What Is a Recessive Gene? Why Are Some Genes Recessive and Some Genes Dominant?
What is a recessive gene? Did you know that there are different types of genes?
Before asking more questions and confusing you, let’s first get some terms out of the way.
Genes are sort of the basic building blocks of life. I say “sort of” because “gene” is a generalized term that stands for “the sequence of DNA that is the chemical basis for heredity.” So, in yet other words, genes are the sequences of DNA that determine all the outward characteristics that are visible. This is by virtue of making proteins that are the foundations for all those outward traits. Genes are located on chromosomes, half of which are inherited from the mother and the other half from the father. This means that each person’s cell contains half the genetic information from one parent and the other half from the other parent. Furthermore, this means that each individual contains two copies of each gene within their genome, which is also known as the entire gene pool that makes them who they are. The proteins that are made from the genes are created based on both existing gene copies. Once the proteins are made, the cell typically cannot tell from which gene copy that protein came from.
But before we get into too many details, let’s get back to the concept for recessive vs. dominant genes. And this is where things get a bit more complicated. Typically, when a scientist uses the terms recessive versus dominant to refer to a gene, it means that they are talking about variations of a gene (also called alleles). Alleles are different sequences of the same gene that affect the function of the protein that they code for.
In short, a dominant allele becomes a trait even if just one copy of it is present. A recessive allele does not become a trait unless both copies of the gene, one from mom and one from dad, are present. If one dominant allele and one recessive allele are present, the dominant allele trait will be expressed.
But you might still be asking yourself why that is, which is totally OK. It actually comes down to the protein level of things. In order for you to completely understand things, let’s define the term “loss of function.” It is simply what it says it is: losing a function. In terms of traits, it means a protein’s function is lost and the trait is not expressed. This is especially true for eye color, whereby brown eyes are a dominant trait and blue eyes are a recessive trait. So, when one brown eye allele and one blue eye allele are present together, the individual will have brown eyes.
In terms of proteins, that simply means the following: recessive alleles are the result of a loss of function in the protein. This means that there is a mutation in the allele itself that stops the protein from being expressed. This loss of function could be caused by something as simple as the swapping of an important amino acid or it could be something as complicated as a mutation that stops the coding and subsequently the expression of the protein completely. But, let’s get back to the example of eye colors. In the case of the blue eye allele, the protein that it codes for is not actually the ‘blue eye protein’ but rather a brown eye protein that has not be expressed.
Now if you are completely thrown for a loop, think of it this way. Brown eyes are brown because of the pigment in them, namely melanin. The brown eye allele codes for the expression of the pigment, melanin. In the case of a loss of function of the brown eye allele, there is no pigment expression, which means that there is no pigment whatsoever and the result will be a blue eye. So, if you think about this in terms of dominant and recessive and everything we have covered so far, it makes sense that the dominant allele will be expressed even if just one is present. This is because that allele causes the melanin to be made which is visible even if the other allele is recessive as that allele stops the melanin from being made. However, if there is no dominant allele and there are only two recessive ones present, there is now the complete loss of function of protein or complete lack of melanin creation, and the ultimate result is a blue eye.
As if this alone was not complicated enough as it is, there are other types of dominance within alleles. There are things such as incomplete dominance when two traits that are inherited from the parents share dominance and that is manifested in the offspring. This is best evident in certain plants that give mixed colored offspring whereby a red flower and a white flower create pink flowers. There’s also codominance, in which case a red plant and a white plant give rise to splotchy offspring (both colors are expressed). There is also mixed dominance, which is mostly seen in blood types, whereby a parent of blood type A and a parent of blood type B create offspring that have the AB blood type.
So, while biology has a way of seeming complicated, things get very simple once the precise details are teased apart and all the pathways are clarified properly. However, at a broader level, things do seem complicated. But it is not because they are actually complicated. Rather, they are complex and there are different parts of information that all must be covered in order for the whole thing to make sense—just like recessive and dominant genes. In short, when it comes to genes, things are not always as they seem on the surface.