A gene is an inheritance unit of us while the allele is an alternative form of it.
Confused? Don’t worry after reading this article your fundamentals of genetics and related terminologies become stronger and your concept of gene vs allele becomes more clear.
So let’s start with some basics,
Gene vs Allele:
Genes are the functional part of the DNA- a polynucleotide chain.
DNA is made up of the phosphate, sugar and bases, the bases are nitrogenous mainly purines and pyrimidines. Two single-stranded DNA joined together by the hydrogen bonds (three between G and C and two between A and C) and arranged spirally, see the image below.
There are two types of DNA present in our genome. The coding sequences make proteins that are 3% portion while the non-coding sequences are just junk and can’t form proteins. It covers around 97% portion of a genome and functionally it regulates gene expression.
Those coding sequences are our genes.
If you want to learn more about genes and DNA please read our previous article: DNA vs Gene.
The elaborated definition of a gene is as stated,
“A gene is a polynucleotide chain of DNA- a functional portion- having introns and exons, encodes protein or group of proteins via mRNA transcript.”
For example, a gene for eye color, a gene for hair color, a gene for height, etc.
Now, this sounds more specific and scientific.
On the other side, the alleles are the alternative forms of a gene.
The alternative forms are two or more than two, for example, the OCA2 gene is located on chromosome number 15 and plays an important role in the development of eye color along with HERC2 gene.
Therefore, the eye color is developed from the activity of the OCA2 gene while various shades of it like a blue eye, black eye, and red eye are developed by different alleles of it.
Mutation- a small variation in a sequence of DNA is the reason to originate different alleles for a gene. Related article: Different Types Of Genetic Mutations.
Though more than two alleles can be possible for one gene, alleles can only be inherited in a pair.
Thus, we can say, genes are inherited as a single entity while alleles in a pair.
Interestingly, more than one gene is responsible for the production of a single protein and more than one protein can be encoded by a single gene.
The gene is responsible for a particular trait while the alleles are responsible for variations in that particular trait.
We will take the example of the OCA2 gene for eye color with us throughout the article.
For example, the OCA2 gene is responsible for the production of eye color traits while blue eye, red eye, black eye, are variation occurs due to different alleles.
Some other examples of gene and alleles are,
|Eye color||A black eye, red eye, blue eye or green eye|
|Hair color||Black hair, blonde hair, brown hair|
|Blood group (ABO)||AA, AB, BB, OB, OA, OO etc|
|Hight||Short height or long height|
A gene comprises two different alleles while the allele can be either dominant or recessive.
Alleles inherited in a pair one from father and one from mother if two dominant alleles inherited together the condition is known as homozygous dominant contrary to this if two recessive alleles inherited together the condition is referred to as homozygous recessive.
If one dominant and one recessive allele inherited together the condition is known as heterozygous.
Again let’s take an example of eye color,
OCA2 is a gene for eye color and OCA2a, OCA2b, OCA2c and OCA2d are different alleles for different shades of eye color.
Suppose the OCA2a allele is for the brown eye while the OCA2b allele is for the green shade eye.
Once the gene OCA2 inherited with the two OCA2a alleles (OCA2a/OCA2a), it is called a homozygous dominant condition that inherited the brown-eye trait in the offspring.
On the other side, the OCA2b allele is for green eyes, when the OCA2 gene carries two OCA2b/OCA2b alleles it inherited the green eye trait called autosomal recessive condition.
But when both the allele OCA2a and OCA2b are inherited together, it inherits the only brown eye trait (OCA2a/OCA2b) called heterozygous dominant alleles.
Here, the OCA2a/OCA2a are the homozygous dominant alleles, OCA2b/OCA2b are homozygous recessive alleles and OCA2a/OCA2b are heterozygous.
These are the alternative forms of the gene OCA2. Now you understand the difference between gene vs allele.
Genes are located on the chromosome and so the alleles are!
The gene OCA2 is located on chromosome 15 which means one allele for gene OCA2 is located on one of the chromosome 15 while the other allele is located on another chromosome 15.
Because the chromosomes are present in a pair, a total of 46 chromosomes- 23 pairs of it- are present in a somatic cell. The germ cells (egg or sperm) contain only a haploid set of it i.e. only 23 chromosomes.
One set of chromosomes from father and one set of chromosomes from mother are inherited in the offspring which means one allele from father and one allele from mother inherited to offspring.
However, the dominant effect of the alleles are totally unknown, it depends on the environmental and other factors, that, which allele becomes dominant and which becomes recessive.
A phenotype type is an observable form of the trait governed by different alleles, different phenotypes of a particular trait originate due to different combinations of alleles.
While the genetic constitution related to the phenotype or trait is called genotype which creates a gene for a trait. You can read our article on Genotype vs phenotype here.
Another difference between gene vs allele is the prevalence,
Genes are present in almost all known organisms, for example, several metabolic- enzyme coding genes are present in all organisms but in some organisms it expresses, and in some, it does not.
On the other side, not all the alleles are present in all organisms, for example, the blue eye allele is present in one particular population but not in others.
The dark skin color allele is commonly present in the populations living in extreme heat while that allele is not present in the population living in cold places.
Wild type allele vs mutant allele:
A phenotype related to the allele which is present normally in the entire population is called a wild type allele while the new allele or harmful allele which creates an entirely new variation in the population is called a mutant allele.
Now this is very interesting, for some the wild type allele might be mutant allele or for some, the mutant allele is wild type.
Let’s understand it by taking an example, the TRS gene encodes a protein called tyrosinase which is majorly responsible for the human skin color.
(this is just an example, not the exact mechanism)
Homozygous dominant alleles TRS1/TRS1 produces a dark skin phenotype which is very essential for the population living in the high-temperature area.
While the homozygous recessive alleles TRS2/TRS2 produce a fair skin color commonly observed in the population living in the lower temperature areas.
Those two conditions are wild type and called wild type alleles in individual populations.
But if Alleles TRS2/TRS2 found in some individuals living at a higher temperature, it may suffer from skin damage or skin cancer because the melanin which protects the skin from harmful sun rays are less in fair skin population, thus the TRS2 allele is the mutant allele for a population living at a higher temperature.
In addition to this, some phenotypes are governed by multiple alleles while some traits are governed by multiple genes.
The best example of multiple alleles is the ABO blood group system, but I think we will understand the genetics of the ABO blood group system in some other article.
Summary of the article:
- Gene is a functional piece of DNA for a specific trait while alleles are a different variation of a gene.
- A gene makes a protein while alleles produce different phenotypes related to it.
- Gene is a single unit for one trait while the alleles occur in pairs.
- The genes are present in all known organism while not all the alleles are present in all the organism
- Genes govern a trait or a group of traits while the alleles produce different phenotypes for different traits.
Allelic variation is required for the origin of new phenotype and thus for survival of us. Over a period of time, different genes mutate under different environmental conditions, and new alleles are created.
Some mutations or changes are harmful but some are beneficial and help us to live. Even, some mutations are harmful temporarily but can be helpful in the future.
Nature creates new variations in genes, different alleles are originated and unwanted alleles are eliminated. Each new allele gives a new power to us for survival, after all, the aim of doing all this is to survive on earth.