The principle of the procedure is that the mitochondria in a host mother’s egg is defective, preventing her from having children. By the introduction of ooplasm, or the selective contents of an egg from a healthy donor to the host, the mitochondrial defect is overcome as the defective organelles are replaced by healthy ones.
Mitochondria carry their own set of genes, distinct from the nuclear DNA, and tests confirmed that children from this procedure carried the genetic material from the birth mother, the father and the donor of the ooplasm. In other words the children had inherited an extra set of genes, and these altered the germline, those genes that they will eventually pass onto their offspring.
As a matter of ethics any procedure that tends to alter the germline of human beings i.e. a change in the genetic material that could be inherited, is illegal in most countries and considered unethical by many; in a large part this is due to the unknown effects of such genetic modification.
However, others point out that this procedure would not lead to 'designer babies' as the nuclear DNA, which is responsible for the traits we inherit (except those that are regulated by mitochondria genes) is not altered by this technology. Dr. Cohen asserted at the time that the procedure did not manipulate genes, it was simply the addition of harmless genes from the mitochondria.