A self-cross of two Yy heterozygotes gives a phenotypic ratio of:

1. A self-cross of one Yy heterozygous offspring can be represented in a two by two Punnett square. 2. Each parent can donate one of two different alleles, giving YY, Yy, Yy and yy. 3. Because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect a phenotypic ratio of 3 yellow to 1 green. 4. The genotypic ratio underneath it is one to two to one, which is a different thing. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 12 'Mendel's Experiments and Heredity', sections 12.1-12.3_