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
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.
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. This is why the brown eye phenotype is denoted by X (capital) while the green eye allele is denoted by x.Now that you know this information, can you determine the likelihood that the child will have green eyes? Create a Punnett square to help you track the possible combinations of alleles of the offspring.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, Xx. For the child to have green eyes, they would have to be homozygous recessive. This only happens on one occasion in the different combinations, hence: 1/4.Can the homozygous dominant offspring ever produce an offspring with green eyes?As mentioned above, for the child to have green eyes they would have to be homozygous recessive, xx. If both parents are homozygous dominant, XX, this combination would be impossible, so 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 one. 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 IA IB and IO.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 IA and IB, while IO is the recessive allele.1 To simplify, we say that blood types are A, B, O, or AB.
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. 2021
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