Heterozygous and homozygous differences

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If you have read about genetic diseases, have you come across the terms "homozygous" or "heterozygous"? But what do these terms mean? And what are the practical results of being "homozygous" or "heterozygous" for a particular gene?

Clinical Cytogeny Unit, Addenbrooks Hospital / Scientific Photo Library / Getty Images

What is a gene?

Before identifying homozygous and heterozygous, we must first think about the genes. Each of your cells contains very long stretches of DNA (deoxyribonucleic acid). This is the hereditary material that you receive from each of your parents.

DNA is made up of several individual components called nucleotides. There are four different types of nucleotides in DNA:

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

Inside the cell, DNA is generally linked to chromosomes (23 pairs).

Genes are very specific DNA segments with a specific purpose. These segments are used by other mechanisms within the cell to produce certain proteins. Proteins are building blocks used in many critical functions within the body, including structural support, cell signaling, facilitation of chemical reactions, and transport.

The cell produces proteins (from its basic components, amino acids) by reading the nucleotide sequence found in DNA. The cell uses a kind of translation system to use information in DNA to create specific proteins with specific structures and functions.

Certain genes play different roles in the body. For example, hemoglobin is a complex protein molecule that carries oxygen to the blood. The cell uses several different genes (found in DNA) to create the specific forms of protein necessary for this purpose.

You inherit your parents' DNA. Generally speaking, half of your DNA belongs to your mother and the other half to your father. For most genes, you inherit one copy from your mother and one from your father.

However, there is an exception associated with a specific pair of chromosomes called sex chromosomes. Because of the way sex chromosomes work, men only inherit one copy of certain genes.

Genetic variations

The genetic code of humans is very similar: more than 99 percent of the nucleotides that make up genes are the same for all people. However, there are some variations in the nucleotide sequence in specific genes.

For example, one variation of a gene can start with the sequence A T TGCT and another variation can start with the sequence A C TGCT. These different variations of genes are called alleles.

Sometimes these variations don't affect the final protein, but sometimes they do. They can cause a slight difference in protein, which makes it work a little differently.

A person is considered homozygous for a gene if they have two identical copies of the gene. In our example, it will be two copies of the version of the gene that begins with "A T TGCT", or two copies of the version that begins with "A C TGCT".

Heterozygosity simply means that a person has two different versions of a gene (one inherited from one parent and the other from the other parent). In our example, the heterozygote will have one version of the gene that begins with "A C TGCT" and another version of the gene that begins with "A T TGCT."

Homozygous: You inherit the same version of a gene from each parent, so you have two genes that match.

Heterozygous: inherits a different version of a gene from each parent. Do not match.

Diseases Mutations

Many of these mutations are not significant and simply contribute to normal human variability. However, other specific mutations can cause human disease. This is often what people talk about when they mention "homozygous" and "heterozygous" – a specific type of mutation that can cause disease.

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

This causes a major change in the hemoglobin configuration. Because of this, the red blood cells that carry 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 an 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 do not have symptoms of sickle cell disease. However, people homozygous for the abnormal β-globin gene have symptoms of sickle cell anemia.

Heterozygotes and genetic diseases

Heterozygotes may have a genetic disorder, but this depends on the type of disease. With some types of genetic diseases, a heterozygous person is almost certainly sick.

In diseases caused by so-called dominant genes, a person only needs one faulty copy of a gene to have problems. An example is Huntington 's neurological disease.

A person with an affected gene (inherited from one parent) is almost certain to contract Huntington's disease as heterozygous ( a homozygous who receives two abnormal copies of the disease from both parents will also be affected, but this is less common for genes of the dominant disease.)

However, with recessive diseases such as sickle cell anemia, heterozygotes do not get sick. (However, they can sometimes have other subtle changes, depending on the disease.)

If the dominant gene is causing the disease, the disease may be heterozygous. If the recessive gene causes the disease, the heterozygote may not develop the disease or have fewer consequences.

What about the 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. Therefore, a woman can be considered homozygous or heterozygous for a certain trait on the X chromosome.

Men are a bit confusing. They inherit two different sex chromosomes: X and Y. Since the two chromosomes are different, the terms "homozygous" and "heterozygous" do not apply to these two chromosomes in males.

You may have heard of gender-related diseases like Duchenne muscular dystrophy . They exhibit a different pattern of inheritance than standard recessive or dominant diseases inherited through other chromosomes (called autosomes).

Heterozygous advantage

For some disease genes, the heterozygote may confer certain benefits on the individual. For example, it is believed that a heterozygous for the sickle cell gene can protect against malaria to some extent compared to people who do not have an abnormal copy.

Inheritance

Suppose there are two versions of a gene: A and a. When two people have a child, there are several possibilities:

  • Both parents are AA : all their children will also be AA (homozygous for AA).
  • Both parents are aa : all their children will also be aa (homozygous for aa).
  • One parent is Aa and the other parent is Aa : Your 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 : Your 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 : Your child has a 50 percent chance of being AA (homozygous) and a 50 percent chance of being Aa (heterozygous).

Get the word of drug information

The study of genetics is complex. If you have a genetic disorder in your family, don't hesitate to ask your doctor what this means for you.

Frequently asked questions

  • Alleles are different forms or variants of a gene. They help identify traits that are inherited from our parents, such as eye color, skin pigmentation, blood type, height, and more.

  • Heterozygosity is when different forms of a particular gene are inherited from each parent. Homozygosity is when the gene inherited from each parent is identical. Basically heterozygous means different and homozygous means the same.

  • Genotype is a term used to define all of a person's genes. Humans have mostly similar DNA, but there are variations in certain sequences. These species are what separate us from each other.

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