After the jury is passed for cause, both parties alternately, beginning with the people, may take their peremptory challenges. But no challenge is lost by failure to alternate if the panel is opened by the other party; and each party is entitled to a full panel before exercising a peremptory challenge. Provided, however, in the discretion of the court, the number of jurors who will hear the case, plus a number of jurors representing the total number of possible peremptory challenges, may be called and examined for cause before the parties begin to exercise their peremptory challenges.