Consider a hypothetical flower called the Purple Beauty whose wild-type phenotyp

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.

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:

Precusor molecule Inte rmediate compound Inte rmediate compound Final product P -A-> (colorless) 12 C-> (pink) DP (dark puple) (colorless)

Explanation / Answer

a.
Genotype: AaBbCc
Phenotype: Dark purple
Since the given alleles are recessive in nature, they show mutant phenotype only in the homozygous condition.

b.
Genotype: AABBcc
Phenotype: Pink
Gene C is required for the conversion of pink pigment to dark purple pigment.
If C is absent, flowers would be pink.

c.
Genotype: aaBBCC
Phenotype: Colourless
Gene A is required for the first step of the pigemnt synthesis.
If gene A is absent, the entire pathway is shut. Hence no pigment production.

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