If you were to survey a random group of men about their greatest health concerns, I doubt the length of their sperm cells would even make the list.

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They identified two genes - zipper and lgl - which they found worked together to change the make up of sperm cells, making it impossible for them to fertilise uninfected eggs.

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Instead, they used a measure called recombination distance, a gauge of how likely genes are to be reshuffled when sperm and egg cells are produced.

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In mammals, this kind of epigenetic modification has been assumed to last only during the life span of a particular animal, because such changes are usually erased in the sperm and egg cells.

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At the moment, would-be parents are offered a sperm "washing" technique prior to insemination, which removes cells and other detritus which can harbour the virus.

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My attention was drawn to this market by a review at the Feminist Law Professors Blog on the newest book in the gamete-trade, Sex Cells: The Medical Market for Eggs and Sperm.

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Building on earlier research, which showed that egg cells from rabbits could be fertilised in test tubes when sperm was added, Edwards developed the same technique for humans.

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Sperm, by contrast, are made continuously throughout life, and each division of their precursor cells brings risk of a misinterpretation of the DNA, and thus a mutation.

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South Korea's researchers have taken egg cells from volunteer women, removed the nuclei from those cells (which contain only half of the genetic complement required to make a human being, since the other half is provided by the sperm), and replaced each nucleus with one taken from one of the volunteer's body cells (which contains a full genetic complement).

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