"Sometimes it works out that a foreign mitochondrial component" Every embryo has 1/2 of genes foreign to mother's mitochondrias - with no gene or combination i

"Sometimes it works out that a foreign mitochondrial component" Every embryo has 1/2 of genes foreign to mother's mitochondrias - with no gene or combination is guaranteed to be non-foreign. If it doesn't cause problems, why would having all foreign genes suddently cause them?