How does the genotype ratio of 1:2:1 relate to the phenotype ratio of 3:1?

The 1:2:1 genotype ratio and the 3:1 phenotype ratio are both expressions of the same underlying genetic inheritance observed by Gregor Mendel in his experiments with pea plants.1

When two heterozygote individuals (genotype Rr) are crossed, the law of segregation dictates that their genes separate during the formation of sex cells. The random union of these gametes results in an F2 generation with three distinct genotypes in a 1:2:1 ratio:12
* 1 homozygous dominant (RR)
* 2 heterozygous (Rr)
* 1 homozygous recessive (rr)

The 3:1 phenotype ratio occurs because of the law of dominance. Because the dominant allele (R) masks the presence of the recessive allele (r), both the RR and Rr genotypes display the same dominant physical trait. Consequently, the 1 (RR) and 2 (Rr) genotypes combine to form the 3/4 dominant phenotype, while the 1 (rr) genotype forms the 1/4 recessive phenotype, resulting in the observed 3:1 ratio.21