The science

Bioethics: Human-animal hybrid embryos

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.

Possible types of animal/human hybrid embryos

Cytoplasmic hybrid embryos: embryos created through cell nuclear replacement using animal eggs

embryos created through cell nuclear replacement using animal eggs Hybrid embryos: embryos created by mixing human sperm and animal eggs or human eggs and animal sperm

embryos created by mixing human sperm and animal eggs or human eggs and animal sperm Human chimera embryos: human embryos which have animal cells added to them during early development

human embryos which have animal cells added to them during early development Animal chimera embryos: animal embryos which have human cells added to them during early development

animal embryos which have human cells added to them during early development Transgenic human embryos: human embryos which have animal genes inserted into them during early development

Source: Human Fertilisation and Embryology Authority

Proportions of human and animal DNA

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

Public opinion

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.