why is important to use an organism with a recessive phenotype in a cross with an organism of unknown Genotype

One of the central conclusions Mendel reached after studying and breeding multiple generations of pea plants was the idea that "[you cannot] draw from the external resemblances [any] conclusions as to [the plants'] internal nature." Today, scientists use the word "phenotype" to refer to what Mendel termed an organism's "external resemblance," and the word "genotype" to refer to what Mendel termed an organism's "internal nature." Thus, to restate Mendel's conclusion in modern terms, an organism's genotype cannot be inferred by simply observing its phenotype. Indeed, Mendel's experiments revealed that phenotypes could be hidden in one generation, only to reemerge in subsequent generations. Mendel thus wondered how organisms preserved the "elementen" (or hereditary material) associated with these traits in the intervening generation, when the traits were hidden from view.