Final answer:
The law of segregation states that paired unit factors (genes) must segregate equally into gametes. This is represented in the Punnett square by the separation of alleles into different gametes. It is the physical basis for accurately predicting the offspring of parents with known genotypes.
Step-by-step explanation:
The part of the Punnett square that best represents the law of segregation is the separation of alleles into different gametes. According to Mendel's law of segregation, paired unit factors (genes) must segregate equally into gametes, meaning that offspring have an equal likelihood of inheriting either factor. This is why we can accurately predict the offspring of parents with known genotypes using the Punnett square. The physical basis of this law is the first division of meiosis, where homologous chromosomes with their different versions of each gene are segregated into daughter nuclei.