UPDATE: It’s a girl! Baby panda’s gender, father revealed

Is it Gao Gao, the slim San Diego stud with the chunk out of his ear?

Or the hefty local boy, Tian Tian, with poor technique and track record?

The Smithsonian National Zoo said it will end the Giant Panda gossip Thursday by announcing which black-and-white dude fathered its newborn cub.

The zoo will also say which male sired the stillborn twin, and reveal the sex of the surviving cub.

Mei Xiang gently places her cub on the floor of her den. The panda cub has a round belly, which indicates that it is nursing well. (Smithsonian National Zoo)

The 12-day-old cub, born Aug. 23, appears to be thriving and is beginning to develop the classic black-and-white giant panda coloring on its fur. But infant cubs are so small that their sex can’t be determined right away.

The cub’s paternity is important because its lineage determines how common its genes are in the giant panda gene pool. Zoo experts did a mouth swab on the cub to extract DNA during its first medical exam last month.

In captivity, inbreeding with commonplace genes can pass along genetic mutations that could lead to potential health problems, the zoo has said. So the less common the genes, the better.

The zoo’s female giant panda, Mei Xiang, was artificially inseminated in March with sperm from two giant pandas — the National Zoo’s Tian Tian, and the San Diego zoo’s Gao Gao.

Gao Gao has proved to be a successful breeder, fathering four cubs in San Diego, including one born last summer.

Gao Gao was rescued from the wild in 1992 in China when he was less than a year old. He was sent to the San Diego zoo in 2003.

He weighs about 180 pounds, undersized for an adult panda, especially a male, and has an ear that was partly bitten off in the wild, the San Diego zoo said on its Web site.

Mei Xiang’s cub should follow this basic schedule of panda cub growth.

Because he was obtained from the wild, his genes are less common and more valuable, the zoo says.

In 2007, the zoo tried using his sperm on Mei Xiang, but it was ineffective.

Tian Tian, who was born in captivity in China, is a hefty 264 pounds but has been a relatively ineffective breeder, with poor natural technique, zoo experts have said.

He has managed to father only the zoo’s beloved Tai Shan in 2005, and a premature female cub last year that died after six days. Tai Shan was sent to a breeding program in China in 2010.

In addition, Tian Tian’s genes are not that valuable because his father has fathered dozens of offspring in the captive population, the zoo has said.

Tian Tian’s father was the famous Pan Pan, who has sired at least 32 cubs in captivity. He has sired multiple cubs a year — seven in 1997, including three sets of twins.