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Difference between Homozygous and Heterozygous

There are several terms in biology that hold a deeper meaning in response to the human system. These terms are related to our genes and other parts of our body. Biology is a distinctive subject that focuses on several parts of the body, like the nervous system, respiratory system, intestines, etc. So, today we will be discussing about a much deeper aspect in biology, i.e., homozygous and heterozygous. But before going on with the explanation of two terms, it is important for us to know the meaning of zygosity. Well, zygosity is defined as a similarity between two or more genes that have been arisen due to mutation (change in the structure of a gene). These two genes are often found at a particular place on a chromosome. Now, let us discuss the basic meaning of the given terms.


Homozygous is defined as a genetic condition wherein a person inherits a similar DNA of a gene from both mother and father. The term 'homozygous' is often used in the context of a disease. A person is said to be homozygous if he has a similar DNA of a particular gene. For instance, if an organism has two identical genes like 'YY' or 'yy,' it is said to be a homozygous organism. There are several diseases related to homozygous like sickle cell anaemia, cystic fibrosis, etc. A homozygous gene can affect two body parts, i.e., the color of the eye and hair.

Homozygous vs Heterozygous


Heterozygous is defined as a genetic condition in which an organism acquires two different types of genes from mother and father. For instance, a child can have mixed hair color, i.e., one hair can be golden, and another hair can be black. These are two genes acquired by a child through his/ her parents. A child can have other mixed characteristics of the biological parents.

There are three kinds of homozygous genes, i.e., complete dominance, incomplete dominance, and co-dominance. There are related diseases, too, with the heterozygous genes, which we will be discussing later. Now, let us take a look at some of the primary differences between homozygous and heterozygous genes.

1. Homozygous is defined as a genetic situation in which a person inherits the same DNA from his/ her biological parents. On the other hand, heterozygous is defined as a situation in which a person has two different kinds of genes acquired by biological parents.
2. There are many disorders associated with homozygous like Cystic fibrosis, Phenylketonuria, etc. There are diseases associated with heterozygous like Huntington's disease, Marfan's syndrome, etc.
3. Homozygous genes are often represented as:
  • 'AA' for homozygous dominant.
  • 'aa' for homozygous recessive.
Heterozygous genes are represented by the 'Aa' genotype.
4. Two or more genotypes are possible with homozygous recessive and homozygous dominant situations. On the other hand, in the heterozygous condition, only one gene is primarily dominant.
5. There is a single kind of gamete released from homozygous genotype. Heterozygous genotype releases two different kinds of gametes.
6. The same characteristics are produced in an organism by the homozygous genotypes over generations. Heterozygous genotypes produce different characteristics over generations.
7. There is no hybridity in homozygous genes. Heterozygous genes possess a hybrid nature.
8. There are two kinds of homozygous conditions, i.e., homozygous dominant and homozygous recessive. There are three kinds of heterozygous conditions. They are:
  • Complete Dominance
  • Incomplete Dominance
  • Co-dominance
9. Organisms having homozygous genotypes are called homozygotes. Organisms having heterozygous genotypes are called heterozygotes.
10. Homozygous genotypes are seen in animals and plants having asexual reproduction. Heterozygous genotypes are seen in organisms having sexual reproduction.
11. Example:
The color of the eyes is considered to be a homozygous condition.
The hair color can be considered as a heterozygous condition (one hair strand can be golden, and another hair strand can be black).

So, these are some of the differences between homozygous and heterozygous genotypes. Well, as discussed earlier, several diseases and effects are associated with the homozygous and heterozygous genotypes. So, let us discuss them briefly.

The appearance of Homozygous Condition

The homozygous condition appears in different parts of the body. They are:

  1. Eyes: The color of the eye is a homozygous condition. The color of the eyes is the same, be it blue, black, or red. Thus, the homozygous condition is primarily seen in the eyes.
  2. Hair: The color of the hair is also considered to be a homozygous condition. When the hair color of one of the parents is dominant, the child is born with that hair color.
  3. Freckles: Freckles are considered to be the brown spots that appear on the skin. These spots are commonly known as pigmentation. Freckles are a kind of homozygous condition that appears in the body due to the dominant nature of the genes.

Disorders associated with Homozygous Condition

  1. Cystic Fibrosis: Cystic Fibrosis is a disorder in which the protein present in the body controls the movement of the fluid in the cells. This protein movement is regulated by CFTR, i.e., cystic fibrosis transmembrane conductance regulator. This disorder results in lung infection, pancreas damage, issues related to digestion, etc.
  2. Sickle Cell Anaemia: In this condition, beta-globin is produced in the red blood cells. This beta-globin is produced through HBB, i.e., haemoglobin subunit beta. This condition results in insufficient blood supply and reduced red blood cells in the body.
  3. Phenylketonuria: In this disorder, the gene produces an enzyme called phenylalanine which breaks the amino acids in the body. This disorder can result in skin rashes, foul smell of the breath and urine, hyperactivity, psychiatric diseases, etc.

So, these are the disorders associated with the homozygous genotypes. Now, let us look at the types of heterozygous genotypes along with the related diseases.

Types of Heterozygous Genotypes

  1. Complete Dominance: Complete Dominance is defined as a situation in which the dominant gene overpowers the recessive gene. For instance, if a child has brown eyes, then it means that the brown color is dominant in the eye; rather than the black color.
  2. Incomplete Dominance: Incomplete Dominance is defined as the situation in which the dominant gene and the recessive gene blend equally. For instance, the hair can have two different colors blended together. One can be black, and the other can be brown. Incomplete dominance is primarily seen in the hair color.
  3. Co-dominance: Co-dominance is defined as the situation in which the dominant and regressive genes are present together. They do not blend; instead, they are equally present. For instance, if a person has one gene of A blood and another gene of B blood, instead of creating a third blood type, the body equally places and produces both blood in the body. In this way, the person produces an AB blood group.

Diseases associated with Heterozygous Condition

  1. Huntington's Disease: The heterozygous gene produces a protein called huntingtin that connects all the nerve cells in the brain. In this disorder, the genes cause a neurodegenerative disorder. The common symptoms of this disease are emotional breakdown, reduced cognition, problems in walking, speaking, eating, etc.
  2. Marfan's Syndrome: Marfan's syndrome is defined as a medical condition in which the strength of the body is highly affected. The most common symptoms of this disease are hypermetropia, heart problems, overgrowth of an arm/ leg, etc.
  3. Hypercholesterolemia: In this disease, the cholesterol levels of the body increases manifold at an early age. The person suffers from several artery diseases.

So, these are some of the diseases associated with heterozygous genotypes. Both homozygous and heterozygous genotypes have some related disorders. But if they are diagnosed and treated early, there is a chance to get rid of them. It is essential to know that either of the two genotypes, i.e., homozygous and heterozygous, exists in the human body. They have several effects on the body. Thus, both homozygous and heterozygous genotypes play a fundamental role in the formation of genes.

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