A true breeding plant that produces yellow seeds is crossed with a true breeding plant that produces green seeds. All of the seeds of the F1 generation are yellow. Why

Because the allele for yellow colour is dominant over the allele for green colour

Explanation:

Assuming the true breeding yellow plant has the genotype YY while the true breeding green plant has the genotype yy. When the two are crossed:

YY   x   yy = Yy, Yy, Yy and Yy. All the progeny are yellow which means the allele Y representing yellow colour is dominant over allele y representing green colour. On the other hand, the y allele is recessive to Y allele and as such, its effect are masked phenotypically.

This is in agreement with Mendel who postulated that when the two alleles representing a gene are different, one will be dominant and the other will be recessive.