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Special note “Beyond Mendelian genetics” ***Remember that some traits result from non-nuclear inheritance! 1. Chloroplasts and mitochondria are randomly assorted to gametes and their daughter cells; thus, traits determined by chloroplast and mitochondrial DNA do not follow simple Mendelian rules 2. In animals, mitochondrial DNA is transmitted by the egg and not by the sperm; as such, mitochondrial-determined traits are maternally inherited Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today's eukaryotic cells. This theory is called the endosymbiotic theory. Interestingly, the mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Also, most mitochondria are present at the base of the sperm's tail, which is used for propelling the sperm cells; sometimes the tail is lost during fertilization (and so none of dad’s mDNa get into the egg, usually). In 1999 it was reported that paternal sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo. Some in vitro fertilization techniques, particularly injecting a sperm into an oocyte, may interfere with this. Recall: the story about the possibility of 3 parent babies (woman donates her mitochondrial to be inserted into the egg of another woman) http://www.bbc.co.uk/news/health-23079276 The fact that mitochondrial DNA is maternally inherited enables genealogical researchers to trace maternal lineage far back in time. ALSO, some traits are Sex-limited….Expression depends on the sex of the individual. Only female mammals can produce milk. Also, male pattern baldness is inherited from the mother’s side.