Every human cell with a nucleus contains chromosomes, tightly packaged structures made largely of DNA and associated proteins. Chromosomes organize genetic information and help ensure that DNA is copied and distributed accurately when cells divide.
Humans typically have 46 chromosomes arranged in 23 pairs. Of those pairs, 22 are autosome pairs, while the remaining pair consists of the sex chromosomes. This basic distinction explains an important difference in how genetic information is organized and, in many cases, how certain inherited traits and disorders are passed through families.
What are autosomes?
Autosomes are chromosomes that are not classified as sex chromosomes. Humans normally have 22 pairs of autosomes, numbered chromosomes 1 through 22.
Each person usually inherits one copy of every autosome from their biological mother and one from their biological father. The two copies of a particular chromosome are called homologous chromosomes. They contain genes for many of the same biological functions, although the versions of those genes can differ.
Autosomes carry most of the human genome’s genes. These genes influence an enormous range of characteristics and biological processes, including metabolism, development, immune function, cell structure, and the production of proteins and enzymes.
Because both males and females normally have the same 22 pairs of autosomes, autosomal genes generally follow inheritance patterns that do not depend on whether a person is male or female.
How autosomes are numbered
The autosomes are conventionally labeled 1 through 22, roughly in order of size, with chromosome 1 being the largest and chromosome 22 among the smallest.
The numbering does not mean that higher-numbered chromosomes are less important. Each chromosome contains many genes with diverse functions, and changes to any chromosome can potentially affect health or development.
A genetic condition caused by a variant in an autosomal gene is described as an autosomal disorder. Depending on the particular gene and variant, such a condition can follow dominant, recessive, or other inheritance patterns.
What are sex chromosomes?
Sex chromosomes are the chromosome pair involved in the biological systems that contribute to sex development and sex-related traits. In humans, the sex chromosomes are called X and Y.
Most people have either:
- XX
- XY
The X chromosome contains many genes that have functions unrelated to sex development. The Y chromosome is much smaller and contains fewer genes, including the SRY gene, which typically initiates a pathway of development toward testes during embryonic development.
The familiar XX/XY pattern is useful for understanding typical human development, but it does not describe every person’s chromosomes. Some people have chromosome patterns such as XXY, X, or XXX, among others. Differences in sex chromosomes can arise through changes in chromosome number or structure and can have varying effects.
Chromosomal sex is also not identical to every aspect of biological sex. Sex development involves chromosomes, genes, hormones, gonads, and anatomy, and these components can sometimes vary in ways that do not fit neatly into an XX-versus-XY description.
The key differences between autosomes and sex chromosomes
| Feature | Autosomes | Sex chromosomes |
|---|---|---|
| Human chromosome pairs | 22 pairs | 1 pair |
| Usual chromosome numbers | 1–22 | X and Y |
| Present in typical XX and XY individuals | Yes | The pair differs |
| Main genetic role | Carry most genes involved in general body functions and development | Carry genes involved in sex development as well as many other functions |
| Typical inheritance | One copy from each biological parent | Depends on the chromosome and the parent’s sex-chromosome complement |
| Examples of chromosome-related conditions | Trisomy 21 | Turner syndrome and Klinefelter syndrome |
The most important distinction is therefore not that autosomes “control the body” while sex chromosomes “control sex.” Both types carry genes involved in many aspects of human biology. The difference is primarily how the chromosomes are classified and how their inheritance and biological roles differ.
How autosomes are inherited
During the formation of eggs and sperm, a process called meiosis reduces the chromosome number by half. Each egg or sperm normally receives one chromosome from each autosome pair.
At fertilization, the egg and sperm combine their chromosomes, restoring the usual total of 46.
For example, a child receives one chromosome 1 from each biological parent, one chromosome 2 from each parent, and so on through chromosome 22. This produces two copies of each autosome.
Autosomal inheritance can therefore be described using familiar patterns. In an autosomal dominant condition, a disease-causing variant in one copy of a gene may be sufficient to cause the condition. In an autosomal recessive condition, a person generally must inherit disease-causing variants in both copies of the relevant gene.
