Answer:
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.
