In general, humans have the same genes. A number of genes are varied. These control our physical traits and health.

Each variation is called an allele. You inherit two alleles for each gene. One comes from your biological mother and one comes from your biological father.

If the alleles are identical, you’re homozygous for that specific gene. For instance, it could mean you have two alleles for the gene that causes brown eyes.

Some alleles are dominant, while others are recessive. The dominant allele is expressed more strongly, so it masks the recessive allele. However, in a homozygous genotype, this interaction doesn’t occur. You either have two dominant alleles (homozygous dominant) or two recessive alleles (homozygous recessive).

Read on to learn about the homozygous genotype, along with examples and disease risks.

The term “heterozygous” also refers to a pair of alleles. Unlike homozygous, being heterozygous means you have two different alleles. You inherited a different version from each parent.

In a heterozygous genotype, the dominant allele overrules the recessive one. Therefore, the dominant trait will be expressed. The recessive trait won’t show, but you’re still a carrier. This means you can pass it on to your children.

It’s the opposite of being homozygous, where the trait of the matching alleles — either dominant or recessive — is expressed.

A homozygous genotype can appear in various ways, such as:

Eye color

The brown eye color allele is dominant over the blue eye allele. You can have brown eyes whether you’re homozygous (two alleles for brown eyes) or heterozygous (one for brown and one for blue).

This is unlike the allele for blue eyes, which is recessive. You need two identical blue eye alleles in order to have blue eyes.


Freckles are tiny brown spots on the skin. They’re made of melanin, the pigment that gives color to your skin and hair.

The MC1R gene controls freckles. The trait is also dominant. If you don’t have freckles, it means you’re homozygous for a recessive version that doesn’t cause them.

Hair color

Red hair is a recessive trait. A person who is heterozygous for red hair has one allele for a dominant trait, like brown hair, and one allele for red hair.

They can pass the red hair allele to their future children. If the child inherits the same allele from the other parent, they’ll be homozygous and have red hair.

Some diseases are caused by mutated alleles. If the allele is recessive, it’s more likely to cause disease in people who are homozygous for that mutated gene.

This risk is related to the way dominant and recessive alleles interact. If you were heterozygous for that mutated recessive allele, the normal dominant allele would take over. The disease may be expressed mildly or not at all.

If you’re homozygous for the recessive mutated gene, you have a higher risk of the disease. You don’t have a dominant allele to mask its effect.

The following genetic conditions are more likely to affect people who are homozygous for them:

Cystic fibrosis

The cystic fibrosis transmembrane conductance regulator (CFTR) gene makes a protein that controls fluid movement in and out of cells.

If you inherit two mutated copies of this gene, you have cystic fibrosis (CF). Every person with CF is homozygous for this mutation.

The mutation causes thick mucus to build up, resulting in:

  • frequent lung infections
  • pancreas damage
  • scarring and cysts in the lungs
  • digestive issues

Sickle cell anemia

The hemoglobin subunit beta (HBB) gene helps produce beta-globin, which is part of hemoglobin in red blood cells. Hemoglobin makes it possible for red blood cells to deliver oxygen throughout the body.

In sickle cell anemia, there are two copies of an HBB gene mutation. The mutated alleles make abnormal beta-globin, which leads to low red blood cells and poor blood supply.


Phenylketonuria (PKU) occurs when a person is homozygous for a phenylalanine hydroxylase (PAH) gene mutation.

Normally, the PAH gene instructs cells to produce an enzyme that breaks down an amino acid called phenylalanine. In PKU, the cells can’t create the enzyme. This causes phenylalanine to accumulate in the tissues and blood.

A person with PKU needs to limit phenylalanine in their diet. Otherwise, they can develop:

  • skin rashes
  • neurological problems
  • musty-smelling breath, skin, or urine
  • hyperactivity
  • psychiatric disorders

Methylenetetrahydrofolate reductase (MTHFR) gene mutation

The MTHFR gene instructs our body to make methylenetetrahydrofolate reductase, an enzyme that breaks down homocysteine.

In a MTHFR gene mutation, the gene doesn’t make the enzyme. Two notable mutations include:

  • C677T. If you have two copies of this variant, you’ll likely develop high blood levels of homocysteine and low levels of folate. Approximately 10 to 15 percent of Caucasian people from North America and 25 percent of Hispanic people are homozygous for this mutation.
  • A1298C. Being homozygous for this variant isn’t associated with high homocysteine levels. However, having one copy each of C677T and A1298C has the same effects as having two C677T.

While scientists are still learning about MTHFR mutations, it’s been associated with:

We all have two alleles, or versions, of each gene. Being homozygous for a particular gene means you inherited two identical versions. It’s the opposite of a heterozygous genotype, where the alleles are different.

People who have recessive traits, like blue eyes or red hair, are always homozygous for that gene. The recessive allele is expressed because there isn’t a dominant one to mask it.