Flowers of sweet pea plants can be
purple or white. The diagram provided shows the
biochemical pathway that allows the production of purple flowers in sweet pea
plants. Which of the following conditions
has to be true for the purple pigment to be produced? (A) Only gene A must be present and
functioning correctly. (B) The genes can be absent, but
precursors 1 and 2 must be present. (C) The genes can be absent, but
both enzymes must be functioning. (D) Only gene A and precursor 1
must be present. (E) Both gene A and gene B must be
present and functioning.
This question asks about genetics
and requires an understanding of complementary genes to answer correctly. The diagram provided shows how
purple pigmentation in the flowers of the sweet pea plant is a two-step process that
requires the presence of enzyme A and enzyme B. The bubbles above describe the
colors of the flower during each step of this process.
We can immediately eliminate answer
(B) — the genes can be absent, but precursors 1 and 2 must be present — because we
know that the precursor bubbles in the diagram describe the state that the flower is
in and not the genes that are present. We can read from the diagram that
genes A and B are necessary for producing the enzymes A and B, respectively. Therefore, we can also eliminate
answer (C) — the genes can be absent, but both enzymes must be functioning — because
both enzymes would not be present if both genes were absent.
Now let’s take a look at the
remaining answers. Answer (A) states that purple
pigmentation can occur only if gene A is present and functioning. If gene A is present, that means
that the flower will only produce enzyme A, which will convert precursor 1 to
precursor 2. However, enzyme B is required for
the second step in the pigmentation process. So a flower with only gene A will
have precursor 2 and will be colorless. It needs enzyme B to turn precursor
2 to the purple pigment. So answer (A) is incorrect. In answer (D), “Only gene A and
precursor 1 must be present” is very similar to answer (A). It also says nothing about the
presence of gene B or enzyme B, which is required for the second stage of the
pigmentation process. So answer (D) is incorrect.
Gene A and gene B are complementary
genes that work together in a two-step process to produce the purple pigment of the
flower. Gene A is necessary for the first
step, which allows for the next step to happen. Gene B is necessary for the second
step. If, and only if, both gene A and
gene B are functional, then the purple pigment will be produced. The correct answer is therefore
(E). Both gene A and gene B must be
present and functioning.