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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
Recall: the story about the possibility of 3 parent babies (woman donates
her mitochondrial to be inserted into the egg of another woman)
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.