acid is alanine as based on data from other animal species. Therefore, the "true" sequence of bovine mtDNA in the germ line could be cytosine at nt 9682, and the originally published sequence could be a rare substitution in the cow that was used. No significant difference could be expected from all the different genotypes of mtDNA in the protein and tRNA coding regions;The data delineated the unique characteristics of mtDNA sequence variations in the D-loop, rRNA, and protein coding regions and provided the evidence of random passing of mtDNA from ancestor to progeny.