An autosome is any chromosome that is not a sex chromosome. In people, the autosomes carry nearly all of the genetic instructions. Because males and females have the same autosomes, a trait or condition located on one can pass to a son or a daughter with the same odds. So when “autosomal” appears on a test result, it mostly tells you how something is inherited, not how serious it is.
How Many Autosomes Humans Have and How They Are Numbered
A typical human cell holds 22 pairs of autosomes and one pair of sex chromosomes.1National Human Genome Research Institute. Karyotype That makes 44 autosomes and 46 chromosomes in total. Each pair has one chromosome from your mother and one from your father. Doctors check this layout with a karyotype, a picture of a person’s chromosomes lined up in pairs.2National Human Genome Research Institute. Karyotype
On a report, the chromosomes are photographed and then arranged by size into numbered pairs. A typical result is written 46,XX or 46,XY. The number counts all the chromosomes, and the letters name the sex chromosomes. If an autosome is changed, the change is written after that, using the chromosome’s number.
The pairs are numbered 1 to 22 roughly in order of size, from largest to smallest.3Wikipedia. Autosome Chromosome 1 is the biggest and carries the most DNA. The order is only approximate, though: chromosome 21 is actually a little smaller than 22.
Autosomes hold the instructions for building and running most of the body. Genes for eye color, blood type and most diseases sit on them. Each gene usually comes in two copies, one on each chromosome of a pair, which is why one faulty copy is often covered by the other.
Size says little about importance. Some small autosomes carry many genes while some large ones carry relatively few.4PubMed Central. Gene-dense autosomal chromosomes show evidence for increased selection No pair is spare, and a change on a small chromosome can affect the body as strongly as one on a large chromosome.
Autosomes Versus Sex Chromosomes
Females usually have two X chromosomes, and males usually have one X and one Y.5National Human Genome Research Institute. Karyotype Autosome pairs, by contrast, look the same and carry the same genes in everyone. So a male and a female differ only in the sex chromosome pair, not in their 22 autosome pairs. Many traits that differ between the sexes, such as height, still involve genes on autosomes, which both sexes carry.
The line between the two kinds is not perfectly sharp. Small tips of the X and Y match each other and swap pieces like autosomes do.6PubMed Central. The Human Pseudoautosomal Region (PAR): Origin, Function and Future Genes in these tips are therefore not passed down strictly along sex lines.7PubMed Central. The Human Pseudoautosomal Region (PAR): Origin, Function and Future
The difference matters most for inheritance. A father has one X and one Y, so each child gets one or the other depending on which sperm fertilizes the egg. All his daughters receive his X, and all his sons receive his Y. He therefore cannot pass an X-linked condition to a son, and conditions on the X often affect boys more. Autosomes work differently: a father passes one copy of each pair to every child, son or daughter, with the same odds.
What Autosomal Dominant Inheritance Means
In an autosomal dominant condition, a single changed copy of a gene is enough to cause the condition.8Genetics Principles. Haploinsufficiency vs Dominant-Negative Sometimes the one working copy makes only about half the normal amount of a needed protein, and half is not enough.9Genetics Principles. Haploinsufficiency vs Dominant-Negative Other times the faulty protein actively interferes with the normal one, dropping function below half.10Genetics Principles. Haploinsufficiency vs Dominant-Negative The same label can therefore come from different underlying problems, which can affect severity and treatment.11Genetics Principles. Haploinsufficiency vs Dominant-Negative
An affected parent with one changed copy passes it to each child with a 50% chance, because each child gets one of the parent’s two copies at random. That is why the condition often appears in every generation of a family.
The 50% figure is easy to misread. It applies to each pregnancy separately, so a family can have three affected children or none by chance. “Dominant” does not mean common or severe, only that one copy is enough. A child who did not inherit the changed copy cannot pass it on.
In a family tree, an affected person usually has an affected parent, and affected people appear in successive generations. Father-to-son passing can happen, which rules out an X-linked pattern. A skipped generation can still happen, since some people carry the change but show few or no signs.
What Autosomal Recessive Inheritance Means
A recessive condition appears only when someone inherits two changed copies, one from each parent.12Research and Reports on Genetics. Understanding Autosomal Recessive Inheritance: Unraveling the Genetic Tapestry One working copy usually covers for the changed one. A person with one changed copy is a carrier, usually without symptoms, who can pass the change on.13Research and Reports on Genetics. Understanding Autosomal Recessive Inheritance: Unraveling the Genetic Tapestry
Carriers are not always entirely unaffected. Some show mild versions of the condition they carry.14PubMed. Carriers of autosomal recessive conditions: are they really ‘unaffected?’ Carriers of the metabolic disorder PKU, for example, can have raised blood levels of phenylalanine, especially after protein-rich meals.15PubMed. Carriers of autosomal recessive conditions: are they really ‘unaffected?’
