That each gamete contains a single allele of the eye color gene is an illustration of . Two aliens of the P generation are crossed. One of them has black eyes and the genotype double uppercase O. The other has orange eyes and the genotype double lowercase o. The first alien produces gametes only with uppercase O, while the other produces gametes only with lowercase o. That each gamete contains a single allele of the eye color gene is an illustration of . Two aliens of the P generation are crossed. One of them has black eyes and the genotype double uppercase O. The other has orange eyes and the genotype double lowercase o. The first alien produces gametes only with uppercase O, while the other produces gametes only with lowercase o. Beadle and Tatum's one gene - one enzyme hypothesis Mendel's law of segregation only Mendel's law of independent assortment only Darwin's theory of natural selection Both Mendel's law of segregation and Mendel's law of independent assortment.