Video Transcript
In this video, we will learn how
complementary genes interact to influence an organism’s phenotype. We will learn about some examples
of complementary genes and how disruption to a complementary gene pathway can stop
some traits from being expressed.
Gregor Mendel, sometimes referred
to as the “father of genetics,” is widely credited with demonstrating the basics of
gene inheritance in his pea plants. For example, he showed how traits
like flower color or seed color are inherited from parent to offspring. Mendel hypothesizes that genes are
inherited independently of each other. This would mean that the gene for
flower color would have no impact on the gene for seed color. However, we have since discovered
that this isn’t always the case. And complementary genes are example
of non-Mendelian inheritance.
In humans, a single gene typically
has two or more alleles that give rise to a particular trait. If there are two alleles, one tends
to be dominant over the other. For instance, the allele for
freckles is dominant to the allele that does not get freckles. If the allele for freckles is
present in a person’s genotype, it’s highly likely to be expressed. We tend to assign letters to
alleles when studying genetics. For example, we may assign the gene
that controls the appearance of freckles the letter F. The dominant allele can therefore
be assigned a capital F and the recessive allele a lowercase f. The combination uppercase F,
uppercase F or uppercase F, lowercase f will give the dominant phenotype of
freckles, while two lowercase f’s will give the recessive phenotype of no
freckles. If a particular trait is controlled
by complementary genes, it requires the dominant alleles of both of these two genes
to be present. Let’s look at an example to better
understand this.
Sweet pea plants can have purple
flowers or white flowers. The color of these flowers is
controlled by two genes; let’s call these genes gene C and gene P. For purple flowers to be displayed,
there must be a dominant allele present for gene C and gene P. So, how does this work? Here, we can see a simplified view
of the complementary gene pathway. Genes C and P work together to
produce the purple pigment; you could say that they complement each other. Gene C controls the expression of
enzyme C and enzyme C catalyzes the reaction that converts precursor number one to
precursor number two, whereas gene P controls the expression of enzyme P. And enzyme P catalyzes the reaction
that converts precursor two into the purple pigment. This purple pigment eventually
gives rise to the purple flower.
Gene P relies on the product
produced by the expression of gene C. So, when there is a dominant allele
of gene C and gene P present, the colorless precursors are converted into a purple
pigment. If we don’t have a dominant allele
for gene C, the plant won’t produce a functional version of enzyme C. And if enzyme C isn’t produced, the
reaction that converts precursor one into precursor two doesn’t happen. And if precursor two isn’t
produced, then the final purple pigment can’t be produced. We may have the dominant allele for
gene C present, but if we don’t have a dominant allele for gene P, a similar thing
will happen. Gene P will not code for enzyme P,
so enzyme P won’t be produced, and precursor two won’t be converted into the purple
pigment. So, in both of these cases where we
don’t have a dominant allele for gene C or for gene P, the phenotype will be white
flowers.
Let’s have a look at the genotypes
that will produce these different phenotypes. We know that at least one dominant
allele for each gene must be present in the genotype to produce purple flowers. So, let’s see what the possible
genotypes are. For white flowers to be produced,
at least one of the genes needs to have two recessive alleles. So, let’s see what possible
genotypes white flowers have. And there’s all the possible
genotypes for these two flowers. Feel free to pause the video to
take a moment to review this.
Using what we know about the
genotypes and phenotypes of flower color and how it is controlled by complementary
gene action, let’s take a look at the inheritance of these alleles. Let’s take two sweet pea plants
that produce white flowers. One has a genotype uppercase C,
uppercase C, lowercase p, lowercase p and the other has the genotype lowercase c,
lowercase c, uppercase P, uppercase P. If these two plants are crossed,
what are the possible genotypes for their offspring? We can complete a Punnett square to
show the possible genotypes.
Since flower color is controlled by
two genes, we need to use a Punnett square that shows dihybrid inheritance. Let’s start by completing the
column and row headers with the alleles that would be found in the gametes produced
by each plant. Now, we can complete each box in
the Punnett square with a combination of alleles that can be inherited from the
parents. You may notice that all the boxes
are the same. This means that there is only one
possible genotype that arises from this cross. The genotype uppercase C, lowercase
c, uppercase P, lowercase p results in purple flowers. This is because for both gene c and
p, there’s a dominant allele present. If we think back to our
complementary gene pathway, this means that both enzyme C and enzyme P are
functional. So, the purple pigment is
produced.
Now, let’s see what happens when we
cross two plants with purple flowers. Again, we’ll start by completing
the column and row headers of our Punnett square with the possible combination of
alleles that will be present in the gametes of these plants. Next, we’ll complete each box in
the Punnett square with the combinations that can be inherited when these gametes
combine to produce new offspring. You may have already spotted that
we have a lot more variation in the genotypes than we did last time. So, let’s go through the genotypes
and see if we can work out the number of different phenotypes produced, and we’ll
record this information here.
Remember any genotype that has a
dominant allele for gene c and gene p will produce a purple flower. If we circle all these genotypes in
our Punnett square, we should have a total of nine possible genotypes that give
purple flowers. Now, let’s see how many will have
white flowers. Any genotype that’s missing a
dominant allele for gene c or gene p will produce white flowers. If we circle all these genotypes in
our Punnett square, we should have a total of seven genotypes that give white
flowers. Another way to say this is in the
form of a ratio. So, the ratio of purple to white
flowers is nine to seven; we call this the phenotypic ratio. Typically, in a dihybrid cross with
genes that are not complementary, we would expect a phenotypic ratio to be nine to
three to three to one. So, this is another sign that
complementary genes do not follow the normal rules of Mendelian inheritance.
Now that we understand the action
of complementary genes, let’s try out a practice question.
Flower color in sweet pea plants is
an example of a characteristic affected by complementary gene action. The flowers can be white or
purple. In this scenario, there must be a
dominant allele present for both genes for the purple flower color to be shown. Which of the following genotypes
would give a plant with white flowers? (A) Uppercase A, uppercase A,
uppercase B, uppercase B. (B) Uppercase A, lowercase a,
uppercase B, lowercase b. (C) Uppercase A, lowercase a,
uppercase B, uppercase B. (D) Lowercase a, lowercase a,
uppercase B, uppercase B. Or (E) uppercase A, uppercase A,
uppercase B, lowercase b.
Complementary gene action refers to
the interaction between multiple genes that control the expression of a particular
trait. Here, we’re told that there must be
a dominant allele present for both genes for the purple flower color to be present
in the phenotype. In genetics, we tend to represent
dominant alleles with uppercase letters and recessive alleles with lowercase
letters. We need to spot the genotype that
will give the phenotype of white flowers. This means that one or both genes
have two copies of a recessive allele.
Option (A) has two uppercase As and
two uppercase Bs. Because this genotype has only
dominant alleles, this will produce purple flowers. So, this can’t be the correct
option. Option (B) has one uppercase A and
one uppercase B. Because there is one dominant
allele present for each gene, this genotype will produce purple flowers. Again, this can’t be the correct
option. Option (C) has one dominant allele
for gene a and two dominant alleles for gene b. This will also produce purple
flowers. So, this is not the correct
choice.
Option (D) has two dominant alleles
for gene b. However, gene a has two recessive
alleles. We know that to produce purple
flowers, there must be at least one dominant allele present for each gene. This genotype will therefore
produce white flowers due to it having no dominant allele for gene a. This looks like our correct answer,
but let’s first double-check option (E) to make sure it’s incorrect. Option (E) has two dominant alleles
for gene a and one dominant allele for gene b. Because both genes have a dominant
allele present, this genotype will produce purple flowers. So, this choice is incorrect. Therefore, we can conclude that the
genotype that will produce a plant with white flowers is (D) lowercase a, lowercase
a, uppercase B, uppercase B.
Now, let’s take a moment to go over
the key points that we covered in this video. Complementary gene action occurs
when two different genes work together to contribute to one trait. These two genes are usually
involved in a specific pathway. If either gene has two recessive
alleles, then that gene produces nonfunctional products. Without the dominant allele in both
genes, that specific trait is not observed. Complementary genes are an example
of non-Mendelian inheritance.
