We all have hundreds of defective genes, study finds

A four-year, £75m project to discover novel variations between human genomes has completed its pilot phase. Among the 1000 Genomes Project's initial findings are each person carries around 250 - 300 genetic mutations. Scientists have previously linked 50 - 100 of these mutations to inherited diseases.

Eight million previously unknown variations were found in SNP (single nucleotide polymorphism) - the building blocks of genes. The study also revealed one million structural variations, such as repeated or deleted DNA sequences and rare differences in the genetic code that occur in only one percent of the population.

The pilot phase of the project sought to develop and compare several different methods of genome sequencing on a sample population of 800 people. The project will go on to sequence the genomes of 2500 people from each of the five major population groups, in the hope at least 95 percent of existing genetic variations will be revealed.

Co-chair of the project Dr Richard Durbin told the Guardian: 'In the last 10 years, DNA sequencing technology has advanced dramatically so it becomes feasible to systematically sequence many people to find genetic variants'. He continued: 'Over half those differences haven't been seen before, and these have provided a more complete catalogue of variation than was available previously'.

179 people had their whole genomes sequenced during the project's pilot phase using 'low-coverage' sequencing - in which a small amount of random DNA is sequenced for each individual. 697 people only had their protein-coding regions called 'exons' sequenced, which are responsible for most important variations, despite accounting for a minority of DNA.

The project found 'low-coverage' and 'exon' sequencing could be used with relatively low error rates. The results of the study were published as part of a series of papers in the journals Nature, Science and Genomic Research.