“In a homozygous condition, a cell contains two same types of alleles for a gene while in a heterozygous condition a cell contains two different alleles for a gene.” 


The structure of DNA is the same in all organisms on earth having DNA as their genetic material, made up of pentose sugar, nitrogenous bases, and trisphosphate. 

The combination of triplets we know it as ‘nucleotides‘, is a monomer unit of DNA. Different organisms have different arrangements of sequences or we can say sequence variations. Due to those variations, we are different from each other. 

The study of DNA, genes, and chromosomes are covered in genetic science, however, terminologies used in it makes it hard to understand. Some of them are gene, DNA, allele, chromatin, homozygous, heterozygous, nullizygous, dominant and recessive etc. 

In the present article, I will try to make you understand two of the common, important confusing terminologies commonly used in genetics, especially in clinical genetics. That is Homozygous and heterozygous. 

I will explain it and also give you some differences between homozygous vs heterozygous so that students can understand it better! 

Related article: Gene vs Allele.

Homozygous vs heterozygous: 

The condition is said to be homozygous when two identical or same types of alleles are inherited together. 

Which means that two similar alleles are involved in making protein. 

Contrary, the condition is said to be heterozygous when two different alleles are inherited together. Which means two different alleles- one dominant and one recessive, are involved in making protein.

Now to make it more clear we have to understand the concept of dominant and recessive. 

A single allele is enough to make a protein or phenotype is known as a dominant allele which masks the effects of the recessive allele in heterozygous conditions.

The recessive allele can only make protein or phenotype when two recessive alleles are present. However, a condition with one dominant and one recessive allele is known as carrier or heterozygous. 

Here, alleles are alternative forms of a gene to make a protein. Let us understand it by taking an example. 

Gene ‘R’ decides the color of some flowers. The ‘R’ allele is dominant and makes red color flowers while allele ‘r’ is recessive and makes white color flowers. So it is obvious that ‘RR’ alleles create red flower color and allele ‘rr’ creates white flower color. 

But as the ‘R’ is dominant over the ‘r’ allele in the heterozygous condition with ‘Rr’ (one dominant and one recessive allele) only red color flowers are produced because the ‘R; is dominant. Now you got the point. 

Read further: Genotype vs phenotype.

Let’s come to the point, 

So in the homozygous condition, a cell possesses either two same type dominant alleles (RR) or recessive alleles (rr). Whereas in the heterozygous condition the cell contains two non-identical alleles (Rr).  

Remember alleles are inherited in pairs. Why so?

Genes or DNA is located on chromosomes, our chromosomes are present in pairs because we are diploid. Humans have 22 pairs of identical autosomes and a pair of sex chromosomes. Therefore genes are also present in pairs. One gene or allele on one and another on the second chromosome.

If we wish to explain the condition by germline cells, the homozygous condition produces a single type of gametes while in a heterozygous condition two different types of gametes are developed.

Due to this reason, extra hybrid vigor is not observed in homozygous conditions unlike the heterozygous. 

Vigour is a tendency to form superior phenotypes to parents. A heterozygous cell has two different types of allele sets thus if we try to crossbreed it, it can make some new superiors phenotypes. 

In plant genetics, different types of heterozygous plant species are created in order to create new phenotypes through cross and back-breeding. 

Notably, In homozygous same trait or phenotype, we get even by self-breeding, on the other side, in heterozygous as two alleles are different,  different traits or phenotypes are produced by self-breeding. 

Only two possible genotype and phenotype conditions are possible for the homozygous, either homozygous dominant or homozygous recessive. However, for heterozygous it isn’t a case.

 In heterozygous, two different alleles can form various different phenotypes such as complete dominance, incomplete dominance or codominance. 

Related article: DNA vs Gene.

Take a quick look:

Complete dominance: 

In the complete dominance, the dominant allele mask or completely suppresses the effect of the recessive allele in heterozygous condition.

Homozygous vs heterozygous

An example of complete dominance.

Incomplete dominance: 

During the heterozygous condition, the dominant allele can’t completely suppress the effect of the recessive allele. 

Homozygous vs heterozygous

An example of incomplete dominance.

Codominance: 

The effect of both the alleles expressed and shown their effectiveness in the heterozygous condition is known as codominance. 

Homozygous vs heterozygous

An example of co-dominance.

Now let us understand it by taking a real example. 

Thalassemia is an autosomal recessive blood disorder occurring by the mutation in the HB gene. 

A person with two dominant HBHB alleles remains unaffected, homozygous dominant. But having the presence of two HbHb recessive causes disease. 

Contrary to this, in the heterozygous condition with alleles HB and Hb the individual does not have symptoms of a disease but he or she can transmit it to their offspring and hence they are considered carriers for the disease

Both homozygous and heterozygous genotypes can be determined using DNA sequencing or PCR. 

In PCR, sequence-specific primers for each allele are used to amplify the gene. Here, in case of either homozygous dominant or recessive one prominent DNA band is observed while in heterozygous conditions, two different DNA bands for two different alleles are observed in a gel.  

Summary 

Homozygous  Heterozygous 
When two identical alleles are present. When two different alleles are present.
Both are either dominant or both are either recessive.  Both alleles are different, one is dominant and one is recessive. 
Produce a single type of gametes  Produce two different types of gametes. 
Phenotype is either homozygous dominant or homozygous recessive.  Phenotypes are completely dominant, incomplete dominant or codominant. 
No extra vigor  Can have extra vigor 
Self breeding produces the same type of phenotypes.  Self breeding produces different types of phenotypes every time. 
Example of alleles are TT, tt, GG or gg Examples of alleles are Tt, Gg. 

Related article: Gene vs genome.

Conclusion: 

We need changes to survive on earth. To do so, new alleles must be created by mutations that are not lethal to us. Heterozygous organisms are a great source of new variations. 

In plant genetics especially, new heterozygous plants are created from homozygous one and by self and back-cross, desirable phenotypes can be produced. 

Nonetheless, back and self-cross are not possible for humans and animals.