When both parents are carriers, each child has a 1 in 4 chance of being affected, a 1 in 2 chance of being a carrier, and a 1 in 4 chance of neither. That is why recessive conditions can appear suddenly in a family with no known history.
Carrier screening before or during pregnancy can bring this to light. A blood or saliva test checks carrier status for common recessive conditions. If one partner is a carrier, the other is usually tested next, and if both are, prenatal testing options can be discussed. Many carriers only learn their status through screening like this.
Relatives having children together raises the risk, because relatives are more likely to share the same changed copy. That raises the chance both partners are carriers. The risk is real, but most children of cousins are healthy.
What Happens When There Is an Extra or Missing Autosome
Most wrong autosome counts lead to miscarriage.16Wikipedia. Aneuploidy These changes are found in about 27 to 30% of miscarriages.17The Principles of Clinical Cytogenetics. Autosomal Aneuploidy An autosome imbalance is far more harmful than an extra or missing sex chromosome, which helps explain why so few survive.18Wikipedia. Aneuploidy
When a chromosome finding explains a miscarriage, it does not usually point to a problem in either parent. Most of these errors happen by chance when the egg or sperm forms. The risk of extra chromosomes does rise with the mother’s age, and doctors may test the parents after repeated losses.
The most common extra autosomes in live births are 21, 18 and 13.19Wikipedia. Aneuploidy An extra chromosome 21 causes Down syndrome, 18 causes Edwards syndrome, and 13 causes Patau syndrome. Each is a trisomy, meaning three copies of a chromosome instead of two. These three are the ones most often seen in live births, partly because they carry relatively few genes, though Edwards and Patau syndromes are usually life-limiting. An extra copy means every cell carries three copies of each gene on that chromosome rather than two, which disturbs the balance the body expects.
When Autosomes Join or Swap Pieces
About 1 in 800 people carry a Robertsonian translocation, where two small autosomes are fused.20PubMed Central. The formation and propagation of human Robertsonian chromosomes The person usually has 45 chromosomes but most of the needed genetic material. Even so, it can cause infertility or a child with an extra chromosome, such as some cases of Down syndrome.21PubMed Central. The formation and propagation of human Robertsonian chromosomes
“Balanced” and “unbalanced” are the key words here. In a balanced translocation, all the genetic material is present but rearranged, so the carrier is usually healthy. An unbalanced one has extra or missing material, which is what can cause problems in a pregnancy or child.
Other swaps happen too. In a reciprocal translocation, two chromosomes swap pieces and no material is lost. Unlike the Robertsonian type, this can involve almost any pair of autosomes. Larger autosomes are involved more often, simply because they offer more places to break.22PubMed. Influences of chromosome size, gene density and nuclear position on the frequency of constitutional translocations in the human population The carrier has the usual 46 chromosomes, so it may only be found on a chromosome picture.
Genetic counseling explains your specific pregnancy risks. IVF with embryo testing is one option some carriers use.
How Autosome Problems Are Detected
Karyotyping and chromosomal microarray are the main diagnostic tests for chromosome problems, including before birth.23ARUP Consult. What Is an Autosome? Definition, Function, and Inheritance A karyotype can be made from blood, cord blood, bone marrow, amniotic fluid or tissue.24IntechOpen. Background, Diagnosis, Types, Management/Prevention and Implications of Chromosomal Abnormalities It cannot see very small missing or extra pieces, so molecular tests such as microarray and FISH are used to find those smaller changes.25PubMed. Gene dosage methods as diagnostic tools for the identification of chromosome abnormalities
To make a karyotype, cells are grown in the lab for a few days and stopped while dividing, when chromosomes are most visible. A technician photographs them and arranges them by size and banding pattern. That growing step explains why results take days to weeks, not hours.
What you experience depends on the setting. In pregnancy, samples come from amniocentesis or placenta sampling. Adults usually give a blood sample. A screening blood test in pregnancy estimates risk but does not diagnose.
Which test you are offered depends on the question. A karyotype is chosen when a translocation is suspected, while microarray is often first for developmental delay or birth differences. Your doctor may order both.
What Autosomal DNA Means on an Ancestry Test
Autosomal DNA tests give a fuller view of ancestry than Y-chromosome or mitochondrial tests, because those follow only a single line, father-to-son or mother-to-child.26MedlinePlus. What is genetic ancestry testing? Autosomal tests read DNA from all your ancestral lines, so they can match you with cousins on both sides of your family.
The company compares your DNA with that of other customers and reports shared stretches. The amount shared is used to estimate how closely you are related, such as a first cousin or a more distant one. A match tells you that you share ancestors, but not which relative links you.
Matches fade with distance because autosomes shuffle pieces each generation when eggs and sperm form. Shared DNA roughly halves with each generation, so close relatives share long matching stretches and distant ones share short ones.
The results have limits. Each parent passes on a random half, so siblings get different results. Distant ancestors may leave little or no DNA in you. Ethnicity estimates depend on the company’s reference groups, and these tests are not medical diagnoses.