Video Transcript
In labradors, the allele for black fur is dominant to the allele for yellow fur. Which of the following best describes what dominant means? (A) If a dominant allele is present, it will always be expressed in the phenotype. (B) If a dominant allele is present, it will never be expressed in the phenotype. (C) A dominant allele will only be expressed in the phenotype if two copies of it are present. Or (D) if a dominant allele is present, it will blend with the recessive allele.
This question is asking us to select the answer choice that best defines the word “dominant.” If you recall what this scientific term means, the rest of the information in the question is not necessary to choose the correct answer. We’ll use this information about labradors to come up with the correct answer together.
An allele can be defined as a version of a gene. In this case, the gene that we’re describing is for color. In this question, we’re told that the allele for black fur is dominant. When we’re describing the alleles that an organism possesses, we’re describing that organism’s genotype. Dominant alleles by scientific convention are usually represented by uppercase letters. So, we’ll call the allele for black fur uppercase B since it is dominant. We can infer that the allele for yellow fur must be recessive. So, we will represent this allele with a lowercase b.
While the genotype tells us what alleles an organism has, the phenotype describes which trait is expressed. So, the phenotype of the dog on the top is black fur, and the phenotype of the dog on the bottom is yellow fur. Next, let’s determine the genotypes that go along with these two phenotypes.
A recessive allele is an allele that will only be expressed if no dominant alleles are present. So in order for a labrador to express the phenotype of having yellow fur, it would have to have the genotype lowercase b lowercase b or two recessive alleles. That means that in order to express the dominant phenotype of having black fur, a labrador could either have the genotype uppercase B uppercase B or uppercase B lowercase b.
We can see from our example that whenever even one dominant allele is present, it’s expressed in the phenotype of the dog. That means we have enough information to choose the correct answer. The answer choice that best explains what dominant means is “If a dominant allele is present, it will always be expressed in the phenotype.
