As the public debate in the UK over hybrid embryos nears its climax in a House of Commons debate next month, some of the facts, shrouded up to now in a battlefield haze of gunpowder smoke, are becoming clearer. One assertion which has slipped through the media with little comment is the fact that hybrid embryos (or cybrids, or human admixed embryos, etc) are 99.9% human or, conversely, only 0.1% animal, depending on the audience. However, an article by Roger Highfield, science writer for the London Telegraph, points out that percentages mean almost nothing.

"The 99.9% figure probably refers to the fact that there are 37 instructions (genes) in mitochondrial DNA, compared with 29,000 in nuclear, which means 0.1% animal instructions. But this is hardly reassuring. Decades of work has shown that even one genetic spelling mistake in the three billion letters of the nuclear code can be fatal - just 0.0000001 per cent. And mitochondria are important: faults in them are responsible for around 50 metabolic disorders that affect one in every 6,500 people. This includes fatal liver failure, stroke-like episodes, blindness, mental retardation, muscle weakness, diabetes and deafness.

"Some claim that using animal mitochondrial DNA in cybrids would be like changing a battery in a computer, leaving the ‘hard disk’ -- the nuclear DNA -- unaffected. But Dr Marc Vermulst of the University of Washington in Seattle, who has linked changes in mitochondria to premature ageing, says: ‘In flies, if you mix the mitochondrial DNA of one strain with the nuclear DNA of another strain, the mitochondria of the mixed strain work less efficiently than they normally would. By evolving together, mitochondria and nuclei have become very finely tuned to each other. I am not sure how well human and cow DNA would communicate with each other. That would be very important.’" ~ London Telegraph, Apr 8