Why is "Gr" a normal genotype in a gamete?

Why is "Gr" a normal genotype in a normal gamete, but "GgRr" isn't?

DNAunion2013-04-02T18:54:40Z

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Gametes are haploid; they have only 1 set of chromosomes. Therefore, they have only 1 allele for any given gene. So a gamete could have a G allele or a g allele, but not Gg alleles. And a gamete could have an R or an r allele, but not Rr alleles.