In the 1973 film "Sleeper," Woody Allen is pursued by a 20-foot-tall, genetically engineered chicken while carrying off a banana the size of a canoe.

His pithy observation: "That's a big chicken."

But was it a kosher chicken? Considering the questions raised about the effects of bioengineering on Jewish dietary laws, that's a tough one to call.

"Different sets of genes and, in the future, whole chromosomes will be able to be placed in other species. This blurs distinctions. Kosher laws are about distinctions, categories of foods that cannot be mixed," said author and activist Jeremy Rifkin, one of biotechnology's harshest critics for the past three decades. "There's nothing to prevent scientists from putting genes from, say, a pig into other foods. That becomes a violation of kosher laws."

Rifkin, an instructor at the University of Pennsylvania's Wharton School of Business and the founder of the Foundation on Economic Trends, will be speaking Thursday evening at Congregation Emanu-El in San Francisco. He is the author of 15 books, including "The Biotech Century" and "The Green Lifestyle Handbook."

While concern about the effects of genetic engineering on kashrut was a fairly hot topic in the early 1990s, Rifkin says interest seems to have waned. But the issues have not been resolved to his satisfaction.

"Most rabbinical responses have been knee-jerk, not very well thought out at all," he said in an interview from his Washington, D.C., office. "You can go to five different rabbis and they'll all have different ideas."

Since kosher laws were written eons before genes were identified, Rifkin points out that myriad problems exist for scientists and biblical scholars who try to apply the age-old restrictions to cutting-edge science. For example, how many genes need to be shared between two separate animals or plants before a violation of kashrut occurs? One? Two? One million? Who knows?

"At what point is there contamination?" asked Rifkin. "When do we cross the categories of Jewish law? We need to ask this, at least."

While those questions have yet to be resolved, Rifkin, who was raised in the Reform movement, stresses that the No. 1 genetic issue facing the Jewish community is the possible application of eugenics via "designer babies."

While acknowledging the benefits of genetically eradicating diseases such as leukemia or Tay-Sachs, Rifkin wonders where genetic screening will end.

"Every parent wants the best for their children's sake. But this changes the parent-child bond. Parents become the eugenical architects and children become the ultimate shopping experience of the 21st century," he said.

"How would we treat a child who is not genetically engineered in a world where the rest of them are? We may lose our most precious gift of all, empathy. In terms of Jewish culture, empathy is pretty basic. When you've lost the ability to empathize with a fellow human being, you've lost your humanity."

While he was once an uncompromising critic of bioengineering who called for a near-prohibition on such technology, Rifkin's stance has softened over the past couple of decades. If properly applied, he believes the science could reap enormous societal benefits.

"What's important here is my issues aren't with science but how we apply it. I think this could be a renaissance. It doesn't have to be 'Brave New World,'" Rifkin said. "The same science used for genetic foods and designer babies could be used for preventive health and organic food. I'd never tell a youngster not to learn about genes. That knowledge is valuable. But you must understand that genes are not all-powerful. There is nurture as well as nature."