What could make a baby bison white?

Photos of a white bison calf in Yellowstone National Park have generated excitement as well as questions: How does that happen?

A park visitor said she took the photographs in the park earlier this month, showing a fuzzy white youngster being nuzzled by its dark brown mother. Park officials said this week that they hadn't yet spotted a white calf in the sprawling park in Wyoming and Montana, home to about 5,000 bison, also called American buffalo.

In the wild, there are two genetic variations that may result in unusually light-colored animals—leucism and albinism. In both conditions, the animal inherits two copies of the gene mutation—one from each parent, who usually appear normally colored themselves.

Leucistic animals lack some cells that otherwise produce melanin, a natural pigment that gives color to fur, eyes, feathers and skin. Their bodies may appear almost entirely white, or only white in patches, and they generally have normal or dark-colored eyes.

Albinism, which is more rare, results from the complete absence of melanin. Albino animals are nearly totally white, and they may have light pink or orange-colored eyes and reduced vision. Albino bison will lack dark colors in their eyes, noses and hooves, said James Derr, a geneticist at Texas A&M University.

The Yellowstone calf, with its black nose and eyes, is not albino, said Jim Matheson, executive director of the National Bison Association.

There's a third possibility: A light-colored calf could be the result of a bison crossed with a white domestic cow. In that case, the calf may be light tan-colored, with brown eyes and a black or brown nose, said Derr.

Matheson said that it's unclear how often white bison calves are born in the wild.

"We just don't know how often it happens because we've never tracked this in history," he said.

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