Question

Consider a hypothetical flower called the Purple Beauty whose wild-type phenotype consists of dark purple pigmentation. The purple pigment is the product of a biosynthetic pathway involving a number of enzymatic steps in which a colorless precursor compound, P, is sequentially converted into two intermediate molecules, I(1) and I(2), and then into the final product, the dark purple pigment, DP. The following is a diagram of this biosynthetic pathway, where gene A is responsible for the production of enzyme A, gene B is responsible for enzyme B and gene C is responsible for enzyme C. The a, b , and c mutant alleles are recessive mutants that when homozygous fail to produce the A, B, or C enzyme, respectively. Flowers that have no pigment (i.e., contain only colorless compounds) are white; flowers that have I2 but no DP are pink; flowers that contain DP are dark purple.

Precursor Molecule Intermediate Compound Intermediate Compound Final Product

P -----------A---------------> I1 ------------------B-------------> I2 -------------------C------------> DP

(colorless) (colorless) (pink) (dark purple)

What would be the phenotypes of individuals of the following genotypes? Explain your answers.

a. Aa Bb Cc:

b. AA BB cc:

c. aa BB CC:

Answer 1

a. Aa Bb Cc: The phenotype of this genotype is purple. This is because the heterozygous genes A,B and c encode the final poruct with DP(dark purple pigment).

b. b. AA BB cc: The phenotype of this genotype is white. This is because the genes A and B encode intermediate products I1 and I2 but homozygous recessive c(cc) gene does not encode for DP pigment.

c. aa BB CC: The phenotype of this genotype is purple. This is because the Homozygous aa genotype does not encode for I1 intermediate product. But, BBCC and encode I2 product and DP pigment. Thus, phenotype is purple.