Heterozygous vs. Homozygous: What's the Difference?

Homozygous and heterozygous describe the pairs of genes you inherited from your parents. If you are homozygous for a particular gene, it means you inherited the same version of that gene from both your mother and father. If you are heterozygous for a particular gene, it means you inherited two different versions of the gene, one from your mother and one from your father.

For example, if you inherited the gene for blonde hair from your mother and the gene for blonde hair from your father, you are homozygous for the gene that controls hair color. If, however, you inherited the gene for blonde hair from your mother and the gene for brown hair from your father, you are heterozygous for that gene. 

Heterozygous vs Homozygous

The designations of heterozygous and homozygous don't refer to individual people, but to individual genes. For example, you can be heterozygous for the gene that controls hair color, but homozygous for the gene that controls eye color. This just means that you have two different versions of the hair color gene, and two identical copies of the eye color gene.

Each version or variation of a gene is called an allele. You get one allele from each parent. Sometimes the alleles are identical and sometimes they are different.

When you inherit different alleles, the dominant allele is the one that is expressed in your body. For example, if you are heterozygous for the hair color gene, you might have one copy of the blonde hair allele and one copy of the brown hair allele. Because the brown hair allele is dominant, you will have brown hair.

A person with blonde hair, however, must be homozygous for the hair color gene, because the blonde hair allele is recessive. This means there need to be two copies of the blonde hair allele in order for that trait to be expressed.

What Is a Gene?

Genes are very specific segments of DNA (deoxyribonucleic acid). Each of your cells contains very long stretches of DNA. This is heritable material that you get from each of your parents.

DNA is composed of a series of individual components called nucleotides. There are four different types of nucleotides in DNA:

• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T)

Inside the cell, DNA is usually found bundled up into chromosomes (found in 23 different pairs).

Genes are used by other machinery inside the cell to make specific proteins. Proteins are the building blocks used in many critical roles inside the body, including structural support, cell signaling, chemical reaction facilitation, and transport.

The cell makes protein (out of its building blocks, amino acids) by reading the sequence of nucleotides found in the DNA. The cell uses a translation system to use information in the DNA to build proteins with highly specialized structures and functions.

Specific genes in the body fulfill distinct roles. For example, hemoglobin is a complex protein molecule that works to carry oxygen in the blood. Several different genes (found in the DNA) are used by the cell to make the protein shapes needed for this purpose.

Because of the way sex chromosomes work (males are XY, and females are XX), males only inherit a single copy of genes located on the X chromosomes.

Variations in Genes

The genetic code of human beings is quite similar: Well over 99 percent of nucleotides that are part of genes are the same across all humans. Alleles, however, have variations in the sequence of nucleotides in specific genes.

For example, one allele might begin with the sequence ATTGCT, and another might begin ACTGCT instead.

Sometimes these variations don’t make a difference in the end protein, but sometimes they do. They might cause a small difference in the protein that makes it work slightly differently.

In this example, a person who is homozygous for this gene would have two copies of the version of the gene beginning “ATTGCT” or two copies of the version beginning “ACTGCT.” A person who is heterozygous for this gene, however, would have one version of the gene beginning “ACTGCT” and also another version of the gene beginning “ATTGCT.”

Disease Mutations

Many gene variations aren’t a big deal and just contribute to normal human variation. However, gene variations can lead to human disease—these are considered mutations.

One example is sickle cell anemia. In sickle cell anemia, there is a mutation in a single nucleotide that causes a change in the nucleotide of a gene (called β-globin gene).

This causes an important change in the configuration of hemoglobin. Because of this, red blood cells carrying hemoglobin begin to break down prematurely. This can lead to problems like anemia and shortness of breath.

Generally speaking, there are three different possibilities:

• Someone is homozygous for the normal β-globin gene (has two normal copies)
• Someone is heterozygous (has one normal and one abnormal copy)
• Someone is homozygous for the abnormal β-globin gene (has two abnormal copies)

People who are heterozygous for the sickle cell gene have one unaffected copy of the gene (from one parent) and one affected copy of the gene (from the other parent).

These people usually don’t get the symptoms of sickle cell anemia because they have one gene that codes for a healthy protein. However, people who are homozygous for the abnormal β-globin gene do get symptoms of sickle cell anemia.

Heterozygous Genetic Diseases

You can sometimes get genetic diseases if you have heterozygous genes. In some types of genetic diseases, heterozygous genes almost always cause the disease.

In diseases caused by dominant genes, a person needs only one copy of a disease-causing gene to have problems. One example is the neurological disorder Huntington’s disease.

A person with only one affected gene (inherited from either parent) will still almost certainly get Huntington's disease. Someone who receives two abnormal copies of the disease from both parents would also be affected, but this is less common for dominant disease genes.

For recessive diseases, like sickle cell anemia, a person with heterozygous genes would not get the disease. However, sometimes they may have other subtle changes, depending on the disease.

What About Sex Chromosomes?

Sex chromosomes are the X and Y chromosomes that play a role in gender differentiation. Women inherit two X chromosomes, one from each parent. So a female can have a homozygous or heterozygous pattern of genes for any trait on the X chromosome.

Men inherit two different sex chromosomes: X and Y. Because these two chromosomes are different, the terms “homozygous” and “heterozygous” don’t apply to these two chromosomes in men.

You may have heard of sex-linked diseases, like Duchenne muscular dystrophy. These display a different inheritance pattern than standard recessive or dominant diseases inherited through the other chromosomes (called autosomes).

Inheritance

Let’s assume two versions of a gene: A and a. When two people have a child, there are several possibilities.

• Both parents are AA: All of their children will be AA as well (homozygous for AA).
• Both parents are aa: All of their children will be aa as well (homozygous for aa).
• One parent is Aa and another parent is Aa: Their child has a 25 percent chance of being AA (homozygous), a 50 percent chance of being Aa (heterozygous), and a 25 percent chance of being aa (homozygous)
• One parent is Aa and the other is aa: Their child has a 50 percent chance of being Aa (heterozygous) and a 50 percent chance of being aa (homozygous).
• One parent is Aa and the other is AA: Their child has a 50 percent chance of being AA (homozygous) and a 50 percent chance of being Aa (heterozygous).

A Word From Verywell

The study of genetics is complex. If a genetic condition runs in your family, don’t hesitate to consult a genetic counselor or your health professional about what this means for you.