In this explainer, we will learn how to explain what occurs when alleles do not have complete dominance and interpret Punnett squares that show this.
Austrian monk Gregor Mendel is often hailed as the father of modern genetics. His experiments with pea plants helped us understand how traits are inherited. However, the patterns of inheritance that he demonstrated do not always apply. One situation in which they do not apply is if alleles do not have complete dominance.
Key Term: Allele
An allele is an alternative version of a gene. For example, a gene may code for flower color and the alleles may be white flowers or purple flowers.
Let’s look at an example of an allele having complete dominance using a Punnett square. Assume that, in pea plants, the allele that gives the flowers a purple color can be represented by P, and the allele for white flowers can be represented by p. In figure 1, we are provided with a Punnett square showing the predicted genotypes of the offspring produced by two plants that are heterozygous (Pp) for flower color being crossed. We can see that there is a 1 in 4 chance that the offspring will inherit two alleles for purple flowers (PP). However, there is a 2 in 4 chance that the offspring will be heterozygous, so have one allele for purple flowers and one allele for white flowers (Pp). If this occurs, what color flowers will they have? Because the allele for purple flowers shows complete dominance, any plant that has the dominant P allele in its genotype will have purple flowers. The only genotype that will give a plant with white flowers is pp.
Definition: Complete Dominance
An allele has complete dominance when it masks the expression of the recessive allele in heterozygous individuals.
However, this is not always the case. In some species of chicken, feathers can come in various colors, including black, white, and blue. If a chicken is homozygous for the black feather allele, it will have black feathers. If a chicken is homozygous for the white feather allele, it will have white feathers. The blue feather color is produced when an individual has one white allele and one black allele, which “mix” to produce a new color. One allele now no longer shows complete dominance over the other; instead, when they are both present in the genotype, a new phenotype is created. This is known as incomplete dominance since the two traits appear to “mix” together. Figure 2 outlines how this happens. In this case, we will use CW to represent the white feather allele and CB to represent the black feather allele. The C tells us the alleles are controlling the characteristic “color,” and the superscript tells us that the traits are “white” and “black.” This writing convention helps us distinguish between the inheritance of traits that are determined by complete dominance and the inheritance of traits when there is a lack of dominance.
Definition: Incomplete Dominance
Alleles show incomplete dominance when an intermediate phenotype is created in an organism heterozygous for a trait.
Example 1: Using a Punnett Square to Demonstrate Incomplete Dominance
The flower color of snapdragon plants shows incomplete dominance. CR indicates red flowers, and Cw indicates white flowers. A Punnett square showing a cross between two parent plants is given below.
State the probability of their offspring inheriting pink flowers.
The question states that the flower color in snapdragon plants shows incomplete dominance. Incomplete dominance is when one allele for a particular trait does not have complete dominance over another, and when an organism is heterozygous for these alleles, the two traits combine into an intermediate phenotype. In the question, we are given two colors—red and white—so we can assume neither the allele for red nor that for white flowers will be completely dominant over the other. In fact, we can assume from the information given that when a red and a white allele are present, the resulting flower color will be pink.
Let’s have a look at the Punnett square provided. We can see that along the topmost row and the left-hand side are the alleles from each parent plant. They are split into single alleles to represent the gametes (sex cells), which only contain one allele for each trait.
Now, we need to fill in the empty cells. We do this by taking the alleles from the column head and the row head for each cell.
We can see that the 4 cells that are representing the possible genotypes of the offspring all have the combination CR CW. That means all possible combinations will have one allele for red flowers and one allele for white flowers. Remember that when there is one allele for each color present, we get a new, intermediate phenotype of pink flowers. So now we just need to translate this into a percentage, as the question asks for the probability .
Therefore, the probability that the offspring will inherit pink flowers is .
Another example of alleles not showing complete dominance is a pattern we call “codominance.” In this scenario, an individual that inherits two different alleles for a specific characteristic expresses both of them in its phenotype. Let’s look back at our chicken example: if a chicken inherited the allele for black feathers from its father and the allele for white feathers from its mother, and it demonstrated a new, intermediate phenotype (e.g., blue feathers), this is incomplete dominance. However, if the mix of the white and the black allele resulted in a “speckled” phenotype that had both white and black feathers, this is codominance. Figure 3 outlines how this phenotype can be produced from the gametes of two homozygous parents.
Codominance occurs when both alleles are expressed simultaneously, and without blending, in the phenotype of an organism heterozygous for a trait.
Example 2: Using a Punnett Square to Demonstrate Codominance
Shorthorn cows show codominance in their coat color. They can be red, white, or roan—a mixture of the two (pictured). Which of the following Punnett squares shows the correct cross when a cow homozygous for a white coat and a bull homozygous for a red coat breed?
To answer this question, we first need to understand some of the key terms. Codominance occurs when an allele does not have complete dominance over another. An organism that is heterozygous for this allele therefore shows a mixture of alleles in its phenotype (physical appearance).
