Because some trait's phenotypes don't indicate their genotypes so in order to predict the phenotype of the offspring you have to know the genotypes of the parents
Answer: In the process of meiosis, alleles separate and are added to the gametes in an independent manner. While each gamete (egg or sperm cell) therefore contains one allele for each gene, it's impossible to know which allele, because alleles segregate in a random manner. However, it's possible to calculate probabilities that the offspring will have a given allele using Punnett squares.
