What is Homozygous and its Example?

Definition of homogenous

homozygous is a genetic condition where an individual having two identical alleles of a particular gene or genes. That means an organism inherited the same DNA sequence for the particular gene from both its biological mother and father.

If the alleles are identical, that means both the gene is either recessive or a dominant gene. Then a person or organism is homozygous. For example, a person has identical alleles for the gene expression that are responsible for the cause of brown hair.

But what if your alleles are not identical, in that case, the gene has two different alleles one is called the dominant allele and the other is a recessive allele. However, in homozygous conditions, this interaction doesn’t occur. As I say above you either have two dominant alleles or two recessive alleles.

Now let’s move on to what actually a homozygous condition or genotype is, which we discuss along with the examples and the risks of the diseases.

What is the homozygous?

As we discussed above homozygous alleles may be dominant or recessive. A homozygous dominant allele variation contains two dominant alleles and expresses the dominant phenotype. Whereas homozygous recessive allele combination contains two recessive alleles and expresses the recessive phenotype.

For example, in pea plants, the gene for seed color is present in two forms, one form (or allele) for yellow seed color(Y) and the other for wrinkled seed color. The yellow seed color is prominent (Dominant) and the green seed color is recurring (recessive).

A homozygous plant contains either of the following alleles for seed color: (YY) or (gg). The (YY) genotype is homozygous dominant and the (gg) genotype is homozygous recessive for seed color

3 Principles of Mendelian Genetics | by The Human Origin Project ...

In the image above, a monohybrid cross is made between plants that are asymmetrical to the color of the round seed.

The predicted inheritance pattern of offspring results in a 1: 2: 1 ratio of progeny. About one-fourth will be homozygous dominant for yellow seed color (YY), half will be heterozygous for yellow seed color (Yg), and one-fourth will have the homozygous recessive green seed color(gg).

The phenotypic ratio in this cross is 3:1. About three-fourths of the offspring will have yellow seeds and one-fourth will have green seeds.

Examples of homozygous

A homogenous genotype can appear in different 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.

Homozygous genes and disease

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 have inherited two mutated copies of this gene, you have cystic fibrosis (CF). Everyone with CF is homologous to 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 an 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 trusted Source 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:

• cardiovascular disease
• blood clots
• pregnancy complications, such as preeclampsia
• pregnancies with neural tube defects, such as spina bifida
• depression
• dementia
• osteoporosis
• migraine
• polycystic ovary syndrome
• multiple sclerosis

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