Nearly everyone — with rare exceptions — is born with 23 pairs of chromosomes that were passed down from parents through combinations of their 46 chromosomes.

X and Y, the two most popularly known chromosomes, are part of the 23rd pair of chromosomes. They’re also called the sex chromosomes because they determine what biological sex you’re born with. (However, this binary is not as simple as it seems.)

The rest of the 22 pairs are called autosomes. They’re also known as autosomal chromosomes. Autosomes and sex chromosomes contain a total of about 20,000 genes.

These genes are essentially 99.9 percent identical in every human being. But small variations in these genes determine the rest of your genetic makeup and whether you inherit certain traits and conditions.

Within these 22 autosomes are two categories of genes that pass on different traits and conditions from your parents. These categories are called autosomal dominant and autosomal recessive. Here’s a quick breakdown of the difference.

Autosomal dominant

With this category, you only need one of these genes to be passed onto you from either parent to receive that trait. This is true even if another gene in the same autosome is a completely different trait or a mutation.

Inheritance

Let’s say your father has just one copy of a mutated gene for an autosomal dominant condition. Your mother doesn’t. There are two possibilities for inheritance in this scenario, each with a 50 percent chance of occurrence:

  • You inherit the affected gene from your father as well as one of your mother’s unaffected genes. You have the condition.
  • You inherit the unaffected gene from your father as well as one of your mother’s unaffected genes. You don’t have the condition, and you aren’t a carrier.

In other words, you only need one of your parents to pass an autosomal dominant condition on to you. In the scenario above, you have a 50 percent chance of inheriting the condition. But if that one parent has two affected genes, there’s a 100 percent chance you’ll be born with it.

However, you can also get an autosomal dominant condition without either parent having an affected gene. This happens when a new mutation occurs.

Autosomal recessive

For autosomal recessive genes, you need one copy of the same gene from each parent for the trait or condition to be expressed in your genes.

If just one parent passes on a gene for a recessive trait, such as red hair, or condition, such as cystic fibrosis, you’re considered a carrier.

This means that you don’t have the trait or condition, but you may have the gene for a trait and can pass it on to your children.

Inheritance

In the case of an autosomal recessive condition, you need to inherit an affected gene from each parent in order to have the condition. There’s no guarantee that will happen.

Let’s say both of your parents have one copy of the gene that causes cystic fibrosis. There are four possibilities for inheritance, each with a 25 percent chance of occurrence:

  • You inherit an affected gene from your father and an unaffected gene from your mother. You are a carrier, but you don’t have the condition.
  • You inherit an affected gene from your mother and an unaffected gene from your father. You are a carrier but you don’t have the condition.
  • You inherit an unaffected gene from both parents. You don’t have the condition, and you aren’t a carrier.
  • You inherit an affected gene from both parents. You have the condition.

In this scenario where each parent has one affected gene, their child has a 50 percent chance of being a carrier, a 25 percent chance of not having the condition or being a carrier, and a 25 percent chance of having the condition.

Here are some examples of common conditions in each category.

Autosomal dominant

Autosomal recessive

Autosomal DNA testing is done by providing a sample of your DNA — from a cheek swab, spit, or blood — to a DNA testing facility. The facility then analyzes your DNA sequence and matches your DNA to others who have submitted their DNA for testing.

The larger a testing facility’s database of DNA, the more accurate the results. This is because the facility has a larger pool of DNA for comparison.

Autosomal DNA tests can tell you a lot about your ancestry and your chances of getting certain conditions with a pretty high level of accuracy. This is done by finding specific variations in your genes and putting them in groups with other DNA samples that have similar variations.

Those who share the same ancestors will have similar autosomal gene sequences. This means that these DNA tests can help trace your DNA and the DNA of those distantly related to you back to where those genes first came from, sometimes back several generations.

This is how these DNA tests can suggest your ancestral ethnic and racial background and what regions of the world your DNA comes from. This is one of the most popular uses for autosomal DNA kits from companies like 23andMe, AncestryDNA, and MyHeritage DNA.

These tests can also tell you with almost 100 percent accuracy whether you’re a carrier of an inherited condition or have the condition yourself.

By looking at the traits within the genes on each of your autosomal chromosomes, the test can identify mutations, either dominant or recessive, associated with these conditions.

The results of autosomal DNA tests can also be used in research studies. With large databases of autosomal DNA, researchers can better understand the processes behind genetic mutations and gene expressions.

This can improve treatments for genetic disorders and even lead researchers closer to finding cures.

Cost of testing

Autosomal DNA test costs vary widely:

  • 23andMe. A typical ancestry test costs $99.
  • AncestryDNA. A similar test from the company behind the Ancestry.com genealogy website costs about $99. But this test also includes nutrition data that can tell you what foods are best for your particular DNA sequence as well as what you may be allergic to or what can cause inflammatory responses in your body.
  • MyHeritage. This similar test to 23andMe costs $79.

Autosomes carry a majority of your gene information and can tell you a lot about your ancestry, your health, and who you are at the most biologically personal level.

As more people take autosomal DNA tests and testing technology becomes more precise, the results of these tests are becoming more accurate. They’re also shedding crucial light on where people’s genes really come from.

You may think that your family is of a certain heritage, but your autosomal DNA results can give you an even more granular identification. This can validate your family’s stories or even challenge your beliefs about your family’s origin.

When taken to its logical extreme, a huge database of human DNA may be able to locate the origin of the first human beings and beyond.

Autosomal DNA testing may also provide the DNA necessary to research just how a number of genetic conditions, many of them disruptive to people’s lives, can finally be treated or cured.