Lesson Video: Lack of Dominance Biology

In this video, we will learn how to explain what occurs when alleles do not have complete dominance, and interpret Punnett squares that show this.

15:12

Video Transcript

In this video, we will learn how to explain what occurs when alleles do not have complete dominance and interpret Punnett squares that show this. We will learn about two main ways alleles can show a lack of dominance, incomplete dominance and codominance, and describe some examples of each. We will also discuss how multiple alleles can influence the trait and how this can play a role in codominance.

You might remember the word dominance from learning about the discoveries of Gregor Mendel and the strides he made towards our current understanding of genetics. Mendel’s experiments on pea plants helped us to understand how traits are inherited from a parent to their offspring. The patterns he demonstrated do not always apply, however. And one situation where this is the case that we will be exploring in this video is if alleles do not have complete dominance.

An allele is an alternative version of a gene. For example, a gene may code for the color of the peas produced by a certain pea plant. And different alleles for this gene may code for the peas being green, like those produced by this parent plant or for the peas being yellow, like those produced by the other parent plant. Let’s look at the offspring the pea plant parents in this example might produce by using a Punnett square.

Punnett squares like the one pictured here are useful tools in the study of genetics to visually represent how alleles are inherited and predict the genotypes and phenotypes of offspring produced by crossing parents with known genotypes. Remember, the set of alleles an organism possesses for a characteristic is their genotype. The genotype of the parent that produces yellow peas would be two lowercase g’s, while the genotype of the parent that produces green peas would be two uppercase G’s.

The phenotype is the observable trait that the genotype produces for this parent plant producing peas with a yellow coat and for this parent plant producing peas with green coats. So far in a Punnett square, we can see the gametes of the green pea producing parent in the left column and the gametes of the yellow pea producing parent in the top row. Because the allele for a green coat shows complete dominance, any plant that has the dominant uppercase G in its genotype will have a green coat.

Complete dominance occurs when an allele like the one for green coat color will always mask the expression of the recessive allele, like the yellow coat color when it is present in the genotype. Therefore, the only genotype that will produce a yellow phenotype like the peas produced by this parent is two lowercase g’s. We can see that all of the offspring produced by this cross will have at least one uppercase G in their genotype. As a green allele shows complete dominance, all of these offspring will therefore produce green peas. But as we’ve said, there are exceptions to this rule.

One such exception is when in complete dominance determines the inheritance of traits. So let’s look at this now. Incomplete dominance is when alleles produce an intermediate phenotype when an organism is heterozygous for a trait, meaning that one of each allele is present. Let’s see what this means with an example. The Andalusian chicken shows incomplete dominance in its feather color. Each chicken can either have feathers that are black, that are white, or that are a mix of the two, which makes their feathers appear blue. A chicken’s feathers will appear black if it is homozygous for the black feather allele, which means they have two of these alleles in their genotype. A chicken’s feathers will appear white if it is homozygous for the white feather allele.

The C tells us that the alleles are controlling the characteristic color and the superscript letter tells us whether the feathers are black for B or white for W. This notation helps us to distinguish between the inheritance of traits that are determined by complete dominance, like in the pea plants, and the inheritance of traits where there is lack of dominance, like in the Andalusian chicken feather color.

The blue feather color is produced when an individual has one white allele and one black allele, so it is heterozygous for the feather color trait. One allele does not show complete dominance over the other. And instead, when they are both present in the genotype, a new blended intermediate phenotype is created, blue feathers. So, if the black feather chicken and white feather chicken were to reproduce, their offspring would inherit one black feather allele and one white feather allele, giving the offspring a genotype with one of each feather color alleles and a blue-feathered phenotype.

Another exception to complete dominance that we will explore in this video is when codominance determines the inheritance of a trait. Codominance occurs when both the alleles are expressed in the phenotypes simultaneously when an organism is heterozygous for a trait. This differs from incomplete dominance, as both the alleles are expressed instead of the alleles causing a blended phenotype. Let’s look at an example in the human body to help us understand this term better.

We rely on the biconcave disk shape of our red blood cells, which you can see pictured here, so that they can carry the maximum volume of oxygen to our body cells. Sickle cell anemia is a recessive inherited disorder that changes the shape of red blood cells, making them sickle shaped like these ones. This means that they cannot carry as much oxygen. Let’s see how this example can show codominant inheritance of alleles.

One parent does not have sickle cell anemia and has biconcave red blood cells. The alleles in their genotype will be represented as HbA HbA. Hb stands for hemoglobin, which is the pigment found in red blood cells. The superscript A stands for hemoglobin A, which shows that these red blood cells are their normal biconcave shape. The other parent does have sickle cell anemia, and their genotype is represented as HbS HbS. This time, the superscript S shows that these red blood cells contain hemoglobin S, which functions very differently to hemoglobin A and makes the red blood cells sickle shaped.

