Answer:
The blue allele is dominant to the yellow allele.
Explanation:
This is a cross involving a single gene coding for flower colour in a particular plant. The flower colour gene possesses two alleles viz: blue allele (B) and yellow allele (b). A truebreeding plant is that which produces only one type of offspring and contains the same allele for a particular trait (colour in this case).
According to the question, a truebreeding blue flower (BB) is crossed with a truebreeding yellow flower (bb) to produce an all blue-flowered offspring in the F1 generation. This is because the blue allele (B) is dominant over the yellow allele (b) in that gene i.e. the blue allele will mask the phenotypic expression of the yellow allele in the heterozygous F1 offsprings (Bb) that were produced.
In his law of dominance, Mendel called the allele that masks the expression of another like the blue allele in this question, DOMINANT allele while the allele that is phenotypically masked or covered up like the yellow allele in this case, RECESSIVE allele.
