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 black fur? (A) Uppercase A, lowercase a,
uppercase B, lowercase b. (B) Uppercase A, lowercase a,
lowercase b, lowercase b. (C) Uppercase A, uppercase A,
lowercase b, lowercase b. (D) Lowercase a, lowercase a,
uppercase B, uppercase B. Or (E) all of the above.
This question asks about
complementary genes. Let’s review this type of
inheritance and some basic terms to answer this question. You might remember that a gene
refers to a segment of DNA that codes for a particular protein. Most individuals have two copies of
each gene, one from the biological mother and one from the biological father. Different versions of the same gene
are called alleles, which we can write as uppercase A to indicate the dominant
allele or lowercase a to indicate the recessive allele. A dominant allele is always
expressed if it is inherited, while a recessive allele requires two recessive copies
in order to be expressed.
Complementary genes are two or more
genes that work together to produce a particular phenotype. In this question, fur color is
affected by complementary genes, and we’re told that a black fur coat will only
arise when both genes have a dominant allele present. We can imagine that the production
of black fur pigment is a two-step process The first step is controlled by a gene
that we’ll call gene A, which produces protein A that converts our first precursor
into the second precursor, while gene B controls the conversion of precursor two
into the black fur pigment. Each of these genes have a dominant
and recessive allele. And as long as we have a dominant
allele for each, the black fur pigment is produced. But if one gene carries two
recessive alleles, then that particular protein won’t be produced and the precursor
won’t be converted. So the fur will stay white.
Therefore, we are looking for an
answer choice that has at least one dominant allele for each gene. This is answer choice (A) uppercase
A, lowercase a, uppercase B, lowercase b.
