The fertility watchdog cleared a team at the University of Newcastle upon Tyne yesterday to conduct an experiment to prevent genetic disease by merging single-cell embryos with donated eggs.

The decision to approve the procedure on appeal, after two previous applications were rejected, is controversial because it could eventually lead to the birth of children who carry genes from two mothers and a father.

It also opens the possibility of “germ-line” genetic engineering, because any children born would carry added genes that would be passed to successive generations.

At present, gene therapy to alter defective DNA is permitted only when such changes would not be passed on.

The licence awarded yesterday by the Human Fertilisation and Embryology Authority (HFEA) allows only experimental use of the technique and not the implantation into a womb of any resultant embryo.

The Newcastle team does not envisage applying for permission to conduct such procedures for reproductive purposes until several years of research have shown it to be effective and safe, though the ultimate goal is to employ it to create healthy children.

Some observers said the HFEA had overstepped its remit when the Government has begun a review of embryology legislation. A legal challenge is likely because the appeal turned on reinterpreting a ban on altering the genetic structure of a cell.

The researchers, led by Professor Doug Turnbull and Mary Herbert, want to prevent inherited diseases caused by mutations in the DNA in cell structures called mitochondria, which provide a cell’s energy.

About one in 5,000 people carry defects in the mitochondrial DNA. Most lead only to mild effects, but in rare cases defects can cause miscarriage, or fatal brain, liver and kidney damage in offspring. One patient the team hopes to help has had five miscarriages, and eventually gave birth to a child with profound brain damage.

Mitochondria are inherited from the mother. The team plans to replace defective mitochondria in eggs with working ones from donor eggs.

First, an egg from a woman carrying mitochondrial defects will be fertilised in vitro using her partner’s sperm. At the point of fertilisation, two “pronuclei” containing genetic material from the mother and father will be removed, and injected into an unfertilised egg from which the nucleus has been removed.

This donated egg will contain healthy mitochondria, but none of the nuclear DNA that makes up most of the human genetic code.

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