Difference Between Homozygous and Heterozygous

Homozygous vs Heterozygous

Before understanding the difference between homozygous and heterozygous, a simple introduction about these would make it convenient. The inherited genes from the parental chromosomes control all the characters or traits of animals, plants, and all other living beings. That is the core reason of showing parental characteristics in the progeny. Most of the eukaryotic organisms have two sets of genes known as maternal genes from mother and the paternal genes from father. Therefore, the genetic situation is known as diploid (two sets of chromosomes). That means all the traits have the genetic components from both mother and father. However, those genes could be either dominant or recessive over each other, and that is where the homozygous and heterozygous characteristics are important to know.


Homozygous genes are composed of two similar type paternal and maternal genes. However, the dominant and recessive characters are important to consider. As an example, let the dominant trait, indicated upper case ‘S’, has been inherited from mother and the same from father for a child. Then, the child has the genetic background (genotype) as ‘SS’ for the particular character or phenotype, and this state is a homozygous genotype. If the inherited traits from mother and father were both recessive, indicated by lower case ‘s’, then the genotype of that phenotype would be homozygous (ss). Therefore, the homozygous genotypes could have either two dominant or two recessive traits. The ‘SS’ situation is known as the dominant homozygous genotype while the ‘ss’ situation is the recessive homozygous genotype. The dominant homozygous genotype expresses the dominant phenotypes while recessive homozygous genotype expresses the recessive phenotype.


As the terms depicts, the heterozygous genes have different types of genes for a particular phenotype. That means the genetic makeup of a particular character or phenotype does not contain similar but different types of genes. There are two basic types of genes known as dominant and recessive. Therefore, heterozygous genotypes or alleles have one dominant gene with one recessive gene responsible for a particular character. However, in the case of a heterozygous genotype, only the dominant gene is expressed as the phenotype, the externally visible or functional character. That is the reason the expressed one of the two genes is called the dominant gene. There is no rule that the dominant gene should come from the maternal or paternal genes, but any type of expressivity (either dominant or recessive gene) could be inherited from any parent. If the dominant gene ‘S’ from one parent couples with the recessive gene ‘s’, then the progeny would be heterozygous (indicated as ‘Ss’). Thereafter, only the dominant gene ‘S’ would be expressed but not the recessive gene ‘s’.


What is the difference between Homozygous and Heterozygous?

• As the terminologies of the words mean, the homozygous genotype contains the same type of genes responsible for a particular phenotype, whereas heterozygous genotype contains one dominant gene with one recessive gene in the diploid genetic setup.

• There are two types of homozygous genotypes known as dominant homozygous and recessive homozygous. On the other hand, heterozygous genotype has only one type.

• In homozygous genotypes, there are two types of phenotypes expressed while only one type is expressed in heterozygous genotypes.

• For a recessive gene to be expressed as a phenotype, it must have to be a recessive homozygous genotype, but not a heterozygous genotype.