An autosome is a non-sex chromosome. If you imagine the complete set of human chromosomes as a 23-volume encyclopedia containing all the instructions for building and operating a human body, the autosomes would be the first 22 volumes. These volumes hold the vast majority of our genetic information, dictating everything from height and eye color to the intricate functions of our cells.
Each human cell contains 46 chromosomes organized into 23 pairs, a collection geneticists refer to as our karyotype. The autosomes comprise 22 of these pairs, while the final pair consists of the sex chromosomes. These autosomal pairs are numbered 1 through 22, in descending order of size, with chromosome 1 being the largest. This numbering system provides a universal map for locating specific genes.
The primary distinction between autosomes and sex chromosomes lies in their functions. The 22 pairs of autosomes carry the genetic code for most of an individual’s traits and biological processes, from metabolic functions to physical features. In contrast, the 23rd pair, the sex chromosomes, determines an individual’s biological sex. A combination of two X chromosomes (XX) results in a female, while an X and a Y chromosome (XY) combination results in a male.
Autosomes Versus Sex Chromosomes
Autosomes are numbered 1 through 22, largely based on their size in base pairs. The members of each autosomal pair in a diploid cell are homologous, meaning they have the same size, shape, and carry genes for the same traits in the same locations. In contrast, the sex chromosomes in males (XY) are not homologous, as the X and Y chromosomes differ significantly in size and the genetic information they carry. Females have a homologous pair of XX chromosomes.
The genetic information housed within the autosomes is collectively referred to as autosomal DNA (atDNA). This atDNA contains tens of thousands of genes that code for proteins and RNA molecules essential for cellular functions, metabolism, growth, and development. These genes are responsible for building and maintaining the body’s structures and systems.
While sex chromosomes have a specialized role in sex determination, autosomes also contain genes that influence sexual development. For example, the SRY gene on the Y chromosome initiates male development by activating the SOX9 gene, which is located on autosome 17. This demonstrates that the development of complex traits often involves an interaction between genes on both autosomes and sex chromosomes.
The Role of Autosomes in Inheritance
The mechanism of inheritance for autosomes ensures that offspring receive a combination of genetic material from both parents. For each of the 22 autosomes, an individual inherits one copy from their mother’s egg and one from their father’s sperm. This results in 22 homologous pairs, with each pair containing one paternal and one maternal chromosome. This pairing is a foundation of diploid organisms, including humans.
This system of biparental inheritance is the basis for genetic diversity. During the formation of sperm and egg cells, a process called meiosis “remixes” the DNA between the two copies of each autosome. The result is a unique set of chromosomes that contains a mixture of genetic material from both of the individual’s parents. When these gametes combine during fertilization, they create a genetically distinct individual.
The genes located on these autosomes come in different versions, or alleles. An individual can inherit two identical alleles or two different alleles for a single gene. The interaction between these alleles determines how a trait is expressed. For example, in the case of earlobes, the allele for unattached earlobes is dominant, while the allele for attached earlobes is recessive. A person only needs to inherit one copy of the dominant allele from either parent to have unattached earlobes.
To have attached earlobes, an individual must inherit two copies of the recessive allele, one from each parent. This illustrates the principles of Mendelian inheritance as they apply to autosomal traits. This pattern of dominant and recessive inheritance governs thousands of characteristics, from simple physical features to more complex biological functions, all encoded on the 22 pairs of autosomes.
Autosomal Disorders
Abnormalities in the number or structure of autosomes can lead to a variety of genetic conditions. These disorders are broadly categorized into numerical and structural abnormalities. The study of these chromosomal changes is a field known as cytogenetics, which often involves analyzing an individual’s karyogram to identify large-scale disruptions.
Numerical abnormalities, or aneuploidy, occur when a person has an incorrect number of autosomes. This is the result of nondisjunction, an error during cell division where chromosomes fail to separate properly into sperm or egg cells. One of the most recognized examples of autosomal aneuploidy is Trisomy 21, also known as Down syndrome. Individuals with Trisomy 21 have three copies of chromosome 21 instead of the usual two. Another example is Patau Syndrome, which is caused by having three copies of chromosome 13.
Structural abnormalities involve changes in the physical structure of an autosome. These can include deletions, where a segment of a chromosome is missing; duplications, where a part of a chromosome is repeated; or translocations, where a piece of one chromosome breaks off and attaches to another. The effects of these structural changes can vary widely depending on the specific genes involved and the size of the chromosomal segment that is altered.
These conditions are referred to as autosomal disorders because the affected gene or chromosome is one of the 22 autosomes. Autosomal disorders can be inherited from parents or can occur spontaneously. Dominant disorders require only one copy of an altered gene to cause the condition, while recessive disorders require two copies. Understanding the nature of these autosomal changes helps in diagnosing and managing the associated health conditions.