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In May 2008 a cross-party attempt to ban hybrid human animal embryos was defeated on a free vote in the House of Commons, by 336 to 176. MPs had been debating the Human Fertilisation and Embryology Bill, which would allow regulated research using hybrid or 'admix' embryos, where the nuclei of human cells are inserted into animal eggs. The resulting embryos would be kept for up to 14 days to harvest stem cells.

In the present state of science, hybrid embryos are produced as research tools, and only kept alive for 14 days or fewer. The article below only deals with the ethical issues of this case, and not with the ethics of producing new creatures that are a combination of animal and human.

A hybrid embryo is a mixture of both human and animal tissue. There are several types of hybrid embryo (listed below), but recent controversy has focused on cytoplasmic embryos.

These are created by transferring nuclei containing DNA from human cells into animal eggs that have had almost all of their genetic information removed.

The resulting embryos are more than 99% human, with a small animal component, making up around 0.1% (more detail below).

The embryos are grown in the lab for a few days, then harvested for stem cells: immature cells that can become many types of tissue. The embryonic stem cells are used in research into different diseases as a way of addressing the shortage of human eggs available for research.

Scientists do not intend to actually create living animal-human hybrids. [Such beings are often referred to as chimeras after a creature in Greek mythology which had a lion's head, a goat's body, and a serpent's tail.]

Some 200 medical charities have urged MPs to support legislation allowing the creation of animal-human embryos.

Source: Human Fertilisation and Embryology Authority

In cytoplasmic hybrid embryos, all the DNA in the cell nucleus is human. The remaining animal DNA is found only in the mitochondria, which are small rod-shaped organelles found outside the nucleus that produce energy for the cell. Mitochondrial DNA is separate from nuclear DNA; it is not involved in cellular division or reproduction.

So the main genetic material is 100% human. Embryos produced by this technique are considered fully human - a "live human embryo" - by the Human Fertilisation and Embryology Act.

A combination of human nuclear DNA and animal mitochondria is likely to make a difference to the resulting cells, but not enough is known about mitochondrial disorders to predict what the effects might be. The House of Commons Select Committee report acknowledged this and viewed it as a positive point.

However, rather than deem this a reason not to conduct such research, [Dr Justin St John from Birmingham University] believes that the creation of human-animal chimera or hybrid embryos may actually "offer us the opportunity to elucidate some of the causes of mitochondrial DNA disease" and that "not to allow this work to go ahead would considerably disadvantage experimental work in these fields".

House of Commons Select Committee on Science and Technology Fifth Report, June 2007

In 2007, a poll conducted by the HFEA found that 61% of the public supported the research after having the process and goals explained to them, while a quarter was opposed to the research.

We believe, too, that we are God's partners in the work of creation, both as innovators and as protectors, and that this partnership obliges us to be guided by ethical principles.

Office of the Chief Rabbi, Memorandum to Select Committee on Science and Technology November 2004

There are strong arguments on both sides of the ethics of creating human/animal embryos. We've listed some of the arguments put forward below.

In some cases the arguments seem to be similar but expressed in a different way.

In some cases arguments on the same side of the case seem to conflict with each other.

Quotations on both sides of the argument can be found at BBC News - In quotes: The ethics of embryos

...the creation of an animal-human being represents a natural border that has been violated, the most grave of violations.

Bishop Elio Sgreccia, president of Pontifical Academy for Life, May 2007

We think there is nothing illegal, immoral or unethical about this.

While we understand the concerns, we think they are largely founded on misinformation. People think we are generating some sort of hybrid animal. This is just cells, just for science. No animal is ever going to be created.

Professor Chris Shaw, Kings College London, 2008