In pea plants, plant height, flower color, and pod color are governed by three independently assorting genes. The three genes have dominant and recessive alleles, with tall (T) dominant to short (t), axial flower (A) dominant to terminal (a), and green pod (G) dominant to yellow (g). If two pea plants that are heterozygous for all three genes are crossed, what is the probability that their offspring will have the genotype TtAaGg?