Video Transcript
Assume that the color of fur in
mice is determined by two genes and their complementary action. The table provided shows the
genotypes of different mice. Which of the following options
correctly replaces X, Y, and Z in the table? (A) X: white, Y: functional, and Z:
nonfunctional. (B) X: black, Y: functional, and Z:
nonfunctional. (C) X: black, Y: nonfunctional, and
Z: functional. (D) X: white, Y: nonfunctional, and
Z: functional.
This question is asking us about
complementary genes, which display a non-Mendelian mode of inheritance. You may recall that a gene
typically has two alleles, or gene versions, that each control a different
trait. These allele are combined to make
the corresponding genotype. And each allele can be either
dominant or recessive.
A dominant allele is one that is
always expressed if at least one copy is present in an organism’s genotype. And a recessive allele is only
expressed in the absence of a dominant allele and requires two copies for its
phenotype to be expressed. When we’re talking about
complementary genes, a dominant allele usually produces a functional protein, while
a recessive allele usually produces a nonfunctional protein.
Now let’s put together what we see
in this table. We have two genes that we’ll call
gene A and gene B. These genes contribute to producing
the color of fur in these mice, so let’s explore that in more detail. The first genotype is uppercase A
uppercase A uppercase B uppercase B. This genotype contains two dominant
copies of gene A and B. Because both dominant alleles are
present for genes A and B, this means that functional enzymes A and B will be
produced. So now we know that Z in this table
corresponds to the word functional. So we’ll indicate that down
here.
Now let’s look at the
phenotype. The color of the fur from this
genotype is black when both enzymes A and B are functional. This tells us that when these two
enzymes are functional, black fur is produced by their action. To make the black color, certain
precursors need to be converted. And we don’t know what the color of
these precursors are yet, so we’ll indicate these as precursor one and two.
In the next genotype in our table,
both alleles are recessive. When this happens, both genes
produce nonfunctional enzymes. We now know that this letter Y
corresponds to the word nonfunctional. So we’ll indicate that below.
When both enzymes are
nonfunctional, precursor one can’t be converted to precursor two. So the mouse will have fur that’s
the color of precursor one. We can see from the table that this
is white.
Now let’s look at the final
genotype. These mice are heterozygous for
both alleles. So they produce functional copies
of enzymes A and B. And so black fur will be
produced. So this letter X here corresponds
to black. And we’ll indicate that below.
Therefore, the option that
correctly matches X, Y, and Z is given by answer choice (C) X: black, Y:
nonfunctional, Z: functional.
