In the study of genetics, the term homozygous refers to inheriting identical versions of a particular gene from both biological parents. Genes contain the instructions that determine specific traits, and these different versions of a gene are known as alleles. When an individual receives two matching alleles for a specific trait, one from each parent, their genetic makeup for that trait is considered homozygous.
This genetic state can be imagined as having two identical copies of a recipe for a certain characteristic. For instance, an individual might inherit the same instruction for eye color from both parents.
Homozygous vs. Heterozygous Genotypes
To understand what homozygous means, it is useful to compare it with its opposite: heterozygous. While being homozygous means having two identical alleles for a gene, being heterozygous means having two different alleles for that same gene. The set of alleles an individual has for a gene is called their genotype.
This difference is often represented using letters. For a homozygous genotype, the letters are the same, such as “BB” or “bb”. A heterozygous genotype is represented by two different letters, such as “Bb”. This notation visually captures the core distinction: “homo” means same, while “hetero” means different.
An individual’s genotype, whether homozygous or heterozygous, provides the underlying blueprint for their traits. The interaction between these inherited alleles dictates which physical characteristics, or phenotypes, are ultimately expressed.
Dominant and Recessive Homozygous Traits
A homozygous genotype can be either dominant or recessive, which determines how a trait is expressed. A homozygous dominant genotype means an individual has two copies of the dominant allele for a trait, and having two of them ensures the dominant trait appears. Using the example of eye color, the allele for brown eyes is dominant, and an individual with a “BB” genotype is homozygous dominant and will have brown eyes.
A homozygous recessive genotype occurs when an individual has two copies of the recessive allele. A recessive trait only appears when no dominant allele is present to mask it. For instance, the allele for blue eyes is recessive, and an individual must have the “bb” genotype to have blue eyes. People with recessive traits like red hair or blue eyes are always homozygous for that specific gene.
The inheritance of these genotypes can be visualized using a tool called a Punnett square. This diagram shows the potential genotypes of offspring based on the parents’ genotypes. If both parents contribute a “B” allele, the child will have a “BB” genotype, and if both pass on a “b” allele, the offspring will have a “bb” genotype.
Relationship to Genetic Conditions
The concept of being homozygous is relevant to understanding certain genetic health conditions. Many of these conditions are passed down through recessive inheritance, and a person must inherit two copies of the same mutated, or non-working, allele to be affected.
Cystic fibrosis is a well-known example of a condition linked to a homozygous recessive genotype. The CFTR gene provides instructions for a protein that manages fluid movement in cells. If an individual inherits two mutated copies of the CFTR gene, they will develop the condition. A person who inherits only one mutated copy is considered a carrier but does not show symptoms.
Sickle cell anemia is another condition that follows this pattern. It is caused by inheriting two mutated copies of the HBB gene, which is involved in producing hemoglobin. Individuals who are homozygous for the mutated gene will develop sickle cell anemia.