Google’s investment in 23andMe, a company that aims to allow users to trawl their DNA online, points at a central strand in the search giant’s intellectual helix.

The start-up’s ultimate aim – to discover drugs that can be prescribed according to an individual’s unique genetic make-up – pinpoints Google’s number one fixation: the importance of personalisation.

The company’s vast fleet of datacentres already holds a huge amount of information on its users. This ranges from the contents of e-mails in its Gmail service, to credit card details through Google Checkout, its online payment system.

However, Eric Schmidt, Google’s chief executive, has insisted that he does not yet “know enough about you”. To put that right, and further sharpen Google’s ability to target consumers with the ads they are most likely to respond to, the company is stepping up its efforts to collect personal information on the web.

“This is the most important aspect of Google’s expansion,” Mr Schmidt said. He envisaged a day when Google will be able to advise users on everything from career moves to how they should spend their free time, based on the collected queries they tap into Google.com.

Despite the Big Brother connotations, it is a vision embraced across the industry.

Yahoo! has invested heavily in Panama, an advertising platform that will track in greater detail than ever before a user’s search history prior to an online transaction. The details extracted will guide advertisers on what search terms they should buy.

Meanwhile, new markets that trade records of online behaviour are emerging. WunderLOOP, a company backed by Niklas Zennström, the billionaire internet entrepreneur, recently launched the first “exchange for behavioural targeting-based online advertising”.

A “stock market” for logs of browsing habits, it allows website owners to trade records of consumers’ long-term behaviour. So armed, advertisers can ensure that a handpicked audience can see a particular campaign.

Other ploys will be more obvious. Autonomy, the search specialist, is exploring “transaction hijacking”, in which shoppers are monitored and informed, mid-purchase, if a better price is found elsewhere.

Google presents itself as a benign Big Brother. Bowing to pressure, it recently cut the length of time it holds search data to two years. Users have to opt in to its new generation of personal search tools and it will not pass user data on unless faced with “a valid legal order”.

Three quarters of its users are not aware that Google stores data on them, a recent survey found. The Electronic Frontier Foundation, an internet rights group, describes a list of search queries as “practically a printout of what’s going on in your brain”.

Which is exactly why these groups are so keen to get their hands on it.