Whenever people see a child with only one parent, they usually assume the kid is the same ethnicity as that parent. If there's a girl who is half Asian and half White, and people see her with her Asian father, most people will assume she's Asian. But if people see that same girl with only her White mother, they'll likely assume she's White.
As if it wasn't bad enough that people make these assumptions, the American government used to do this when collecting census data. Before the 2000 U.S. Census, mixed-race people in the United States could only choose one ethnicity. A woman named Susan Graham was filling out the Census in 1990, and she couldn't find a place on the form for her multiracial children. She asked the U.S. Census Bureau about it and they told her that mixed-race children are counted as the mother's ethnicity!
Graham is white. Her husband is black. When she received her 1990 census form, she complained to the census bureau that there was no place for her children. An official told her that children take the race of the mother because "in cases like these we always know who the mother is, but we don't know who the father is."
About the same time, her son was starting kindergarten. Her husbnd took him to school. [sic] A teacher, filling out a school form, concluded her son was black.
"We had the same child who was white on the census, black in school and multiracial at home," Graham said. "And I thought there's something wrong with this picture."
Like she says, when the same person is White on the Census, Black in school, and mixed-race at home...there's something wrong.
This problem was fixed in the U.S. Census in 2000, but it still exists elsewhere. Many people still assume a person's mother and father are always the same ethnicity, and when they see someone who is biracial, they usually force them into one side. A well-known example of this is how the media usually refers to Tiger Woods as African-American. He's actually only 1/4 Black and is more Asian than any other race, but almost nobody calls him Asian-American.