The allele for blue flowers is “B” and the one for yellow flowers is “Y”.
A true-breeding specimen must be homozygous, so the blue flowers must have the genotype “BB” and the yellow ones must be “YY”. If you cross a flower with genotype “BB” with one with genotype “YY”, all the offspring will be “BY”. If a “BY” flower appears blue, then the blue allele must be the dominant to the yellow allele.
