What are the possible results of breeding a dark brown mouse with a light brown mouse?

Most biology students have used a Punnett Square, diligently filling in each cell with combinations of genetic alleles. But when asked to explain the Punnett Square in Figure 1, which shows the probability of fur color in the offspring of a dark brown mouse and a medium brown mouse, students often say, "So, if the parents have four babies, two will always be dark brown." This is not always true.

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