Nearly a fifth of all human genes have been patented - the majority by private biotechnology companies, according to a survey of patent records published today.

The extent to which companies claim ownership of human genes has raised alarm among researchers and led to warnings that by asserting commercial rights over crucial genes, companies risk stifling research into diseases such as breast cancer, diabetes and obesity.

Legal cases triggered by disputes over who owns specific genes and how access to working on them is restricted are also likely in future, the scientists warn.

Kyle Jensen and Fiona Murray at Massachusetts Institute of Technology calculated the proportion of human genes that had been patented by comparing the genetic sequences claimed in US patents to genes listed in the National Centre for Biotechnology Information gene database. They were surprised to find so many had already been patented. "The stories that we hear in the media only concern a very small number of human genes," said Dr Murray. "But it turns out that a high number have been patented, including some of the more obscure ones."

Writing in the journal Science today, the researchers report that nearly 20% of the human genome, or 4,382 of the known 23, 688 human genes, have been patented, with over half owned by private companies. Around 63% of the patents are assigned to private firms, with one firm, Incyte Pharmaceuticals/Incyte Genomics, having intellectual property rights covering 2,000 human genes.

The researchers also found that the rights to some human genes, such as BRCA1, the gene linked to breast cancer, are claimed by more than one company, with as many as 20 patents asserting rights to various uses.

As yet it is not clear how gene patenting will affect medical research, but many scientists fear that by monopolising specific genes, companies will exclude other researchers from studying them.

John Harris, a professor of bioethics at the University of Manchester and member of the Human Genetics Commission, said: "The pharmaceutical industry tend to argue they need to protect the products of their research, otherwise they would not invest in future research. However, I worry that this kind of patenting could have impacts on the cost of health and the freedom to access it."

According to Helen Wallace of the pressure group Genewatch, the patent grab threatens to skew research into areas where intellectual property rights are easy to secure. "It encourages a search for genes, when many problems with health could be addressed by better research into diet, social and economic factors."

Although Dr Murray's study only looks at the patenting of human genes in the US, it is likely that the same genes will have been patented in Europe. "The patent systems in Europe and the US are separate, but normally people will apply for patents in all major jurisdictions," said Hilary Newiss, a solicitor who specialises in intellectual property law and a member of the Human Genetics Commission.