Video Transcript
A pea plant with a tall stem can
have the genotype uppercase T uppercase T or uppercase T lowercase t, whereas a
plant with a short stem will have the genotype lowercase t lowercase t. A tall pea plant is crossed with a
short plant to produce four offspring. Which of the following outcomes
would indicate that the tall plant had the genotype uppercase T uppercase T? (A) Three out of four of the
offspring have tall stems. (B) 50 percent of the offspring
have short stems. (C) All of the offspring have tall
stems. (D) None of the offspring have tall
stems. Or (E) all of the offspring have
stems that are halfway between the tall and short plants.
This question presents a
theoretical experiment in which two pea plants are crossed to produce four offspring
and asks us to predict the potential phenotypes of these offspring, given specific
parental genotypes.
Let’s first clarify these key terms
and how the genotype and phenotype relate to each other. A gene is a sequence of DNA that
can encode a particular characteristic. For example, here the
characteristic we are observing is the height of the pea plant stem.
As you may know, for each gene in
the genome, there are two alleles, one inherited from each parent. Alleles are different versions of
the same gene. Here there are two possible alleles
for our stem height characteristic, which are represented as uppercase T and
lowercase t. Note that in our example, one of
these two different alleles codes for tall stems and one codes for short stems.
The term genotype refers to the
genetic makeup, or the combination of alleles, that an organism has for a certain
gene. On the other hand, the term
“phenotype” refers to the physical expression of the genotype. In our case, the phenotype we are
observing is a stem being tall or short.
Now we need to use the clues in the
question to understand better how the combination of different alleles in the
genotype determines the different phenotypes. We are told that a plant with the
genotype uppercase T uppercase T or uppercase T lowercase t can display the
phenotype tall stem. On the other hand, a pea plant with
the genotype lowercase t lowercase t will always display the phenotype short
stem. With these clues, we can presume
it’s likely that one allele is recessive and the other is dominant.
A dominant allele is always
expressed in the phenotype when it is present in the genotype. In this case, the dominant allele
uppercase T will always make the tall stem phenotype appear in the organism’s
physical characteristics, even if the allele lowercase t is present in the genotype
too.
The recessive allele is only
expressed in the phenotype if two copies are present in the genotype and the
dominant allele is absent. So the short stem phenotype will
only appear if the plant’s genotype is lowercase t lowercase t.
So presuming that the allele
uppercase T is dominant to lowercase t, let’s draw a Punnett square to see what the
genotypes and phenotypes of the offspring will be. We need to cross a tall pea plant
with the genotype uppercase T uppercase T, as indicated in the question, with a
short pea plant that will invariably have the genotype lowercase t lowercase t.
When creating the Punnett square,
the two alleles of one parent are shown in the row at the top of the square, while
the two alleles present in the other parent are shown in the far-left column. Therefore, we can write uppercase T
in each box in the top row and lowercase t in each box in the far-left column. Each remaining box in the Punnett
square represents the offspring’s potential genotypes and should combine one allele
from each parent: one from the corresponding top row and one from the corresponding
far-left column.
As you can see, each of the
offspring resulting from this cross will have the same genotype: uppercase T
lowercase t. Since uppercase T is dominant to
lowercase t, we know that the genotype uppercase T lowercase t will produce the tall
stem phenotype in all of the offspring. This means that the correct answer
to this question must be (C): all of the offspring have tall stems.
