The statement "traits will sort themselves into gametes independently of what other traits are doing" refers to Mendel’s law of independent assortment. This law states that alleles of two or more different genes get sorted into gametes independently of one another, meaning the inheritance of one trait does not affect the inheritance of another trait

. Gregor Mendel discovered this principle through dihybrid crosses, observing that the combinations of traits in offspring did not always match the parental combinations, which demonstrated that genes for different traits assort independently during gamete formation

. This independent assortment occurs during meiosis, where homologous chromosomes are randomly distributed into gametes, creating genetic variation

. Thus, the correct completion of the sentence is: "This is Mendel’s law of independent assortment."