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Genetic mutations are changes or variants in DNA that make up genes. If someone inherits an identical gene variant from both biological parents, they have a homozygous genetic predisposition for a condition.

This article examines examples of homozygous gene variants and their risk factors. We also look at how a heterozygous mutation differs from a homozygous mutation.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms, “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

Homozygous, or homozygosity, refers to the inheritance of genes from each biological parent. The variant results from a mutation that changes the DNA sequence making up a gene.

Experts refer to variations in genes as alleles. If the alleles are identical, a person is homozygous for that gene.

If someone inherits two identical copies of a harmful gene variant, they are predisposed to a condition. However, homozygous genes manifest a condition rather than predispose someone to it, such as hemophilia in females.

Closely related parents’ children have an increased risk of homozygous conditions.

Vs. heterozygous

Heterozygous causes a person to inherit two different gene variants from each biological parent. These variations may involve one or many parts of a gene.

The effects depend on the portions of the genes it affects. Some effects may be inconsequential, and some may be important.

Dominant vs. recessive

Dominant versus recessive traits and conditions also play a role in the risk of health conditions.

Dominant genes do not have to be homozygous, while recessive genes do. Even if a trait is homozygous recessive, it may still be incomplete, which means that even if a gene is present, that condition may still not occur.

The following are examples of homozygous conditions.

Hemophilia

Hemophilia is a sex-linked recessive disorder. In females, each parent has an X chromosome with the same atypical gene. However, it only needs one atypical X chromosome for males to develop it.

In rare cases, females can also have hemophilia if it affects both X chromosomes or one is missing or non-functioning.

This condition causes the blood to not clot properly, resulting in sudden or excessive bleeding following an injury or surgery.

Otospondylomegaepiphyseal dysplasia (OSMED)

OSMED, or Nance-Sweeney syndrome, occurs due to homozygous genes.

This rare genetic condition results from mutations in the Collagen alpha-2(XI) (COL11A2) gene.

OSMED may cause:

  • hearing loss
  • distinct facial features
  • bone malformations

Homozygous familial high cholesterol

A 2015 study refers to homozygous familial high cholesterol, a rare, severe genetic condition that causes high levels of low-density lipoprotein (LDL) cholesterol.

LDL is the “bad” type of cholesterol that builds in the wall of blood vessels. Without treatment, this buildup could have serious consequences.

Pulmonary arterial hypertension

Pulmonary hypertension is a severe condition that causes high pressure in the blood vessels transporting blood from the heart to the lungs.

A 2016 study found that a homozygous mutation may link to an aggressive form of the condition.

Depression

According to 2018 research, genetic factors may contribute to the risk of depression. However, the studies have inconsistent results, but some associate homozygosity with depression.

Cancer

A 2021 study found that a particular homozygous gene variant may increase the risk of early onset oral cancer.

Additionally, other findings from 2018 noted links between homozygous genes and some cancers in animals.

Consanguineous marriages occur between two closely related individuals — second cousins or closer.

Parents’ biological children in consanguineous marriage have a higher risk of inheriting homozygous genetic conditions.

Several factors can influence the risk in people with a homozygous predisposition for a condition, such as:

  • genetics
  • lifestyle
  • environmental

Although people cannot change their genetic makeup, they can engage in lifestyle habits, such as:

  • maintaining a moderate weight
  • quitting smoking
  • avoiding exposure to known environmental toxins

Mutations can cause a change in the DNA sequence in a gene, which may result in a harmful gene variant. When people inherit the same gene variant from both biological parents, they have a homozygous genetic change.

Homozygosity can also be two typical alleles that health experts still define as homozygous.

Homozygous genes are more common in offspring of closely related biological parents, such as second cousins.

Sometimes, instead of inheriting two identical gene variants from parents, individuals inherit slightly different ones. This is called a heterogenous genetic change.