As both parents are homozygous for their respective traits, the gametes of the parent without sickle cell anemia would both be HbA and the gametes of the parent with sickle cell anemia would both be HbS. So the offspring produced by this cross would inherit one of each allele from their parents and have a heterozygous genotype HbA HbS. The phenotype would consist of a mix of sickle-shaped and biconcave red blood cells, and this is known as the sickle cell trait. This shows codominance as they have both types of red blood cell in their body as both of the alleles are expressed in their phenotype.

Let’s look at one more example of how lack of dominance, specifically codominance with multiple alleles, can affect the inheritance of traits. Multiple alleles means that there are three or more possible alleles for a trait. But as usual in diploid organisms like humans, only two will be present in a single person’s genotype. One example of multiple alleles showing codominance is in human A, B, O blood groups. Humans have three alleles that determine their blood group, IA, IB, and IO, as we can see in the table here.

Each person will inherit two alleles that determine their blood group, one from their biological mother and the other from their biological father. If a person has two IA alleles in their genotype, this means that their red blood cells all have type A antigens on their surface. Antigens are molecules that are present on the cell surface membrane of all cells, and they can distinguish different cells from each other, such as self-cells which belong to your body and non-self-cells, which might belong to a disease causing microorganism.

If a person has two IB alleles in their genotype, it means that their red blood cells all have type B antigens on their surface. If a person has the alleles IA and IB, they will have both type A and type B antigens on their red blood cells. As both the alleles are expressed in the phenotype, the inheritance of IA and IB alleles is an example of codominance. The allele IO indicates that there are neither type A nor type B antigens on the surface of the red blood cells, like this one here.

Let’s use this information to work out what blood types to fill in to the table. If the genotype contains two IA alleles, the person will have blood group A. And if they inherit one IA and one IO allele, type A antigens will still be present on the red blood cells. So this person will also have blood group A. This is an example of the IA allele showing complete dominance of the IO allele. If both their alleles are IB or if they inherit an IB and an IO allele from their parents, this person will have blood group B as the IB allele also shows complete dominance over the IO allele. If both the alleles are IO, the person will have blood group O as no other alleles are masking the O allele.

Codominance only comes into play if a person inherits one IA allele from one parent and one IB allele from the other parent. A person with a heterozygous genotype IA IB would have the phenotype blood group AB. In this case, both alleles are present in the phenotype, so they’re demonstrating codominance. Let’s have a go at some practice questions to test what we’ve learned about lack of dominance.

Snapdragon flowers (pictured) show incomplete dominance in the color of their petals. The petals can be red, CR CR white, CW CW, or pink, CR CW. Two pink flowers are crossed. What is the probability, in percent, that their offspring will also have pink flowers?

Let’s start by annotating our diagram with the information given to us in the question. We are told that the genotype of red snapdragons is CR CR. The C means color and the R means red. Remember, the set of alleles an organism possesses for a characteristic is their genotype. We are also told that the genotype of white snapdragon flowers is CW CW, where this time the superscript W means white. Finally, we know that the genotype of pink snapdragon flowers is CR CW.

The phenotype is the observable trait that is produced by the genotype. In these cases, the phenotype of each individual plant is its flower color, red, white, or pink. The question states that the inheritance of flower color in snapdragons shows incomplete dominance. Allele show in complete dominance when an intermediate phenotype is created in an organism that is heterozygous for a particular trait, which means that they have one of each allele. In this example, the intermediate phenotype is the pink flower color, as it is a blend between the white and red flowers and has a heterozygous genotype.

Let’s draw a Punnett square to work out the percentage of offspring likely to be pink when these two parent plants with pink flowers reproduce. Punnett squares like this one are used to visually represent how alleles are inherited and predict the genotypes and phenotypes of the offspring produced by crossing parents with known genotypes. We know that pink snapdragons have the genotype CR CW. We put the alleles that will be present in one parent’s gametes in the top row and we put the alleles that will be present in the other parent’s gametes in the left column.

Now we need to fill in the blank cells with the potential offspring genotypes that are able to be produced from a cross between these two pink parents. We do this by taking the alleles in the gametes from each row and column head, for example, the CR allele from this parent and the CR allele from this other parent. Let’s do this for the rest of the cells. We can see in the Punnett square that one out of four of the offspring has the genotype CR CR and so has a red-flowered phenotype. We can also see that one out of four of the offspring has the genotype CW CW and so a white-flowered phenotype. Finally, two out of the four offspring produced in this cross have the heterozygous genotype CR CW and so have a pink-flowered phenotype.

The question asks us to determine the probability in percent that the offspring produced by this cross will have pink flowers. So let’s do this by converting this value into a percentage by multiplying it by 100 percent. Therefore, the probability that the offspring produced by two pink snapdragons will also be pink is 50 percent.

Here are some of the key points that we’ve addressed in this video.

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