Genuine Risk shattered a great myth today: the one that says good fillies cannot beat good colts. She won the Kentucky Derby and became the second member of her sex to achieve that feat in the 106-year history of America's most famous horse race.

But if she debunked one popular Derby notion, she upheld another one: a thoroughbred needs the innate ability to run 1 1/4 miles to win here. While the favorites, Rockhill Native and Plugged Nickle, were struggling through the long Churchill Downs stretch, Genuine Risk was living up to her pedigree.

She surged past the two favorites on the final turn, opened a brief, clear lead, and then withstood the efforts of the stretch-runners to score by a length over Rumbo and another length over Jacklin Klugman. Her time was a respectable 2:02 before a crowd of 131,859.

Genuine Risk had not been considered a principal contender here. Her odds of 12 to 1 reflected feminist sentiment as much as hard-boiled handicapping judgments. And her very appearance in the Derby apparently represented the triumph of her owner's sentiment over her trainer's best judgment.

Only two weeks ago, after Genuine Risk had run against males for the first time and suffered the first defeat of her career, trainer LeRoy Jolley announced emphatically that the filly would not come to Churchill Downs.

But owners Diane and Bert Firestone of Waterford, Va. (near Leesburg), could not resist the chance to run the first filly in the Derby since Silver Spoon in 1959, or to have Genuine Risk join Regret (1915) as the only female winners.

It is not often that such decisions by owners pay off. But today everything came up roses for the Firestones.

Rockhill Native, the 2-to-1 favorite, and Plugged Nickle, the 5-to-2 second choice, both broke alertly and went for the lead, along with longshot Bold 'N Rulling. Jacinto Vasquez was content to let Genuine Risk take a position in the second flight, three or four lengths behind the leaders.

The favorites loped through the first quarter-mile in an unusually slow 24 seconds and the half-mile in 48. Buck Thornburg, on Plugged Nickle, and John Oldham, on Rockhill Native, were in the position where they wanted to be: They were in front of everybody else, and they still hadn't asked their mounts for any serious exertion.

"On the backside," Vasquez said, "the leaders were running awful easy and I didn't know how much horse they had. But I knew I had a lot of horse at the time."

Because the pace was so slow, various improbable horses were able to make at least brief moves into contention. Degenerate Jon, a hopeless longshot, made a short-lived bid along the inside. Then Genuine Risk started accelerating on the outside.

She rushed up four horses wide, losing valuable ground on the turn, and people who had seen her fail to sustain a similar move at Aqueduct two weeks ago might have suspected that her bid for glory was going to be brief.

But, as she moved past Rockhill Native and Plugged Nickle, it became redily apparent that these two weren't going to offer any resistance. The favorites were dead.

Then, just as Genuine Risk got clear, passing the mile mark in 1:37 3/5, two California invaders swing to the outside and launched their challenge. Jaklin Klugman, bidding to make this a storybook Derby for her owner, actor Jack Klugman, suddenly moved within a length of the filly. Jockey Darrel McHargue said, "At the quarter pole, I told myself, 'This is my Derby'."

Genuine Risk may have been a member of the supposedly weaker sex, but in this case she was like a queen being confronted by two hobos. An offspring of Exclusive Native, the leading stallion in the United States whose son Affirmed won the Derby two years ago, she did not figure to be fazed by the most crucial quarter-mile in American racing.

First she shook off the ill-bred Jaklin Klugman, son of a stallion with a $500 stud fee. And then she held off the late challenge of Rumbo, who finished in what has become his customary position: second.

Rumbo did not fail so much because he is a chronic "bridesmaid" but because Genuine Risk was running strong. She covered the last quarter-mile in a swift 24 2/5 seconds -- the fastest last quarter by a Derby winner since Secretariat.

Her final time of 2:02 suggested that she and the rivals behind her are not the inept bunch they have been made out to be all spring. The track was very fast, but Genuine Risk did manage to run two-fifths of a second faster than Spectacular Bid did last year and a fifth faster than Seattle Slew in 1977.

Genuine Risk paid $28.60, $10.60 and $4.80. Rumbo returned $5.20 and $3.40, and Jaklin Klugmann paid $4.40 to show.

Super Moment finished another four lengths back in fourth place, a nose before Rockhill Native. Plugged Nickle was seventh.

Genuine Risk had been purchased by the Firestones as a yearling for $32,000, having been encouraged to do so by their 14-year-old son Matthew. She proved herself a great bargain last year, winning all four of her starts against members of her own sex.

Jolley said that the Derby "was always at the back of our minds," but he didn't train Genuine Risk as if that were the case. Because fillies don't often challenge colts in major American races, trainers aren't in the habit of preparing them to do so.

Instead of putting Genuine Risk through the sort of schedule that he did Foolish Pleasure, his Derby winner in 1975, Jolly ran her twice against weak fields of fillies. After she had extended her winning streak to six, Jolly entered her hesitatingly in the Wood Memorial Stakes.

Even the day before that race he wasn't certain he would run her. And when Genuine Risk finished third, 1 1/2 lengths behind Plugged Nickle, she seemed to suggest that she wasn't quite as good as the males. Jolly indicated that he would run Genuine Risk in New York's triple crown series for fillies.

But when the expected size of the field shrand from 20 to a dozen or so, and when Rockhill Native lookes less than awesome winning his last prep race, the Firestones said they were encouraged to try the Derby, and Jolly said he concurred with the decision.

The Firestones said they will consult with their trainer before deciding whether to try the next leg of the Triple Crown, the Preakness on May 17. No filly has won that since 1924.