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Definition of phenotype expression
An organism's phenotype refers to the organism's physical appearance (physical traits) as well as the type of proteins expressed within the organism's 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 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.
Sickle cell disease results from a mutation in a gene responsible for encoding the haemoglobin molecule found in red blood cells. Glutamate is needed to form a normal haemoglobin molecule. This leads to the formation of a healthy round red blood cell. In the case of sickle cell anaemia, a mutation causes glutamate to be changed into valine. This change leads to the production of a sickle cell, which has a different shape (that of a sickle), since the haemoglobin molecules become sticky.
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.
An allele is one of the variants of a gene, i.e. one of the specific DNA sequences that appear at a certain position on a chromosome. In humans, each gene has two alleles because we have a pair of each chromosome (except for the sex chromosomes X and Y). The alleles can be the same or different in each chromosome.
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. From this, you could already guess that the brown eye allele is dominant, while the green eye allele is recessive. Hence there are four genetic combinations to code for eye colour in this example.
| X | x | |
| X | XX | Xx |
| x | Xx | xx |
Example of phenotypic expression
Let's take a look at another important example of how genotypic
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.
The immune system is the body's defence system against invading pathogens and chemicals. your immune system recognizes infectious pathogens and kills them before they cause bodily harm, but it can also recognise other things. The immune system is made up of antibodies and specialized cells like macrophages, dendritic cells, and T-cells. These cells work together to neutralize and kill pathogens before they spread to healthy cells.
Following this logic, 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 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 IA allele and an IB 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.
Antibodies are specialized proteins that target and bind to specific proteins called antigens. Antibodies are crucial players in the immune system as they bind to antigens found on the surface of pathogens and help destroy them.
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.
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.
Rh Factor: as with the blood type proteins, the Rh factor is a protein that can be found on the surface of red blood cells.
Phenotypic Expression - Key takeaways
- An organism's phenotype refers to the organism's physical appearance as well as the type of proteins expressed within the organism's body.
- Variations in gene expression lead to altered proteins being produced which lead 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. 2021An organisms______ refers to the organisms physical appearance as well as the type of proteins expressed within the organisms body.
Phenotype
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Frequently Asked Questions about Phenotypic Expression
How are genes expressed in the phenotype?
Genes are expressed in the phenotype through transcription and translation. Each gene has two copies of itself in a person's genome, which are called alleles. Alleles are either dominant or recessive. A person can express the dominant allele having both dominant homozygous and heterozygous dominant. In order for a recessive allele trait to be expressed the organisms must have two recessive alleles.
What is a phenotype in which both alleles are expressed?
A phenotype in which both alleles are expressed is a codominant one (meaning that both alleles coding for the trait are dominant).
How is the phenotype of a quantitative trait expressed?
The phenotype of a quantitative trait depends on the expression of many genes, which each add to the amount of expression of the trait.
How do genes and environment affect phenotypic expression?
Genes have multiple variants (alleles). The combination of these variants will condition the phenotype. At the same time, the environment can influence epigenetic changes to the DNA where the genes are, regulating which gene copy is expressed, or if the gene is expressed at all, again influencing the final phenotype.
When are recessive alleles expressed phenotypically?
Recessive alleles are only expressed phenotypically when an organism has two recessive alleles and no dominant alleles.
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