Two parents, both with brown eyes, have four children. One child has brown eyes, like his parents, but the other three all have blue eyes. One of the children has been learning about genetics in science class and knows that brown eyes are dominant to blue eyes in humans. The student developed the following model of a Punnett square to predict the genotypes of his parents and his siblings.

Based on his model, the student claims that only one of the four children should have blue eyes. Which of the following explanations best describes why the Punnett square does not model the traits in the student’s family? (DOK 3)