Before defining homozygous and heterozygous, we must first think about genes. Each of your cells contains very long stretches of DNA (deoxyribonucleic acid). 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 very specific segments of DNA with a distinct purpose. These segments 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 signalling, 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 sort of translation system to use information in the DNA to build specific proteins with specific structures and functions.
Specific genes in the body fulfil distinct roles. For example, haemoglobin 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 specific protein shapes needed for this purpose.
You inherit DNA from your parents. Broadly speaking, half of your DNA comes from your mother and the other half from your father. For most genes, you inherit one copy from your mother and one from your father.
However, there is an exception involving a specific pair of chromosomes called sex chromosomes. Because of the way sex chromosomes work, males only inherit a single copy of certain genes.
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. However, there are some variations in the sequence of nucleotides in specific genes.
For example, one variation of a gene might begin with the sequence ATTGCT, and another variation might begin ACTGCT instead. These different variations of genes are called alleles.
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.
A person is said to be homozygous for a gene if they have two identical copies of the gene. In our example, this would be two copies of the version of the gene beginning “ATTGCT” or two copies of the version beginning “ACTGCT.”
Heterozygous just means that a person has two different versions of the gene (one inherited from one parent, and the other from the other parent). In our example, a heterozygote would have one version of the gene beginning “ACTGCT” and also another version of the gene beginning “ATTGCT.”
Homozygous: You inherit the same version of the gene from each parent, so you have two matching genes.
Heterozygous: You inherit a different version of a gene from each parent. They do not match.
Many of these mutations aren’t a big deal and just contribute to normal human variation. However, other specific mutations can lead to human disease. That is often what people are talking about when they mention “homozygous” and “heterozygous”: a specific type of mutation that can cause disease.
One example is sickle cell anaemia. In sickle cell anaemia, there is a mutation in a single nucleotide that causes a change in the nucleotide of a gene (called the β-globin gene).1
This causes an important change in the configuration of haemoglobin. Because of this, red blood cells carrying haemoglobin begin to break down prematurely. This can lead to problems like anaemia 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 anaemia. However, people who are homozygous for the abnormal β-globin gene do get symptoms of sickle cell anaemia.
Heterozygotes And Genetic Diseases
Heterozygotes can get a genetic disease, but it depends on the type of disease. In some types of genetic diseases, a heterozygous individual is almost certain to get the disease.
In diseases caused by what are called dominant genes, a person needs only one bad copy of a 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 as a heterozygote.2 (A homozygote who receives two abnormal copies of the disease from both parents would also be affected, but this is less common for dominant disease genes.)
However, for recessive diseases, like sickle cell anaemia, heterozygotes do not get the disease. (However, sometimes they may have other subtle changes, depending on the disease.)
If a dominant gene causes a disease, a heterozygote may manifest the disease. If a recessive gene causes a disease, a heterozygote may not develop the disease or may have lesser effects of it.
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 be considered homozygous or heterozygous about a specific trait on the X chromosome.
Men are a little more confusing. They 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).
For some disease genes, it is possible that being a heterozygote gives a person a certain advantage. For example, it is thought that being a heterozygote for the sickle cell anaemia gene may be somewhat protective for malaria, in comparison to people who don’t have an abnormal copy.
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 Brain Labs
The study of genetics is complex. If a genetic condition runs in your family, don’t hesitate to consult your health professional about what this means for you.
As always, stay healthy.