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Phenotypic Expression

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Phenotypic Expression

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

Meaning of phenotypic expression

An organisms phenotype refers to the organisms physical appearance as well as the type of proteins expressed within the organisms body. 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 color fur. An organisms phenotype can be determined based on the parent's genotype using a Punnett square.

Definition of phenotype expression

An organisms phenotypic expression refers to the observable characteristics in an organism which results from the expression of genes. Variations in gene expression leads to altered proteins being produced which leads to changes in physical and structural appearance. Mutations within the genome can also influence an organisms phenotypic expression. This can be seen in the case of sickle cell anemia. The sickle cell disease results from a mutation on a gene responsible for encoding the hemoglobin molecule found on red blood cells. Within the gene, glutamate is needed to form a normal hemoglobin molecule. This leads to the formation of a healthy round red blood cell. In the case of sickle cell anemia, a mutation causes glutamine to be changed into valine. This change leads to the production of a sickle cell since the hemoglobin molecules become sticky (See Figure 1).

Phenotypic expression of genes

Humans, just like all other eukaryotic organisms, have physical characteristics that help 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 genes are dominant while other genes are characterized as recessive. Dominant genes are genes that express themselves more strongly than recessive genes. We know that an organism has half their genes coming from their mom while the other half comes from their dad. As an offspring is growing inside the mothers womb, their genes are being 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 parents genes. Let's take a look at an example below.

An expecting mother is wondering what the possibilities are that her child has green eyes. Both the mother and father have brown eyes however the expecting mother's dad has green eyes while the expecting father's mother also has green eyes.

Xx
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xXxxx

In the case of the example, the expecting mother and the expecting father are both heterozygous dominant. This means that they both possess one allele that corresponds to the brown eye phenotype while the other allele corresponds to the green eye phenotype. In the case of eye color, the brown eye allele is expressed more strongly than the green eye allele. This is why the brown eye phenotype is denoted by X while the green aye allele is denoted by x. Now that you know this information, can you determine the likelihood that the child will have green eyes? If you said 1 in 4 you would be absolutely correct! Notice the different combinations of alleles. There is one homozygous dominant potential offspring denoted by XX while the homozygous recessive is denoted by xx. There are also two heterozygous dominant potential offspring. Can the homozygous dominant offspring ever produce an offspring with green eyes? The answer is no. A homozygous dominant offspring can never produce an offspring with green eyes because it has two dominant alleles. Even if it were to reproduce with a homozygous recessive mate, the recessive phenotype will never be present because the dominant allele will always be expressed stronger than the recessive.

Allele: Two of more alternative forms of a gene.

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 on their blood cells and the antibodies in their plasma. The proteins expresses on a red blood cell is determined by alleles at the I locus.1 These alleles are IA IB and IO.1 We know from the previous section that some genes are dominant while others are recessive. The dominant alleles in the case of determining blood type are both IA and IB while IO is the recessive allele.1 Let's look at Figure 2 to see how the process works.

In Figure 2 you see how a person's genotype affects their blood type. People with one IA allele and one Io allele will still have the A blood type because IA 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.

Moving on in figure 2 you can see that a person with B blood type will either have IB and IO alleles or will have two IB 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 in order to survive. What is interesting is that a person can have both dominant alleles which produces a double dominant phenotype. In figure 2 we see that a person with AB blood type will have an IA allele and an IB allele.1 A person with these genotypes have both A and B antigens on their blood cells. People with this blood type do not have any 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.

The final blood type we will be discussing is type O. People with type O blood have two recessive IO 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.

Phenotypic expression of traits

There is another factor that comes into play when establishing a person's blood type is the RH factor.

RH Factor: An inherited protein that can be found on the surface of red blood cells

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Phenotypic Expression - Key takeaways

    • An organisms phenotype refers to the organisms physical appearance as well as the type of proteins expressed within the organisms body.
    • Variations in gene expression leads to altered proteins being produced which leads to changes in physical and structural appearance.
    • People with this blood type do not have any antibodies in their plasma. As a result, they are the universal recipient.
    • A person's blood type is considered RH positive (Rh+) when they do have the expressed D antigen on their red blood cells.

References:

1. Art of Smart. Phenotype and Gene Expression. 2021

Frequently Asked Questions about Phenotypic Expression

Genes are either dominant or recessive. A person can express the dominant gene having both dominant homozygous and heterozygous dominant. In order for a recessive gene trait to be expressed the organisms must have two recessive genes. 

A phenotype in which both alleles are expressed is the recessive phenotype. 

The phenotype can be quantitatively measures using a punnett square. 

Genes influence which proteins are expressed which thereby influences the organisms phenotype. a person's gene expression can be changed based on their environment in order to promote survival of the organism which means that an organism's environment can influence their phenotype. 

Recessive alleles are only expressed phenotypically when an organism has two recessive alleles and no dominant alleles.   

Final Phenotypic Expression Quiz

Question

An organisms______ refers to the organisms physical appearance as well as the type of proteins expressed within the organisms body.

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Answer

Phenotype

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Question

An organisms phenotype can be determined based on the parent's genotype using a punnet square.

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Answer

True 

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Question

If a mother has dominant XX alleles (brown eyes) while the father has recessive xx alleles (green eyes), what is the likelihood that the offspring will have green eyes?

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Answer

1/4

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Question

Variations in gene expression leads to altered proteins being produced which leads to changes in physical and structural appearance.

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Answer

True

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Question

Mutations within the genome can not influence an organisms phenotypic expression. 

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Answer

False

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Question

Within the human genome, some genes are dominant while other genes are characterized as recessive.

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Answer

True

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Question

______ genes are genes that express themselves more strongly.

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Answer

Dominant

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Question

Two of more alternative forms of a gene is a ______.

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Answer

Allele

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Question

A person's blood type depends on the proteins present on their blood cells and the antibodies in their plasma.

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Answer

True

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Question

In the case of blood phenotype the dominant alleles in the case of determining blood type are both IA and Iwhile IO is the recessive allele.

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Answer

True

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Question

People with A antigens on their blood cells must have anti-____ antibodies in their plasma.

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Answer

B

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Question

A person with B blood type will either have IB and IO alleles or will have two IB alleles.

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Answer

True

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Question

A person with AB blood type will have an Iallele and an IB allele.

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Answer

True 

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Question

Which blood type is called the universal recipient?

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Answer

AB

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Question

Which blood type is the universal donor?

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Answer

O

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