According to the endosymbiotic theory, mitochondria are believed to have initially been free-living bacteria which were engulfed by other bacteria. the majority of mitochondrial genes are believed to have been transferred into the nuclear dna of eukaryotic cells by the process of

Lateral gene transfer.

Explanation:

Gene transfer may be defined as the mechanism by which the gene has been transferred to the other generation or the other organism. Two main types of gene transfer are horizontal gene transfer and vertical gene transfer.

The lateral gene transfer may be defined as the transfer of genes in the organism other than the parents to their offspring. The transfer of mitochondrial genes to the nuclear DNA has occurred by the horizontal gene transfer as the gene has been transmitted between the different organism and not to the next generation.

Thus, the answer is lateral gene transfer.

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