Video Transcript
A pea plant has the following two alleles, uppercase P uppercase P, for purple flowers. Which of the following terms can be used to describe this pea plant? (A) Homozygous recessive, (B) homozygous dominant, (C) heterozygous recessive, or (D) heterozygous dominant.
This question describes the set of alleles that a certain pea plant possesses, then asks us to choose the correct term to describe this plant. From the answer choices, we can see that we are determining whether this plant is homozygous or heterozygous and whether the alleles are dominant or recessive.
When we describe the traits that an organism possesses, such as having purple flowers, we’re describing its phenotype. On the other hand, when we describe the alleles that an organism possesses, we’re describing its genotype. An allele can be defined as a version of a gene.
Let’s take, for example, a species of fruit fly. All fruit flies will possess a gene that determines the color of their eyes. And in our imaginary example, that gene comes in two versions or alleles, either red or black. The dominant red allele is indicated by an uppercase R, and the recessive black allele is indicated by a lowercase r. This means that fruit flies with black eyes have the genotype lowercase r lowercase r, whereas fruit flies with red eyes can have the genotype uppercase R uppercase R or uppercase R lowercase r. There are two options of genotype that lead to the phenotype of having red eyes, because the dominant allele, uppercase R, will always be expressed.
Returning to our question, since the pea plant has the alleles uppercase P uppercase P, we can conclude that it must have the dominant genotype. So we can eliminate the answers that say that the genotype is recessive.
Next, we need to determine whether the pea plant genotype is homozygous or heterozygous. Well, we use the term homozygous when an organism has two alleles that match because the prefix homo- is a word part that means the same. We use the term heterozygous when an organism has two alleles that are different from each other because the word part hetero- means different.
Returning to our question, a homozygous dominant genotype would have two uppercase P alleles because homozygous means that they would match, whereas a heterozygous dominant genotype would have an uppercase P and a lowercase p because heterozygous means that the alleles don’t match.
Therefore, the term used to describe the pea plant in our question would be homozygous dominant.
