It is unfortunate that the picture chosen to illustrate the article "Engineered to perfection" (THES, July 10) about the issues surrounding pre-implantation genetic diagnosis was misleading, as were certain aspects of the text.
The embryo in the pictures has been developing for four weeks, whereas embryos used for pre-implantation genetic diagnosis are only three days post fertilisation. They consist of just eight cells, a tiny ball almost invisible to the naked eye. At this stage, it has not even been determined which cells will form the embryo and which the placenta. This is why it is sometimes called a pre-embryo.
Because the genetic testing is performed at such an early stage, embryos have to be generated by in vitro fertilisation. They are not removed from the mother; this would be impossible to achieve at a sufficiently early stage.
Joy Delhanty Human genetics group University College London
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