In a breeding experiment, a pea plant producing yellow seeds is crossed with a green-seeded pea plant. All the F1 plants are yellow-seeded. If these F1 plants are crossed with the green-seeded parent plant, the progeny consists of
Yellow seeded and green seeded plants in 1:1 ratio
Since all the F1 plants are yellow-seeded, ‘yellow seed’ is obviously the dominant trait. It also shows that both the parents are homozygous for their respective traits (YY) and (yy). If the dominant parent was heterozygous, the F1 generation would have consisted of yellow and green-seeded plants in a 1:1 ratio, which is not the case here. If the F1 hybrid (Yy) is crossed with the recessive parent (yy), this would be the case:
The (Yy) progeny would be yellow-seeded and the (yy) progeny would be green seeded. And as you can see here, progeny with the dominant and recessive characters are produced in equal proportions, i.e., in a 1:1 ratio.