Video Transcript
The diagram shows the process
Mendel used to produce two generations of pea plants. What is the ratio of the dominant
trait to the recessive trait in the second generation?
The question presents us with a
diagram of several generations of pea plants and asks us to find the ratio of
dominant traits to recessive traits in the second generation of offspring. To do this, first, we will need to
decide whether the yellow pea trait or the green trait is dominant. In order to do this, let’s briefly
recall what these terms mean.
You might remember that a trait
refers to a specific version of a characteristic in an organism. And in this case, the
characteristic we are looking at is the color of peas. In the parental generation, there
is one pea plant that produces yellow peas and one that produces green peas,
displaying the two different traits this characteristic can take. When these two organisms join in a
reproductive cross, all of the resulting offspring are yellow.
Now, you might recall that for
certain characteristics, different alleles exist. Alleles are alternative versions of
the same gene, such as the gene that produces pea color, that result in different
traits, such as yellow or green pea coloring.
Dominant alleles are those that
when present in the genotype mask other alleles. This means that the dominant trait
will always be expressed whether there is one or two dominant alleles present in the
genotype. Recessive alleles are those that
can be masked by dominant alleles. This means that there must be two
recessive alleles present in the genotype, and therefore no dominant alleles, for
the recessive trait to be expressed. As the yellow allele masks the
green allele in all of the offspring in the first generation, we can deduce that the
yellow pea color must be the dominant trait. It is always expressed when the
allele that codes for it is present in the genotype, even if only one dominant
allele is present.
It is conventional for dominant
alleles to be represented by uppercase letters and for recessive alleles to be
represented by lowercase letters. So the parent pea plant that
produces yellow peas must have at least one uppercase P to denote its genotype. By knowing that yellow peas is the
dominant trait, we can work out that producing green peas must be the recessive
trait. Therefore, the parent plant that
produces green peas must have two recessive alleles and, as a result, two lowercase
p’s in its genotype.
However, we do not need to know all
of the parents’ and offspring’s genotypes in order to answer this question, as all
it asks us to do is to find the ratio of traits, dominant to recessive, in the
second generation of offspring when one in the first generation is
self-pollinated. We can gather all of this
information from the diagram that is presented in the question.
Ratios are expressed as two
numbers, in this case the number of offspring in the second generation that produce
the dominant trait and then the number of offspring in the second generation that
produce the recessive trait, separated by a colon. As three of the offspring in this
generation express the dominant yellow pea trait and only one of the offspring in
the second generation expresses the recessive green pea trait, we can work out the
correct answer to this question. The ratio of the dominant trait to
the recessive trait in the second generation is three to one.
