Phenotypic Expression

When an organism develops inside the womb there are certain genes expressed in order to create that specific organism with its particular characteristics. Have you ever wondered why every person does not look the same? Some people may have straight hair while others may have curly hair. Likewise, some people may have blue eyes while others have brown eyes. These differences in appearance are due to variations in gene expression. The genes that are present within an organism's DNA are referred to as the organism's genotype while the organism's physical appearance is called the phenotype. There are many different factors that can affect the phenotype of an organism. Let's take a closer look.

Definition of phenotype expression

A cat can have black fur while another can have white fur. These cats do not have the same fur phenotype because they do not have the same colour fur. An organism's phenotype can sometimes be determined based on the parent's genotype using a Punnett square.

An organism's phenotypic expression refers to the observable characteristics in an organism that results from the expression of genes. Variations in gene expression lead to altered proteins being produced which lead to changes in physical and structural appearance. Mutations within the genome can also influence an organism's phenotypic expression. This can be seen in the case of sickle cell anaemia.

Phenotypic expression of genes

Humans, just like all other eukaryotic organisms, have physical characteristics that help to identify them. These characteristics may be having blond hair, curly hair, or straight hair. A person's physical characteristics are based on their gene expression and the way that their genes are expressed.

Within the human genome, some alleles are dominant while other alleles are recessive. Dominant alleles are alleles that express themselves even if there is just one copy of them, while recessive alleles need two copies to be expressed. We know that an organism has half its alleles coming from its mom while the other half comes from its dad (i.e. one copy of the gene comes from each parent). As an offspring is growing inside the mother's womb, its genes are expressed in different combinations. If you would like to know what the offspring's eyes would be, you could use a Punnett square to map out all the given possibilities depending on the parent's genes. Let's take a look at an example below.

Example of phenotypic expression

Let's take a look at another important example of how genotypic expression affects a person's phenotypic blood type. You may know from previous classes that there are a few different blood types depending on a person's genes. A person's blood type depends on the proteins present in their blood cells and the antibodies in their plasma. The proteins expressed on a red blood cell are determined by alleles at the I locus.¹ These alleles are I^A I^B and I^O.¹ We know from the previous section that some alleles are dominant while others are recessive. The dominant alleles in the case of determining blood type are both I^A and I^B, while I^O is the recessive allele.¹ To simplify, we say that blood types are A, B, O, or AB.

People with one I^A allele and one I^o allele will still have the A blood type because I^A is the dominant allele.¹ People with A antigens on their blood cells must have anti-B antibodies in their plasma. If they were to have anti-A antibodies, their immune system would attack their blood cells and the person would not be able to survive. This is why in the case of transfusions, people must receive a blood type that does not have antibodies that kill their blood antigens.

Following this logic, you can see that a person with B blood type will either have I^B and I^O alleles or will have two I^B alleles.¹ These genotypes cause B antigens to be expressed on the person's blood cell. As a result, this person must have anti-A antibodies to survive. What is interesting is that a person can have both dominant alleles which produce a double dominant phenotype: a person with AB blood type will have an I^A allele and an I^B allele.¹ A person with these genotypes has both A and B antigens on their blood cells. Therefore, people with this blood type do not have any anti-A or anti-B antibodies in their plasma.¹ As a result, they are the universal recipient. This means that people with AB blood can receive blood from any blood type because their plasma does not have any antibodies against blood cells.

The final blood type we will be discussing is type O. People with type O blood have two recessive I^O alleles causing them to have no antigens present on their blood cells.¹ Since people with O blood type do not have any antigens on their blood cells, they have both anti-A and anti-B antibodies in their plasma.¹ Having no antigens on their blood cells is what makes people with type O blood the universal donor. This means that they can donate their blood to anybody.

There is another factor that comes into play when establishing a person's blood type: the Rh factor. It is what determines if we are "positive" or "negative", i.e. A⁺, O^-, etc. We will not go into much detail here, but the logic is the same as with all phenotypic traits: the dominant allele will be the one expressed in the case of a heterozygous person. The dominant allele for the Rh factor is the positive one.

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