Let's use A as the dominant allele for green seed colour and a as the recessive allele for yellow seed colour.
If a yellow seeded plant is crossed with a heterozygote,
aa X Aa
the yellow seeded plant would only produce one type of gamete (a) while the heterozygote would produce two different types of gametes (A and a)
If we put this into a Punnett square, we will see that there two two possible genotypes for the offsprings. Either Aa or aa.
Since the allele for green seed is dominant, Aa will exhibit the green seed colour phenotype.
Hence, the chance of getting an offspring with green seed colour is 1/2, or 0.5
