Video Transcript
In a species of pea plants, the
allele for green pods is dominant to that for yellow pods. The diagram provided shows two
parent pea plants being crossed. Which of the following Punnett
squares shows the correct cross? (A), (B), or (C).
In the diagram, we see the genotype
for each color pod. We may recall that when gametes are
produced, the two alleles an organism has for each gene are separated to produce
gametes with a single allele for each gene. If we separate the alleles for each
of these genotypes, we get both uppercase G and lowercase g alleles in the gametes
of the green pod plant and only lowercase g alleles in the gametes of the yellow pod
plant.
Punnett squares are a tool that
allow us to quickly simulate this separation of alleles and then also simulate the
combination of those separated alleles with those of the other parent, to produce
the genotypes of the offspring. Let’s see what the cross of these
two pea plants would look like.
We could put the genotypes of the
green and yellow pod plants either across the top or along the side of this Punnett
square. It does not matter which we put
where. We will get comparable results
either way.
To fill out the Punnett square, we
combine the letter of the column header with the letter of the row header for each
square in the grid. So, if we fill out the remainder of
the Punnett square in the same manner, we should get something like this. If we look among the answer
choices, we will find that the best match is (A). The correct cross shows one of the
parental genotypes as uppercase G lowercase g and the other as lowercase g lowercase
g.
