December 2, 1986

THE genetic makeup of a child is a stronger influence on personality than child rearing, according to the first study to examine identical twins reared in different families. The findings shatter a widespread belief among experts and laymen alike in the primacy of family influence and are sure to engender fierce debate. The findings are the first major results to emerge from a longterm project at the University of Minnesota in which, since 1979, more than 350 pairs of twins have gone through six days of extensive testing that has included analysis of blood, brain waves, intelligence and allergies. The results on personality are being reviewed for publication by the Journal of Personality and Social Psychology. Although there has been wide press coverage of pairs of twins reared apart who met for the first time in the course of the study, the personality results are the first significant scientific data to be announced.

The Nassau County Executive, Francis T. Purcell, announced his retirement today. He said he intended to take a job that was ''an opportunity that comes once in a lifetime.'' Although he would not discuss the job, it was widely believed that the Cablevision Systems Corporation of Woodbury had offered him a job.

THE scholar Ephraim Isaac was arranging a meeting over the telephone with a reporter he had never seen before. They would meet in front of the big fireplace in the lounge of an inn here. And how would a stranger recognize Dr. Isaac? ''Well,'' Dr. Isaac said carefully, ''I wear glasses. I have a beard. And I shall be dressed in a white djellaba. Now tell me how I may recognize you.''

Federal officials investigating insider trading on Wall Street are taking a close look at the takeover earlier this year of the Harris Graphics Corporation, in which Ivan F. Boesky was a large investor, according to a source close to the investigation. Other large investors in Harris included Drexel Burnham Lambert and several institutional clients that have worked closely with the firm. Harris Graphics was one of 12 stocks that investigators sought information about earlier through subpoenas. But the source, who requested anonymity, said that investigators appeared to be particularly interested in the takeover and the trading leading up to it. Harris stands out from the list of stocks on the subpoenas because of the links between the various investors who had a stake in Harris and profited from its $242 million acquisition last May by AM International. At the middle of these links is Drexel, which has acknowledged receiving subpoenas in the investigation but denied any wrongdoing. Yesterday its officials denied any impropriety in the Harris Graphics transaction. $5 Million Profit for Boesky The takeover of Harris Graphics brought Mr. Boesky, who settled the largest insider case ever filed last month, a profit of more than $5 million, according to public documents. And much of this profit resulted from the fact that he was a major buyer of Harris stock long before the public knew that it was engaged in serious negotiations.

Guinness P.L.C., the brewer and one of Britain's largest companies, has apparently become a target of the British Government's widening inquiry into insider trading. In announcing an investigation of Guinness today, the Department of Trade and Industry said its goal was to ''examine circumstances of the affairs of Guinness suggesting misconduct'' but did not detail the reasons for the inquiry. The two inspectors conducting the investigation will have broad powers to question anyone in the company and demand documents. Guinness issued a statement saying it would cooperate with the investigation, which it said would ''inquire into the company's affairs and, in particular, interests in the company's shares.'' Guiness added, ''The inquiry has come as a complete surprise.''