Now that Joanna Kerns has become a success in Hollywood terms—she earns $10,000 a week on a Top 15-rated TV show and is recognized by Bagel Nosh cashiers everywhere—it seems fitting that her hit should be titled Growing Pains. It is a subject Kerns, 33, understands. The third child in a family of jocks, she grew up in the shadow of older sister Donna de Varona, the two-time gold-medal-winning 1964 Olympic swimmer who is now an ABC sports commentator. "Donna was a golden girl on the cover of all the magazines, and she traveled all over the world," says Kerns, who stars as journalist and working mother Maggie Seaver. "She had a very exciting life and I wanted all that."

Kerns got it, but not exactly the way she dreamed. Newfound success—her only previous role of note was on 1984's short-lived TV version of Four Seasons—has taken an unwanted personal toll. Kerns's nine-year marriage to Richard Kerns, a television commercial producer 21 years her senior, broke up last year, and the experience sent her into analysis. Like her character on Growing Pains, Kerns also is a working mother fighting guilt. "I feel I do miss a lot with Ashley [her 7-year-old daughter], but I also know that if I didn't work, we wouldn't have the life-style we have," she says. "More important, I wouldn't know who I am."

Establishing an identity of her own has always been important to Joanna. Born in San Francisco, she grew up in Santa Clara, Calif., the third of four children of an insurance salesman and a clothing-store manager. Joanna hoped to become her generation's Mary Lou Retton. "Dad's attention was focused on those of us who were performing," says sister Donna, now 38. "I started winning so early it must have been very hard for her."

It was, and Joanna moved to Fresno alone at 13, where she lived in a boarding house while studying with a top gymnastics coach. Despite her 5'8" height a foot taller than most of her competitors—she ranked 14th out of 28 competitors vying for the 1968 Olympic gymnastic team. But she failed to make that team and two years later tore cartilage in her left knee while doing a back somersault. Unsure of her future, she tried college but lasted less than a year and decided to take up dancing. She eventually won a part in Clown Around, a Gene Kelly stage vehicle supposedly bound for Broadway; the show got no farther east than San Francisco. Kerns took a job dancing at Disneyland before landing a part in a touring production of Two Gentlemen of Verona. That brought her to New York and earned her $285 a week ("just enough to cover my phone bill"). Eventually she returned to L.A. to concentrate on dramatic acting.

Instead she became something of a queen of commercials, pitching cars, soap, food—"everything but feminine hygiene," she cracks. During production of a series of Dodge Aspen ads in 1974, she met Kerns, a former movie company executive. They married in 1976. Their age difference—and her recent success—contributed to the demise of the marriage, she believes. "A woman changes a lot between 23 and 33," she says. "His career is high-powered, and as I got more successful it was like two corporations under one roof. A lot of friction happens." They called off the merger but share custody of Ashley, who lives in her mom's 10-room cliffside home overlooking the San Fernando Valley.

It was after Four Seasons that Kerns's career went into full throttle. She clinched the part on Growing Pains, a Cosby clone, last year. "She is a jock and a party girl, but she has an edge to her," says Alan Thicke, 38. "Like June Cleaver on diet pills."

Kerns was, naturally, thrilled to pin down the part but was nervous about working with Thicke. "He always predicts doom," she says. "One week we were going to be opposite Rudolph the Red-Nosed Reindeer, and all week long Alan said, 'Oh my God, we're going up against Rudolph. We're going to die.' And we did get beaten by Rudolph." Doom is the last thing Kerns has on her mind. In fact, she is branching out. She joined a writer's group at UCLA and has turned out her first screenplay, Freestyle, now in the hands of an independent producer. It is the story of two sisters trying to make the Olympic swimming team. The younger sister wants to beat the older one so she can win her father's love. It is a story Kerns knows firsthand. And if it's anything like her own life, it will have a happy ending. Says Joanna of her dad: "He has always loved me. I just couldn't see it."