This calculation is looking at the amount of genetic material in a cell by (essentially) weight. Let's take that as the right way to compare these for a moment.

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Nuclear genome (no sex chromosones): 2,881,033,286 base pairs

Sex chromosones (avg): 262,592,623 base pairs (310,541,120 women, 214,644,126 men)

Nuclear genome (avg): 3,143,625,909 base pairs

Mitochondrion genome: 16,569 base pairs

mtDNA in a cell: 500

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The proportion of genetic material in a cell that comes from mtDNA is thus:

(total mtDNA) / ((nDNA no sex) * 2 + (nDNA sex) + (total mtDNA))

(16,569 * 500) / (2,881,033,286 * 2 + 262,592,623 + 16,569 * 500)

which is 0.00137, or 0.137%

If we look at just the amount of information encoded, we get:

(mtDNA) / (nDNA + mtDNA)

(16,569) / (3,143,625,909 + 16,569)

which is 5.2710^-6 or 0.000527%