Video Transcript
Snapdragon flowers, pictured, show
incomplete dominance in the color of their petals. The petals can be red, CRCR; white,
CWCW; or pink, CRCW. Which of the following Punnett
squares shows the correct cross between two pink flowers?
This question asks us to identify
the correct Punnett square for a cross between two pink parent flowers. Have you heard of the term Punnett
square before? These are useful tools for
determining the kinds of offspring that two parents can produce.
Before we create our Punnett
square, we first need to talk about how genes are passed along to offspring. You might have heard the term
“genes” used before. It refers to a segment of genetic
material called DNA that contains instructions to produce a functional unit, such as
a protein. Genes can be responsible for
producing many different characteristics. One example of a characteristic
encoded by a gene is the color of flower petals. Each individual should possess two
copies of each gene. One copy is inherited from the
biological mother and one from the biological father.
Parents can pass on different
alleles. The word allele refers to different
variations of a gene coding for a specific characteristic like flower petal
color. In our question, there are two
alleles of the gene that codes for flower petal color. The allele CR codes for red
flowers, and the allele CW codes for white flowers. Usually, one of these alleles would
be completely dominant over the other. This means that it would always be
expressed in the offspring if it were inherited from either parent.
However, we are told that
snapdragon flowers exhibit incomplete dominance instead of complete dominance. This means that both alleles are
expressed. As you can see, when a red allele
and white allele is inherited, a mix between the two colors happens. The flower color appears pink.
Now, let us return to the Punnett
squares of our question. We need to select the Punnett
square that correctly predicts the possible offspring of two pink flowers. We know from the question stem that
a pink flower has both alleles CR and CW. Each parent only passes on one of
the two alleles. This knowledge already allows us to
exclude the answer options (B), (C), and (D). Punnett square (B) would show the
possible offspring of a red and a pink flower. Punnett square (C) would show the
possible offspring of a red and a white flower. Punnett square (D) supposedly shows
the possible offspring of a red and a pink flower. However, the predicted outcome is
wrong.
To predict the possible offspring
with the help of a Punnett square, the alleles are shifted horizontally from left to
right and vertically from the top downwards in the empty boxes. If we fill in the rest of our
Punnett square, we can find the possible looks of the offspring because of these two
pink flowers. We can now see that the offspring
of two pink flowers can have red flowers, pink flowers, or white flowers. The correct Punnett square showing
the cross between two pink flowers is Punnett square (A).
