I agree that DNA in mitochondria is much smaller than DNA in the nucleus. But in each person there are many mitochondria and they nay have slightly different DNA. And the DNA in mitochondria has a different variation than the DNA in the nucleus. So it's difficult to weight both.
Can we say 2.1 parents? A long time ago I read that most binary classifications are not completely binaries, it's just that 2 options cover almost all the cases. (Are virus alive?) I guess integer classifications also have hidden corner cases.
I also remember from a biology book that in a lab they mixed two blastula(?) of small lizards(?) or something like that. They had different skin color and the baby had patches of both colors. Does that count as 2 or 4 parents?
> I think it would be better to describe this as an ‘organelle’ transplant as it would be easier for people to understand and discuss.
Unlike previous attempts, the donor mitochondria are not transferred into the mother egg. Instead the donor cell is denucleated, and the nucleus from a mother's egg is transferred into the denucleated donor cell. Consequently, there is a wide variety of donor specific material, which may influence the early stages of development and only "wash out" after a number of cell divisions.
> But calling it a 3 person baby is unhelpful and misleading as IMO mitochondria DNA is of a different category to chromosomal DNA.
How so? Arguably, mitochondrial genes are much more essential than most nuclear genes.
1. Mutations in any mitochondrial gene often have dire consequences, whereas variants in nuclear genes are much more frequent.
2. Mitochondrial DNA is the most expressed in pretty much any cell by a huge margin. Mitochondria express 13 (IIRC) protein coding genes and two dozen other RNAs. Those 30 odd genes often make up 1-5 % of a cell's whole transcriptome. Only genes coding for ribosomal RNA are more strongly expressed.
human nuclear genome size (haploid): 3.1 billion bp
mitochondrial genome size: 16 000 bp
1 human nuclear genome per egg -> 3.1 billion bp nuclear DNA
100 000 mitochondria, each with 1-10 genomes per mitochondrion [1] -> 1.6-16 billion bp mitochondrial DNA
So the ratio of mitochondrial to nuclear DNA in human eggs is on the order of 0.5 to 5.
Anyway, it's a very slow procces, IIRC like millions of years. We can ignore it in the human escale.
Also, both DNA use a sligtly different genetic code. From https://en.wikipedia.org/wiki/Human_mitochondrial_genetics
> For most organisms the "stop codons" are "UAA", "UAG", and "UGA". In vertebrate mitochondria "AGA" and "AGG" are also stop codons, but not "UGA", which codes for tryptophan instead. "AUA" codes for isoleucine in most organisms but for methionine in vertebrate mitochondrial mRNA.
So it's not as easy as cut&paste.