According to Mendel's law of independent assortment, the alleles of a gene will separate into gametes independently of one another i.e. one allele of a gene does not hinder the separation of another. The separation occurs via meiosis in such a way that only one allele of a gene is present in each GAMETE.
These gametes are what forms the crossing basis in a punnet square. According to this question involving a dihybrid cross, the genotype of one of the parents is BbTT. The allelic combination of the gametes will be: BT, BT, bT, bT. These gametes will be put on the outside of the Punnett square.
