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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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 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.
1. What are the Types of Chromosomes?
Ans: There are two types of chromosomes that are found in the human body. They are autosomes and sex-chromosomes or allosomes. Autosomes are the chromosomes that are different from the sex chromosomes. The sex chromosomes are the ones that play a significant role in the formation of biological sex which is a genetic combination derived from the chromosomes of parents. The sex chromosomes are different in a male and female and are typically identified as a combination of the X chromosome with the pairing of either X or Y chromosome. In either case, a sex chromosome pair is different between a male and a female. On the other hand, the autosomes are the ones that are common to both sexes and are commonly found in both the male and the female. There is as such no distinction between the chromosomes apart from general genetic diversity except in the cases of disorders. But the functioning of the genes that are found in chromosomes does not necessarily define particular sex although they may hold a regulatory control over those genes that have significance in the defining of the sexual characteristics.
2. What is the Role of Autosomal Chromosomes?
Ans: There is a significant difference between autosomes and sex chromosomes. The chief role of the autosomal chromosome is associated with the various functions of metabolism of any cell except the sex determination. The autosomes are the ones that are usually present in the somatic cells and are singly in gametes. Thus, the role of the autosomal chromosomes is mainly in conducting and defining the functions of the body. The autosomal chromosomes can play a regulatory role in the genes that are responsible for determining the sex of a person or determination of any of the sexual characteristics. The role of autosomal chromosomes thus is more in the sense of daily functions that are common to both the males and females of a species.
3. