Lesson Explainer: Complementary Genes Biology • First Year of Secondary School

In this explainer, we will learn how to describe the effect of complementary genes on gene expression, and apply this to given examples.

Complementary genes are a type of gene interaction between two different genes that work together to create a specific phenotype or visible trait. The use of the word “complementary” is related to the relationship between the two genes that make up the phenotype. The root of the word “complementary” is “complement,” which comes from the Latin word for “to complete.” So, complementary genes can be thought of as genes that help to “complete” each other to create a visible outcome.

Complementary genes are an example of a non-Mendelian gene interaction. The inheritance patterns for such gene interactions are complex because they often require more than one gene for a specific trait. For example, complementary genes help control the color of the flower in sweet pea plants. This means that this color depends on two different genes interacting “to complete a pathway” and make a specific flower color visible.

You may remember that a gene typically has two alleles or gene versions that each control a different trait of a certain characteristic. An individual’s genotype is the combination of the two alleles that contribute the phenotype. While the genotype can tell you what is possible genetically, the phenotype is the actual visible outcome.

Gene interactions like complementary genes defy the well-known Mendelian patterns of inheritance and highlight the fact that not all genes dictate a single unique phenotype.

In a genotype, it is often one allele that helps determine the phenotype. The allele that helps determine the phenotype depends on whether the alleles are dominant or recessive. The dominant allele helps determine the visible outcome of the trait. The trait associated with the recessive alleles is often masked by the dominant allele and does not help determine the phenotype. When a recessive allele is paired with a second recessive allele, the recessive trait is expressed.

When writing out the genotype, the dominant allele is symbolized by an uppercase letter, whereas the recessive allele is symbolized with a lowercase letter. For example, for gene B, the dominant allele is written as “B,” whereas the recessive allele is written as “b.” The genotype of gene B that contains one dominant allele and one recessive allele is symbolized as “Bb.”

In complementary gene action, it is the dominant alleles of the two genes that work together to contribute to the phenotype. If either gene is missing the dominant allele, the phenotype cannot be observed. Dominant alleles in both genes are necessary “to complete” the pathway and produce the specific trait.

To explain this complementary action between genes, let’s take a closer look at the sweet pea flower experiment performed by William Bateson and Reginald Punnett to discover complementary genes.

Bateson and Punnett performed their experiments on Lathyrus odoratus, a sweet pea that normally has purple flowers. In their experiments, they used two varieties of the plant that had white flowers. In the first step of their experiments, they crossed the two varieties of white sweet pea flowers and produced a first generation of flowers that were purple. An example of the two parents used in the first cross and the first generation of flowers is shown in Figure 1.

For the second generation, Bateson and Punnett crossed two of the purple flowers from the first generation. In the second generation of flowers, they counted a total of 382 purple-flowered plants and 269 white-flowered plants. A Punnett square demonstrating the difference between the genotype and the observed phenotypes is shown in Figure 2.

The phenotypic ratio of purple flowers to white flowers was . Generally, the phenotypic ratio helps estimate the likelihood or probability of a phenotype occurring in the offspring. This result of 9 purple flowers to 7 white flowers surprised Bateson and Punnett. They both expected the phenotypic ratio seen in a dihybrid cross (), which is the ratio we typically see when crossing two different genes that control two different traits. The actual phenotype of the sweet pea flowers suggested that, unlike in a dihybrid cross, the two scientists have just observed a new form of inheritance.

Let’s take a closer look at why there is a change in the phenotypic ratios for sweet pea plants and how this modification is only seen for phenotypic ratios but not for genotypic ratios.

Years after Bateson and Punnett first observed the ratio of purple to white flowers, scientists were able to discover the reason for the change in the phenotypic ratio of sweet pea flowers. In sweet pea plants, a two-step biochemical pathway must occur to produce purple flowers.

In sweet pea flowers, two different genes, gene C and gene P, work together in a two-step pathway to produce a purple pigment. Another way to say this is that the products of gene C complement the products of gene P to produce the purple pigment.

Gene C controls the first step of the biochemical reaction and codes for an enzyme that catalyzes the reaction at the first step of the pathway. The final product of this reaction is used to start the second step. The second step of the biochemical reaction is controlled by gene P, which codes for enzyme P, which catalyzes the second reaction. A schematic of this process is shown in Figure 3.

For the biochemical pathway to work normally, a dominant allele of both gene C and gene P must be present in the genotype to code for the production of functional enzymes. Gene C and gene P both have a dominant allele (C and P) and recessive alleles (c and p). The dominant C and P alleles code for functional enzymes that are needed in the production of the purple pigment, whereas the recessive c and p alleles code for nonfunctional enzymes.

If either step of the biochemical pathway controlled by genes C and P were to have two recessive alleles and no dominant allele present, both steps would be unable to produce functional enzymes. Without functional enzymes in step 1 or 2, the biochemical pathway cannot produce the purple pigment and only white flowers are produced. This means that the presence of any homozygous (identical) recessive alleles and absence of either of the two dominant alleles will produce white flowers. So, in genotypes like ccPP, CCpp, ccPp, and Ccpp, the purple pigment does not appear.

Example 3: Determining the Outcome of Nonfunctioning Genes in Complementary Gene Action

Flowers of sweet pea plants can be purple or white. The diagram provided shows the biochemical pathway that allows the production of purple flowers in sweet pea plants.

If a mutation in gene B caused the enzyme it codes for to be nonfunctioning, what would the outcome be?

1. The plant would have white flowers.
2. The plant would have purple flowers.
3. The plant would have no flowers.
4. The plant would wilt and die.

Answer

In the diagram provided, we see a two-step biochemical process that ends with the production of a purple pigment in sweet pea plants. The diagram shows us that a colorless precursor is acted upon by enzyme A (which is expressed by gene A), which catalyzes the first step of the reaction.

The products of the first step of this biochemical pathway are then used in the second step of the pathway to produce a purple pigment in the flowers of sweet pea plants. Gene B codes for enzyme B, which catalyzes the second step of the reaction.

Both gene A and gene B products are needed to produce the purple pigment. If either step is nonfunctional, then no purple pigment is produced, and the affected pea plant can only make colorless/white flowers.

Therefore, if a mutation in gene B caused the enzyme it codes for to be nonfunctioning, the outcome would be A. The plant would have white flowers.

Notice that since the white sweet pea flowers are a result of homozygous recessive alleles for at least one of the genes, only the ratio of the visible trait or phenotypic ratio changes. That is to say, for any genotype in which there is at least one dominant allele for gene C and gene P, a purple flower can be produced. The diagram in Figure 4 shows how the absence of a dominant allele in one of the genes causes a genotype to result in a white phenotype.

Figure 4 shows the genetic cross that produces purple and white sweet pea flowers through complementary genes. There are nine combinations of alleles in the second generation that feature at least one dominant C and one dominant P allele, which would result in a purple flower phenotype. There are seven combinations that result in either cc or pp, which would result in a white flower phenotype.

This gives a phenotype ratio of 9 purple flowers to 7 white flowers because heterozygous genotypes for both genes still have a purple coloring. So, it is the complementary gene action between two dominant alleles that is responsible for the change in the phenotypic ratio without changing the genotypic ratio.

Let’s summarize what we have learned in this explainer.