Video Transcript
The diagram provided shows a simple
summary of the offspring produced when a plant with blue flowers reproduces with a
plant with purple flowers. What can be assumed about the
traits shown by these flowers? (A) The allele for purple flowers
is dominant to that for blue flowers. (B) The allele for blue flowers is
dominant to that for purple flowers. (C) Both parents are homozygous for
their color of flowers. (D) A mutation has caused the
difference in the colors of the offspring.
To answer this question, we need to
understand the diagram presented. Let’s review it together. On top, we can see the parent
plants that reproduce together. This is called a
cross-fertilization as it involves a cross between two distinct parents: one parent
being a purple flowering plant and the other parent being a blue one. The offspring resulting from this
cross make the first generation, called F1. A very remarkable fact that we can
notice about F1 offspring is that they are all blue. This likely means that the blue
allele is dominant to the purple allele.
Then, we can read that F1 offspring
self-pollinate. What does it mean? Self-pollination refers to the
ability of these plants to fertilize themselves. As plants are often hermaphrodites,
they can produce both eggs and sperm gametes. It’s then possible that their sperm
fertilizes eggs from the same flower. When this plant with blue flowers
undergoes self-pollination, it produces a second generation of offspring called
F2. As you can see on the diagram, this
F2 generation doesn’t only show blue but also purple offspring, with a ratio of
three blue flowering plants for one purple flowering plant. Since the purple phenotype
reappeared in the F2 generation, we can assume that the allele for purple was
present in the F1 generation but recessive. This is why no purple flowers
appeared in the F1 generation.
With this F2 generation, we can
assume that the F1 generation organisms had a heterozygous genotype with one
dominant allele and one recessive allele. Let’s refer to the alleles as
uppercase B for blue and lowercase b for purple.
Now, we can create a Punnett square
with this information. Now, we can write all the genotypes
that can result from the self-pollination of F1 flowers in our Punnett square. We can also determine the phenotype
of the offspring. As you remember, we made the
assumption that the allele uppercase B is dominant and the allele lowercase b is
recessive. So, the genotype uppercase B
uppercase B will give blue flowers. The genotype uppercase B lowercase
b will also give blue flowers. And the genotype lowercase b
lowercase b will give purple flowers.
As you can see, when two
heterozygous F1 plants are crossed or self-pollinated, they produce offspring with a
flower color ratio of three blue flowering plants to one purple flowering plant. As you remember, this was exactly
the ratio we had in the diagram.
Now let’s look back at our answer
choices. (B) states the allele for blue
flowers is dominant to the allele for purple flowers. This is likely correct since the
self-pollination of a heterozygous blue flowering plant resulted in offspring in a
three-to-one ratio, which is characteristic of dominant to recessive alleles. This is thus the correct assumption
that could be made about the traits shown by these flowers: the allele for blue
flowers is dominant to that for purple flowers.
