Video Transcript
Assume fur color in mice is an
example of a characteristic affected by complementary gene action. The fur can be white or black; in
this scenario, there must be a dominant allele present for both genes for the black
color to be shown. Which of the following genotypes
would give a mouse with white fur? (A) Lowercase a lowercase a,
uppercase B uppercase B. (B) Uppercase A lowercase a,
uppercase B lowercase b. (C) Uppercase A uppercase A,
uppercase B uppercase B. (D) Uppercase A uppercase A,
uppercase B lowercase b. Or (E) uppercase A lowercase a,
uppercase B uppercase B.
This question requires some
familiarity with genetics and how genotypes are represented. Let’s start by exploring some key
terms that will help us understand how to answer it.
The genotype is the genetic makeup,
or alleles, of an organism. You might recall that for certain
characteristics, like fur color in mice, different alleles exist. Alleles are different versions of
the same gene that result in different traits.
Dominant alleles are versions of
genes that mask other alleles and so are always expressed when they are present in
the genotype. Recessive alleles are the versions
of a gene that can be masked by dominant alleles. So they are only expressed when the
dominant allele is not present in the genotype.
In this example, genes A and B are
of interest to us. Generally, dominant alleles are
represented by uppercase letters, and recessive alleles are represented by lowercase
letters.
Now that we have covered some
basics, we can take a look at the answer choices.
The question tells us that both a
dominant allele for gene A and gene B must be present in an individual’s genotype to
produce a mouse with black fur. As we are looking for the genotype
that would give a mouse white fur, not black fur, we can eliminate all of the answer
choices that include both at least one uppercase A and at least one uppercase B, as
this would result in the mouse having black fur.
The genotype in option (B) —
uppercase A lowercase a, uppercase B lowercase b — contains one uppercase A and one
uppercase B. So this genotype would result in a
black-furred mouse and can therefore be eliminated from our choices.
The genotype in option (C) —
uppercase A uppercase A, uppercase B uppercase B — contains only dominant
alleles. So this genotype would also result
in a black-furred mouse and can be removed from our choices.
The genotype in answer option (D) —
uppercase A uppercase A, uppercase B lowercase b — contains two uppercase As and one
uppercase B. So this genotype would produce the
black fur trait too.
Furthermore, the genotype in option
(E) — uppercase A lowercase a, uppercase B uppercase B — contains one uppercase A
and two uppercase Bs. So this genotype cannot be the
correct answer either.
Answer option (A) — lowercase a
lowercase a, uppercase B uppercase B — has two dominant alleles for gene B, but only
recessive alleles for gene A. As the production of black fur
requires a dominant allele to be present for both gene A and gene B, this genotype
would produce a mouse with white fur.
The correct answer is therefore
option (A): lowercase a lowercase a, uppercase B uppercase B.
