Mitochondrial DNA emerging from a mitochondrion (blue and partially cutaway). Mitochondria are small organelles thought to be descended from internalised bacteria.They contain their own DNA (circular like bacterial DNA). Mitochondria in egg cells replicate as the fertilised egg divides and so the next generation carries mtDNA from the mother (not the father, sperm do not contribute any mitochondria during fertilisation). Family trees can be drawn up based on mtDNA. Certain diseases are inherited via the mitochondria.