Final answer:
The self-pollination of a heterozygous offspring results in a phenotypic ratio of 3 yellow:1 green due to the segregation of alleles during gamete formation and the dominance of one allele over the other.
Step-by-step explanation:
The phenotypic ratio of the offspring when a heterozygous individual self-pollinates is 3 yellow:1 green. This is derived from a genotypic ratio of 1YY:2Yy:1yy, where Y represents a dominant allele and y represents a recessive allele. Using a Punnett square to predict the outcome of this cross, we can see that the parental genotype Yy can produce gametes with either the Y or y allele.
Mendel's law of dominance informs us that the YY and Yy genotypes will both display the dominant yellow phenotype, while only yy will display the recessive green phenotype. Therefore, we would expect a 3:1 phenotypic ratio in the offspring with three parts yellow to one part green when we consider a large enough sample size.