Experiments using mice have shown that the offspring with the new mitochondria carry no information that defines any human attributes.

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The researchers said that offspring of their mice had so far shown no signs of this, although more research was needed to confirm safety.

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When the modified offspring of these mice were examined, she and her colleagues found that the transgene was being expressed in widely varying ways.

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The hybrids were bred again with deer mice, and the type of burrows those "backcross" offspring made show the inheritance patterns of the behaviours - and strong hints of how many genetic changes are required to modify it.

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These eggs were then transplanted into a female mouse and healthy offspring were born who grew into fertile male and female adult mice.

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