As described in this chapter, mothers will pass on a mitochondrial defect to their offspring. In a type of gene therapy, one approach to circumvent this problem is to have two different maternal contributions, with the nucleus of the female with the defective mitochondria being placed in an enucleated egg derived from a female with normal mitochondria. After fertilization, the resulting offspring would have three parental sources of DNA—with nuclear DNA derived from a mother and a father, and mitochondrial DNA derived from another 'mother.' Recently, children with this genetic makeup have been born, but the elimination of defective mitochondria is not complete, with the amount of defective mitochondria derived from the defective mother ranging from 0 to 9%. Discuss potential complications resulting from such a mixture of genomes.