In the genetics or biology of chromosomes, autosomes are chromosomes that are not sex chromosomes. When a pair of autosomes is considered in the diploid cell, they have the same morphology. The collective DNA that is present in the autosomes is known as atDNA or auDNA. For example, in the diploid genome of human beings, the number of pairs of autosomes in humans is 22 pairs of autosomes and one allosome pair hence taking the total number of autosomes in humans along with allosomes to 46. The pairs of the autosomes are typically labelled with the numbers (1-22 in human beings), and the numbering is based on the order of their sizes. They are labelled with letters for identification. The autosomes and sex chromosomes are different as the sex chromosomes consist of two X chromosomes in females and one X and one Y chromosome in males. The following image shows the chromosomes and depicts the clear distinction between what are autosomes and allosomes:
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General Characteristics of Autosomes
It is clear from the introduction, what are autosomes. Although the autosomal definition states autosomes and sex chromosomes or allosomes are different, they still contain some of the sexual determination genes. An example of this includes the SRy gene which is present on the Y chromosome and is responsible for the encoding of the TDF transcription factor that has been found to be vital for the determination of male sex during development. But the functions of the TDF factors are activated by the SOX9 gene which is present on chromosome 17 and its impact is observable in cases when mutations in the SOX9 gene causes humans with ordinary Y chromosomes to develop as females.
When understanding what are autosomes it is necessary to know how they are identified. All of the human autosomes and sex chromosomes have been identified and have been mapped by the extraction of the chromosomes from a cell that was arrested in the metaphase or prometaphase of the cell cycle. The chromosomes were then stained with a type of dye which is most commonly known as the Giemsa stain. For easy comparison, the chromosomes are normally viewed as karyograms. Clinical geneticists usually compare the karyogram of one individual with a reference karyogram in order to discover the cytogenetic basis of certain phenotypes. For example, the karyogram of a patient with Patau Syndrome typically shows three copies of chromosome 13. The drawback of the karyogram techniques is that they can only detect large-scale disruptions to the chromosomes as any of the chromosomal aberrations that are smaller than a few million base pairs are generally not seen in a karyogram.
The Genetic Disorders of Autosomes
The genetic disorders arising out of the autosomes can be because of a number of reasons such as the common reason being the nondisjunction in parental germ cells or the Mendelian inheritance of some of the deleterious alleles from the parents. The genetic disorders of autosomes that follow the rules of Mendelian inheritance can be inherited either in a dominant or recessive manner. The frequency of such disorders being manifested is equal in frequency in both males and females. The autosomal dominant disorders are usually found in both the parent and their child because to inherit the disease the child only needs to inherit a single copy of the deleterious allele and also to express it. But recessive forms of autosomal disorders need two copies of the deleterious allele in order for the manifestation or expression of the disease. Even in certain cases when the disease is not shown by the parents but can manifest in the child if both the parents are careers (or heterozygous) i.e. both of them carry one copy each of the deleterious gene or the mutant version which can come together in the child.
An image is given below that shows the autosomal recessive inheritance from parents:
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Sometimes in conditions of autosomal aneuploidy i.e. the condition where there is an abnormal number of chromosomes, can result in conditions of disorder and diseases. In such a condition the number of chromosomes is different from the normal two sets which form a pair as per the autosomal definition. The aneuploidy of the autosomes is not well tolerated and usually results in miscarriage of a developing fetus. This is clear from the incidences that show that fetuses with aneuploidy of chromosomes that are gene-rich such as chromosome 1 usually never live to a term and fetuses with aneuploidy even in the gene-poor chromosomes like chromosome 21 are miscarried for more than 23% of the time. In conditions of monosomy, the presence of only a single copy of an autosome, there are very few and rare chances of survival past birth but having three copies which are known as trisomy is more compatible with life and although a disorder has chances to survive beyond birth. The most common example of trisomy is Down's syndrome in which there are three copies of chromosome 21 instead of the normal two.
The abnormality of chromosome number from the normal one as per the autosomal definition can also be the result of unbalanced translocations during the cell division cycle of meiosis. The deletions in part of a chromosome can cause some partial monosomies, and on the other hand, duplication can cause parital trisomies. Whenever the duplication or the deletion is significant enough, it is usually detected by the analysis of karyograms of an individual. The translocations of the autosomes are responsible for a large number of diseases that range from cancer to schizophrenia. Unlike the single-gene disorders, these diseases are caused because of the abnormal number of chromosomes and hence arise from improper gene dosage or non-functional gene products.