In this example, there are two alleles that determine coat color in shorthorn cows: one that gives a red coat and one that gives a white coat. If a shorthorn cow has two copies of the allele that gives a white coat, it will have a white coat; if it inherits two copies of the allele that gives a red coat, it will have a red coat. However, if a cow inherits one allele for a white coat and one allele for a red coat, it shows a “roan” coat, which is a mixture of these two phenotypes.
The question states that one parent is homozygous for the white coat allele and one parent is homozygous for the red coat allele. Homozygous means the two alleles are the same, so we are looking for a Punnett square where the top row and the left-hand column have two of the same alleles. The alleles are written as CR and CW, so we are looking for something that looks like this:
We then need to complete the cells of the Punnett square. We do this by taking the allele from the column head and the row head for each cell, as follows:
If we use this format to complete the entire Punnett square, we see that the correct answer is the following:
Another example of codominance, and this time in humans, is the presence of the sickle cell allele. The majority of red blood cells in humans have a biconcave shape, so they look like a disc. This helps maximize the amount of oxygen the red blood cells can carry. However, if a person has sickle cell anemia (a disease that follows a recessive inheritance pattern), their blood cells become a “sickle” shape. A simple illustration to outline the differences between these shapes is given in figure 4. Red blood cells that have a sickle shape are elongated, are flatter, and consequently cannot carry as much oxygen as biconcave-shaped red blood cells.
Figure 5 shows how the alleles for biconcave and sickle-shaped red blood cells can be inherited and expressed as codominant alleles in the phenotype.
As we can see, having an allele for the biconcave shape and an allele for the sickle shape means that both of these types of red blood cells are present in the body.
For a person to inherit sickle cell anemia, they must have two copies of the sickle cell allele. However, if they have one sickle cell allele and one normal allele, they have what we call “sickle-cell trait.” These individuals will possess both biconcave and sickle-shaped red blood cells in their body. This is an example of codominance—both phenotypes are being simultaneously expressed. Interestingly, having sickle-cell trait has been found to provide increased resistance to malaria!
Another example of alleles not showing complete dominance is the ABO blood groups of humans. Humans have three alleles that determine blood group: IA, IB, and IO. Table 1 outlines the combinations of alleles that result in each blood group type. ABO blood groups are also a good example of a trait determined by multiple alleles; this means that there are three or more possible alleles for a trait, but only two are present in a single person’s genotype.
Definition: Multiple Alleles
Multiple alleles are three or more possible alleles that determine a trait, but only two will be present in a diploid organism.
A person will inherit two alleles for blood group, one from their biological mother and the other from their biological father. Looking at table 1, we can see that if both these alleles are IA, the person will have blood group A. If they are both IB, the person will have blood group B. If they inherit IA and IO, the A allele will show complete dominance over the O allele, and the B allele will also have complete dominance over O if IB IO is inherited.
However, if they inherit one A allele and one B allele, instead of one allele showing complete dominance over another, they will have blood type AB. In this case, both alleles are present in the phenotype and are demonstrating codominance. Figure 6 outlines how blood groups are created and how group AB can be inherited.
Example 3: Understanding the Inheritance of Blood Groups
A man claiming to be a long lost child of a recently deceased millionaire argues he has a claim to the inheritance. The millionaire has blood group O and the child has blood group AB. Could the millionaire be the father? Why? Use the blood group table provided to help you.
Blood groups in humans are a good example of alleles showing codominance—this is when two alleles for a specific trait are both expressed in the phenotype. People with blood group AB have inherited the allele for blood group A from one parent and the allele for blood group B from the other parent. Instead of one allele being dominant over the other, both are expressed to give the phenotype of blood type AB.
If we refer back to the question, we can see that the child claiming to be an offspring of the millionaire has blood type AB. So he must have inherited one of these alleles from his biological mother and one from his biological father. This means that, to be the father of the child, the millionaire must have had a genotype that includes at least one allele for blood group A (IA) or at least one allele for blood group B (IB).
However, the millionaire had blood group O. Looking at the table, we can see that the only way this is possible is if his genotype for the blood group allele was IO IO.
Therefore, we can conclude that, no, the millionaire was not the father, as the child would have inherited one allele for blood group O from him.
Although both incomplete dominance and codominance are examples of alleles not showing complete dominance, there are some important distinctions between the two. Alleles showing incomplete dominance will create a new, distinct phenotype in an organism that has two different alleles, for instance, a new flower color or a new color of feathers. Alleles showing codominance do not create new phenotypes in heterozygous organisms, but rather the traits shown by both alleles will be present in the phenotype, for example, having red blood cells with a biconcave shape and also having red blood cells with the sickle shape.
- A “lack of dominance” will occur if no single allele has complete dominance over the expression of another.
- Incomplete dominance occurs when alleles of heterozygous individuals combine to form a new, distinct phenotype.
- Codominance occurs when both alleles of heterozygous individuals are simultaneously expressed in the phenotype, without mixing or blending.
- Multiple alleles are three or more possible alleles that will determine a trait, but only two are expressed in a person’s phenotype.