A gene is a unit of inheritance of us while the allele is an alternative form of it.
Confused? Don’t worry after reading this article your fundamentals of genetics becomes stronger and your concept of gene vs allele becomes more clear.
So let’s start with some basics,
Genes are the functional part of the DNA- a polynucleotide chain.
A DNA is made up of the phosphate, sugar and bases, the bases are nitrogenous mainly purines and pyrimidines.
Two single-stranded DNA joins together by the hydrogen bonds (three between G and C and two between A and C).
Functionally, DNA is either coding sequences or non-coding DNA in which the proteins are encoded by the coding sequences while the gene expression is maintained by the non-coding DNA sequence.
Those coding sequences are our genes.
If you want to learn more about gene 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 colour, a gene for hair colour, 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 plays an important role in the development of eye colour along with HERC2 gene.
Thus, the OCA2 is a gene responsible for the development of eye colour while the blue eye, red eye, black eye are different alleles for it.
A slight variation in a gene sequence called mutation or alteration originates different alleles for a particular 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, genes are inherited as a single entity while alleles in a pair.
Interestingly, more than one genes are responsible for the production of a single protein and more than one proteins can be encoded by a single gene.
The gene is responsible for a particular trait while the alleles are responsible for the variation in that particular trait.
We will take the example of the OCA2 gene for eye colour with us throughout the article.
For example, the OCA2 gene is responsible for the production of eye colour trait while blue eye, red eye, black eye are variation occurs due to different alleles.
Some other examples of gene and alleles are,
|Eye colour||A black eye, red eye, blue eye or green eye|
|Hair colour||Black hair, blonde hair, brown hair|
|Blood group (ABO)||AA, AB, BB, OB, OA, OO etc|
|Hight||Short height or long height|
A gene comprised of two different alleles while the alleles can be dominant or recessive.
Alleles inherited in a pair one from father and one from mother if two dominant alleles inherited together the condition is called homozygous dominant contrary to this, if two recessive alleles inherited together the condition is called homozygous recessive alleles.
If one dominant and one recessive allele inherited together the condition is known as heterozygous.
Again let’s take an example of eye colour,
OCA2 is a gene for eye colour and OCA2a, OCA2b, OCA2c and OCA2d are different alleles for different shades of eye colour.
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 homozygous dominant condition which inherited the brown-eye trait in the offsprings.
On the other side, OCA2b allele is for green eye, when the OCA2 gene carries two OCA2b/OCA2b allele 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 form 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, total 46 chromosome- 23 pairs of it- are present in a somatic cell.
The germ cells (egg or sperm) contain an only haploid set of it i.e. only 23.
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.
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 is originated due to different combination of alleles.
While the genetic constitution related to the phenotype or trait is called genotype which creates a gene for a trait.
Another difference between gene vs allele is the prevalence,
Genes are present in almost all known organism, for example, several metabolic- enzyme coding genes are present in all organism but in some organism it expresses and in some it does not.
On the other side, not all the alleles are present in all organism, for example, the blue eye allele is present in one particular population but not in other.
The dark skin colour 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 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 colour.
(this is just an example, not the exact mechanism)
Homozygous dominant alleles TRS1/TRS1 produce 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 colour commonly observed in the population living in the lower temperature areas.
Those two conditions are wild type and called wild type alleles in an individual population.
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 skin from harmful sun rays are less in fair skin population, thus the TRS2 allele is 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.
- Gene encodes for a particular protein while alleles produce different phenotypes related to it.
- Gene is an individual unit of one trait while the alleles occur in pair.
- Genes create an organism while the alleles create variations in it and because of different alleles, we are different from each other.
- The genes are present in all known organism while not all the alleles are present in all the organism
- Genes govern a trait of a group of trait 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 mutated under different environmental conditions and new alleles are originated.
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 future.
Nature creates new variation 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.