Preimplantation genetic diagnosis probes disease-causing genes in an eight-celled cleavage embryo.

If genetically healthy, cleavage embryo is implanted in woman and develops into a baby.

If genetic disease is inherited, cleavage embryo is not implanted into woman.

WWWlifflBi single cell from each of several 8-celled cleavage embryos, and probed those cells for the disease-causing gene variant that ran in the family. They also scrutinized the HLA genes, which control rejection of a transplanted organ, and chose the ball of cells that would be Adam to implant into Lisa Nash's uterus. A month after the birth, physicians infused the umbilical cord stem cells into his sister. So far, it is working.

Preimplantation genetic diagnosis works because of a feature of many animal embryos called indeterminate cleavage. That is, up until a certain point in early development, a cell or two can be removed, yet the remainder of the embryo can continue to develop normally if implanted into a uterus. Allen

Handyside and colleagues at Hammersmith Hospital in London invented the technology in 1989. The first cases helped a few families to avoid devastating inherited illnesses in their sons. Then in 1992 Chloe O'Brien was born free of the cystic fibrosis that made her brother very ill, thanks to PGD. In 1994 came another milestone, when a girl was conceived and selected to provide umbilical cord stem cells that cured her teenage sister's leukemia.

So far, about 500 children have been born worldwide following PGD. In addition to enabling families to circumvent particular inherited conditions (table 23A), it enables couples who repeatedly lose early embryos due to chromosome abnormalities to select chromosomally normal embryos. Eventually, PGD may become a routine ad

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