Question

suppose you cross a true breeding blue flower with a true breeding yellow flower. in the next generation, all of the flowers are blue. what does this outcome tell you about the allele for blue flowers.

Answer 1

A true-breeding specimen seems to be homozygous, thus, the blue color flowers must exhibit the genotype BB and the yellow ones must have possessed the genotype YY.  

In the given case, when one crosses a flower with genotype BB and other with genotype YY, all the offspring will be BY. As in the next generation, all the flowers appear blue, thus, the blue allele must be dominant.