Video Transcript
In humans, the allele for attached earlobes is recessive to the allele for unattached earlobes. Which of the following best describes what recessive means? (A) A recessive allele will always be expressed in the phenotype. (B) A recessive allele will never be expressed in the phenotype. (C) A recessive allele will only be expressed in the phenotype if two copies of it are present. Or (D) a recessive allele will only be expressed in the phenotype if one copy of it is present.
This question is asking us to choose the answer choice that best defines the term recessive. If you remember this definition, then you don’t need the rest of the information in the question to choose the best answer choice. We’ll go ahead and use this information as an example so we can choose the correct answer choice together.
The question describes two phenotypes related to earlobes in humans: attached and unattached. In this example, the allele for attached earlobes is recessive. An allele can be defined as a specific version of a gene. It’s scientific convention to represent recessive alleles with lowercase letters. And since we’re talking about earlobes, we’ll call this allele lowercase e. Since the allele for attached earlobes is recessive, we can infer that the allele for unattached earlobes will be dominant. Dominant alleles are represented by uppercase letters.
The two genotypes that could give someone the trait of unattached earlobes are uppercase E uppercase E or uppercase E lowercase e. There are two options here because a dominant allele is an allele that is always expressed in the phenotype.
Using the information from this example, we’re now able to define the term recessive. One of our answer choices states that a recessive allele will always be expressed in the phenotype. We know that this answer choice is incorrect because that is the definition of a dominant allele. Another answer choice states that a recessive allele will never be expressed in the phenotype. We know that this answer choice is incorrect because in our example individuals with the genotype lowercase e lowercase e express the recessive trait of having attached earlobes.
The answer choice that states that a recessive allele will only be expressed if two copies are present appears to be correct. But let’s eliminate our last answer choice just to be sure. We can tell that the answer choice that states “A recessive allele will only be expressed if one copy is present” is incorrect because in the example when only one recessive allele was present, the dominant unattached phenotype was expressed.
So the answer choice that best describes what recessive means is “A recessive allele will only be expressed in the phenotype if two copies of it are present.”
