Video Transcript
The diagram provided shows how
flower color is inherited in pea plants. In this example, what is the
recessive trait?
This question provides us with a
diagram showing a parental generation of plants with different-colored flowers and
their offspring. It asks us to work out what the
recessive trait is. Let’s review some key terminology
and how recessive and dominant traits interact in reproductive crosses.
A trait refers to a particular
characteristic, in this case the flower color of the plant being either red or
white. Genes are sequences of DNA which
contain information to produce a characteristic, for example, the gene to produce
flower color. Alleles are two or more versions of
the same gene. Note that in our example, two
different alleles for a particular trait exist: one coding for red flowers and one
coding for white flowers.
From the diagram, we can see that
when a red-flowering parent is combined with a white-flowering parent, all of the
resulting offspring are red. To understand why this is the case,
we need to briefly cover what dominant and recessive alleles are.
Dominant alleles are variants of
genes that, when present in the genotype, mask the presence of other variants of the
gene. It is conventional for dominant
alleles to be represented by capital letters. Assuming that the red trait comes
from the dominant allele, represented by a capital R, a flower is red if its
genotype comprises the alleles uppercase R uppercase R or the alleles uppercase R
lowercase r.
Recessive alleles are those that
can be masked by dominant alleles. So the trait they encode is only
expressed when there is no dominant allele and there are two copies of the recessive
allele present in a genotype. It is conventional for recessive
alleles to be represented by lowercase letters. In our example with flower color,
the recessive allele is represented by a lowercase r. The flowers produced will only be
white if both alleles are recessive, lowercase r lowercase r.
Because all of the offspring
exhibit a red color, red is most likely the dominant trait. And therefore, white would be the
recessive trait. Let’s test this hypothesis by using
a Punnett square, which shows all of the possible allele combinations of the
offspring for this particular trait based on the parents’ possible genotypes.
Let’s first test what would happen
if the red flower had one dominant allele, uppercase R, and one recessive allele,
lowercase r. As we’re assuming that the white
allele is recessive, the white flower must have two copies of the recessive allele,
lowercase r. These parental genotypes would
result in half of the offspring having different-colored flowers, half red and half
white. This is not what we can observe in
the offspring produced by the cross in the question. So both of the parents must have
two copies of their respective alleles.
Let’s test these genotypes in the
Punnett square next to see if it provides us with the appropriate outcome. If the red-flowered parent had two
dominant alleles in their genotype, uppercase R uppercase R, and the other
white-flowered parent had two recessive alleles, lowercase r lowercase r, this would
produce offspring that all had at least one copy of the dominant allele in their
genotype. Therefore, all of the offspring
would display the dominant trait. And as they are all red, we can
conclude that the dominant allele must code for the red flower color.
This means that our assumption
about the red trait being dominant was correct and that we can deduce what the
recessive trait must be. The recessive trait in this example
is white flowers.
