Homozygous and Heterozygous

Genes regulate the health and physical traits of humans. Mostly, they have similar genes, but variations exist, too. Each of these variations is called alleles. You get two alleles for every single gene by birth. One is shared by your biological father, while the other by your biological mother. While reading about genetic conditions, you may often come across the words homozygous and heterozygous. So, when the alleles of a gene are identical, you are said to have a homozygous genotype. On the contrary, you have heterozygotic traits if you inherit two different alleles of a gene. To know further about homozygous and heterozygous in detail, let’s first understand what genes are. 

What is a Gene?

Gene is the fundamental, physical and functional unit of heredity that you inherit from both of your parents. Each cell in your body contains a very long sequence of DNA (Deoxyribonucleic acid). A gene is the particular segment of that long stretch of DNA with specific functions. According to studies, approximately 20,000 to 25,000 genes are present in humans. Each person inherits two copies of each gene from their biological parents. 

Genetic Variations

Over 99 per cent of nucleotide sequences comprising the genes are similar in humans. Although, a few genes are slightly different among people, which makes up for their unique physical characteristics. 

For instance, one variation of a gene may start with the nucleotide sequence ATTTGC, while the other variation may start ATTGCT. The forms of a single gene with slight variations in their sequences are called alleles.  

As genes are used by cells to make proteins, sometimes these variations may cause the latter to work a little differently. 

What is meant by Homozygous?

Homozygous is a term used in genetics, meaning two identical copies of a DNA gene sequence responsible for a specific trait. 

You are homozygous for a gene if you have two similar copies of a single gene. That is, if the alleles are identical for the gene, you have a homozygous genotype. So, considering the above example, you will inherit two copies of the gene starting ATTTGC or two copies of the gene beginning ATTGCT.

Examples of Homozygous Genotype

There are several ways in which homozygous genotype may appear in humans. Some examples are:

• Freckles

These are tiny spots on the skin, made of melanin pigment, controlled by the MC1R gene. The trait is dominant. However, if you don’t have freckles, it means you are homozygous for the recessive version of the gene. Recessive traits are less likely to get expressed if one of the alleles is dominant. 

• Eye colour

The allele responsible for expressing the brown eye colour trait is dominant to the blue eye colour allele. You can have brown eyes whether you’re heterozygous (one allele for brown and one allele for blue) or homozygous ( two alleles for brown). The alleles for blue eyes are recessive, so you will require two identical blue eye alleles to have blue eyes. 

Diseases Related to Homozygous Genes

• Sickle cell anaemia

• Cystic fibrosis

• Phenylketonuria

What is Meant by Heterozygous?

Heterozygous is a term used in genetics, meaning two different versions of a DNA gene sequence. It is seen in diploid organisms, which receive one allele from each of their biological parents. 

You are heterozygous if you have two different alleles of a gene inherited from both of your parents. For instance, you will inherit one version of the gene starting ATTTGC and also another version of the gene starting ATTGCT.

Examples of Heterozygous Genotype

In a heterozygous genotype, the two different alleles of a gene interact to decide how the traits will be expressed. This interaction is typically based on dominance. The allele that strongly expresses itself between the two is ‘dominant’ over the other (recessive). The heterozygous genotype involves 

• Complete dominance

Here, the dominant allele completely masks the effect of the recessive allele. For example, the eye colour. The brown eye allele is completely dominant over the blue eye allele. 

• Incomplete dominance

Here, the dominant and the recessive allele blend to give rise to a third trait. For instance, the hair texture. A curly hair allele and a straight hair allele can create a wavy hair trait.

• Codominance

Here, both the dominant and the recessive traits are expressed equally. They don’t blend, though. An example is the AB blood type.

Diseases Related to Heterozygous Genotype

• Huntington’s Disease

• Familial hypercholesterolemia

• Marfan’s syndrome.

Homozygous and Heterozygous: Difference

Conclusion

Genes are the hereditary trait bearers. They are a particular segment of the DNA nucleotide sequence with specific functions. The genetic code of all humans is quite similar. According to studies, most of the nucleotide sequence in a gene is similar in most humans. Although, exceptions also exist. Each person has two alleles of a single gene. An individual is homozygous if they inherit two identical versions of a gene. On the contrary, these versions are different in the case of heterozygotic individuals.