These patterns concern genes located on autosomes, rather than the chromosomes themselves being inherently dominant or recessive.
How sex chromosomes are inherited
The inheritance of X and Y chromosomes differs from autosomes.
In the usual XX/XY system, an egg normally carries an X chromosome. Sperm normally carry either an X or a Y chromosome. If an X-bearing sperm fertilizes the egg, the resulting embryo is typically XX; if a Y-bearing sperm fertilizes the egg, it is typically XY.
This is why the father’s sex chromosome typically determines whether an embryo has an XX or XY chromosome complement in the conventional system.
X and Y chromosomes also have different inheritance patterns. A father with a Y chromosome passes it to his sons, not his daughters. A father passes his X chromosome to his daughters, while a mother typically passes one of her X chromosomes to each child.
These patterns are important in X-linked inheritance, in which a disease-causing variant occurs on the X chromosome. X-linked conditions can behave differently in people with one X chromosome than in those with two X chromosomes because the number and activity of X chromosomes differ.
Why X chromosomes behave differently
People with two X chromosomes do not generally produce twice as much of every X-linked gene product as people with one X chromosome. One major reason is X-chromosome inactivation.
Early in development, most cells with more than one X chromosome largely inactivate one X chromosome. The inactivated chromosome condenses into a structure called a Barr body. X-chromosome inactivation helps balance the expression of many X-linked genes between cells with one X chromosome and cells with multiple X chromosomes.
The process is not absolute: some genes on the X chromosome escape inactivation. This helps explain why having an unusual number of X chromosomes can sometimes affect development or health.
The Y chromosome does not undergo the same kind of chromosome-wide inactivation because it is structurally and genetically different from the X chromosome.
What happens when chromosome number changes?
Humans normally have two copies of each autosome and, in typical chromosome complements, one or two sex chromosomes. Sometimes an individual has an extra or missing chromosome. This is called aneuploidy.
An extra copy of an autosome is known as a trisomy. For example, trisomy 21, commonly called Down syndrome, results from having an extra copy of chromosome 21 in all or some cells.
Sex-chromosome aneuploidies include conditions such as Turner syndrome, usually associated with a single X chromosome, and Klinefelter syndrome, usually associated with an XXY chromosome complement. Their effects can differ substantially from those of autosomal trisomies because sex chromosomes have different gene content and because mechanisms such as X-chromosome inactivation affect gene expression.
Chromosome number can also vary between cells within the same person. This is called mosaicism and can produce a range of effects depending on which cells carry the chromosome difference.
Autosomal genes and sex-linked genes are not the same thing
A useful distinction in genetics is between autosomal inheritance and sex-linked inheritance.
If a gene is located on an autosome, its inheritance is generally described as autosomal. If it is located on the X or Y chromosome, it is described as sex-linked.
X-linked inheritance can produce characteristic family patterns because males with a single X chromosome have no second X copy of most X-linked genes. A disease-causing variant on their X chromosome can therefore have a direct effect even when the same variant might be recessive in a person with two X chromosomes.
Y-linked inheritance is much less common because relatively few genes are located on the Y chromosome. A Y-linked variant can be passed through the paternal line from a father to his sons.
Not every trait associated with sex is sex-linked. Many traits involved in sex development or sex-related biology depend on genes located on autosomes as well as on the sex chromosomes.
Why the distinction matters
The difference between autosomes and sex chromosomes is fundamental to understanding human genetics. It helps explain why some genetic conditions affect males and females at different rates, why certain variants follow recognizable patterns through families, and why changes in chromosome number can have different consequences depending on which chromosome is involved.
At the same time, the distinction should not be oversimplified. Autosomes are not simply “body chromosomes,” and sex chromosomes are not simply “sex chromosomes” in the sense of containing only sex-related genes. Autosomes and sex chromosomes both carry genes essential to normal development and everyday cellular function. Their differences in gene content, chromosome structure, inheritance, and regulation are what make the distinction biologically important.