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Question Video: Using the Results of a Monohybrid Cross to Determine a Parental Genotype Biology • First Year of Secondary School

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A pea plant with a tall stem can have the genotype TT or Tt, whereas a plant with a short stem will have the genotype tt. A tall pea plant is crossed with a short plant to produce 4 offspring. Which of the following outcomes would indicate that the tall plant had the genotype TT? [A] 3 out of 4 of the offspring have tall stems. [B] 50% of the offspring have short stems. [C] All of the offspring have tall stems. [D] None of the offspring have tall stems. [E] All of the offspring have stems that are halfway between the tall and short plants.

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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.

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