The prospect of mixing together genetic material from human beings and animals triggers a deep and universal instinct of unease. That should make us question rigorously any such proposal even, indeed especially, one which holds out the prospect of finding a cure for devastating diseases. The government's Human Fertilisation and Embryology Bill contains a controversial measure which would allow the creation of human/animal hybrid embryos for medical research. A number of MPs, most prominently three Catholic members of the cabinet, are known to be unhappy at being required to vote for this and are said to be prepared to resign rather than vote against their consciences.

The simmering parliamentary disquiet has been brought to a public boiling point by the decision of Cardinal Keith O'Brien, the leader of Scotland's Catholics, to use his Easter sermon for an attack on the Bill. It is right for religious leaders to argue a moral standpoint; they provide a substantial framework of core values for the ethical debates which affect us all when scientific advance moves the boundaries of possibility. Instead, Cardinal O'Brien has plunged the debate into uninformed polarity by condemning the proposals to allow human cells to be injected into eggs taken from animals as a "monstrous attack on human rights, human dignity and human life" that will enable experiments of "Frankenstein proportion".

This deliberate suggestion of the creation of hybrid creatures has achieved a high profile for the issue, but at the cost of misleading the public. The Bill proposes taking eggs from animals, most probably cows, from which the cow nuclei are removed and cells from humans injected. The resulting embryos - which would be more than 99% human - would not be allowed to develop beyond the 14 days already established as the limit for research using entirely human embryos.

The scientists who want to carry out this research argue that it will allow them to discover how certain genetic mutations go on to cause neurological diseases such as multiple sclerosis, Parkinson's and Alzheimer's. To many people, such work, if successful, would be not only life-enhancing, but life-affirming. The government argues that Britain is currently at the forefront of such medical research, but if further embryo research is outlawed, it will be carried out elsewhere, possibly by British scientists. Cardinal O'Brien believes that the potential for embryo-based research has been overstated and argues that research using adult stem cells has already produced treatments and should, therefore, be the focus of future work. What is most urgently required (and before the second reading of the Bill if possible) is a properly informed debate involving scientists and ethicists. Advances in medical science have always raised ethical dilemmas, and public acceptability is the appropriate brake to apply to scientific possibility. What is acceptable, however, can only be decided on a basis of information, otherwise genuine points of difference are lost in inflammatory confrontation.

In such matters, sticking too rigidly to parliamentary procedures can undermine rather than enhance democracy. It is entirely principled for MPs of any political party and of any religion or none to have misgivings about or objections to aspects of this Bill. The Prime Minister has said he will "respect the conscience of every member of this House". In that case, his own conscience should urge him to announce the free vote which would separate the political from the ethical considerations.