Anautosome is anychromosome that is not asex chromosome.[1] The members of an autosome pair in adiploid cell have the samemorphology, unlike those inallosomal (sex chromosome) pairs, which may have different structures. TheDNA in autosomes is collectively known asatDNA orauDNA.[2]
For example,humans havea diploid genome that usually contains 22 pairs of autosomes and oneallosome pair (46 chromosomes total). The autosome pairs are labeled with numbers (1–22 in humans) roughly in order of their sizes in base pairs, while allosomes are labelled with their letters.[3] By contrast, the allosome pair consists of twoX chromosomes in females or one X and oneY chromosome in males. Unusual combinationsXYY,XXY,XXX,XXXX,XXXXX orXXYY, among other irregular combinations, are known to occur and usually cause developmental abnormalities.
Autosomes still contain sexual determinationgenes even though they are not sex chromosomes. For example, theSRY gene on the Y chromosome encodes the transcription factorTDF and is vital for male sex determination during development. TDF functions by activating theSOX9 gene onchromosome 17, so mutations of theSOX9 gene can cause humans with an ordinary Y chromosome to develop as females.[4]
All human autosomes have been identified and mapped by extracting the chromosomes from a cell arrested inmetaphase orprometaphase and then staining them with a type of dye (most commonly,Giemsa).[5] These chromosomes are typically viewed askaryograms for easy comparison. Clinical geneticists can compare the karyogram of an individual to a reference karyogram to discover the cytogenetic basis of certainphenotypes. For example, the karyogram of someone withPatau Syndrome would show that they possess three copies ofchromosome 13. Karyograms and staining techniques can only detect large-scale disruptions to chromosomes—chromosomal aberrations smaller than a few million base pairs generally cannot be seen on a karyogram.[6]
| Karyotype of human chromosomes | |
|---|---|
| Female (XX) | Male (XY) |
 |  |
| There are two copies of eachautosome (chromosomes 1–22) in both females and males. Thesex chromosomes are different: There are two copies of the X-chromosome in females, but males have a single X-chromosome and a Y-chromosome. | |
Autosomal genetic disorders can arise due to a number of causes, some of the most common beingnondisjunction in parental germ cells orMendelian inheritance of deleterious alleles from parents. Autosomal genetic disorders which exhibit Mendelian inheritance can be inherited either in anautosomal dominant or recessive fashion.[7] These disorders manifest in and are passed on by either sex with equal frequency.[7][8] Autosomal dominant disorders are often present in both parent and child, as the child needs to inherit only one copy of the deleteriousallele to manifest the disease. Autosomal recessive diseases, however, require two copies of the deleterious allele for the disease to manifest. Because it is possible to possess one copy of a deleterious allele without presenting a disease phenotype, two phenotypically normal parents can have a child with the disease if both parents are carriers (also known asheterozygotes) for the condition.
Autosomalaneuploidy can also result in disease conditions. Aneuploidy of autosomes is not well tolerated and usually results in miscarriage of the developing fetus. Fetuses with aneuploidy of gene-rich chromosomes—such aschromosome 1—never survive to term,[9] and fetuses with aneuploidy of gene-poor chromosomes—such aschromosome 21— are still miscarried over 23% of the time.[10] Possessing a single copy of an autosome (known as a monosomy) is nearly always incompatible with life, though very rarely some monosomies can survive past birth. Having three copies of an autosome (known as a trisomy) is far more compatible with life, however. A common example isDown syndrome, which is caused by possessing three copies ofchromosome 21 instead of the usual two.[9]
Partial aneuploidy can also occur as a result ofunbalanced translocations during meiosis.[11] Deletions of part of a chromosome cause partial monosomies, while duplications can cause partial trisomies. If the duplication or deletion is large enough, it can be discovered by analyzing a karyogram of the individual. Autosomal translocations can be responsible for a number of diseases, ranging fromcancer toschizophrenia.[12][13] Unlike single gene disorders, diseases caused by aneuploidy are the result of impropergene dosage, not nonfunctional gene product.[14]
