Is A Chromosome An Allele: 5 Facts You Should Know


To answer the question ” Is a chromosome an allele?”, chromosomes and alleles are related to each other in terms of genetic material.

Chromosomes can likely be called a collection of alleles. A single chromosome can house up to several genes or alleles along its length. Alleles are pieces of genetic information found on a chromosome.

Are alleles and chromosomes the same?

No alleles and chromosomes are not exactly the same, even though they are relatable. Alleles are genes found on a chromosome pair, at the same location to give rise to a specific genetic character, a single chromosome can have more than 2000 of these alleles each.

Structure of a condensed chromosome in metaphase showing (1)Chromatid, (2) Centromere, (3) Short arm and (4) Long arm
Image: Wikipedia

A diploid organism has 2n chromosomes or n pairs, with each having specific alleles in the same location for all the chromosomes. The two matching genes, or alleles, on a pair of chromosomes in a diploid organism may be the same or may have distinct base sequences. In each pair, one allele comes from each parent.

 Some phenotypes, which are the physical manifestation of genetic information, need the interaction of multiple genes, which complicates the interactions between alleles.

How are chromosomes and alleles related?

Alleles can be considered as a part of the chromosomal structure. Alleles are basically genes that are found in a chromosome, and when they occur in a chromosomal pair in the same location they produce a specific trait in the respective organism.

A single chromosome can contain more than 2000 alleles or more, responsible for hair colour, eye colour, hair texture, blood group and all characteristics that are genetically passed on from parents to their children. A trait or quality that is obvious is referred to as a “phenotype.”

ABO blood typing and all their respective alleles
Image: Wikipedia

Both alleles make up a genotype if there is only one gene responsible for the trait. Thus, the collection of alleles that code for each trait in an organism is referred to as its genotype. The genotype of a specific gene can be homozygous or heterozygous depending on the DNA sequence of each allele.

Where an allele is found on a chromosome?

Alleles are varying pairs of genes found in an organism, that are found along the entire chromosomal length in the DNA. A single chromosome has several hundred genes that code for different proteins. Each gene can have more than one allele located in the same locus.

23 chromosome pairs in a normal human male
Image: Wikipedia

Each chromosomal pair has a unique set of genes coding specific proteins located at a specific locus on the chromosome. Now alleles coming from two parents may be the same or different. If they are the same the alleles are homozygous, and if not they are heterozygous.

How many alleles of a gene are present in each chromosome?

A single chromosome can have over 2000 alleles. A single chromosome has a high number of genes that code for various proteins, and each gene is made up of alleles coming from the respective parents in every chromosomal pair.

Hence each chromosome also has the same number of alleles as the number of protein-coding genes. A single chromosome is can go in length from 1 to over 20 micrometres during the metaphase stage of prophase.

Types of alleles:

Alleles are mainly classified into three main types depending on if they are expressed in heterozygous conditions or not.

Dominant allele:

These alleles are expressed in both homozygous and heterozygous conditions. This means this allele would express its protein if one parent passes on the allele or both parents pass it on. For example height, eye colour etc

Recessive allele:

This type of allele is expressed only if both parents pass it on to their offspring. So simply put recessive alleles can only express their protein when in a homozygous condition. For example disorders like genetically transmitted thalassemia, sickle cell anaemia, or rare eye colours like green and blue.

Codominant alleles:

 Some specific alleles are able to coexist in a heterozygous setting. This means the dominant allele does not completely mask the recessive allele but they both express themselves and bring about a result somewhere in between. But this is a rare type of phenomenon that occurs only for some specific alleles in a handful of organisms. For example ABO blood codominance in humans.

is-a-chromosome-an-allele
Codominant alleles of ABO blood typing
Image: Wikipedia

Difference between an allele and a chromosome:

Differences between an allele and a chromosome include:

  Point of difference  Allele  Chromosome
DefinitionAlleles are naturally occurring gene variants that can give rise to phenotypic or genotypic characters in organismsChromosomes are threads of DNA that are tightly coiled acting as genetic material.
OccurrenceGenes and alleles are found along the entire length of the chromosome.Chromosomes are usually paired in a diploid organism and are found in the nucleus
FunctionTwo alleles located on the paired chromosomes in the same location give rise to a specific trait in an organism. This can include outwardly seen traits or other genetic features.Chromosomes are condensed DNA carrying all the required information from one generation to the next.
Usual numberA single chromosome can contain 2000 or more genetic alleles.Humans have the highest chromosome number where each cell contains 23 pairs i.e. 46 chromosomes
Table of comparison between alleles and chromosomes

Conclusion:

So a chromosome and an allele are not the same, but they are closely related where they closely function together in genetic propagation. While one is a block the other is a chain that carries everything together.

Trisha Dey

I am Trisha Dey, a postgraduate in Bioinformatics. I pursued my graduate degree in Biochemistry. I love reading .I also have a passion for learning new languages. Let’s connect through linked in: https://www.linkedin.com/in/trisha-dey-183482199

Recent Posts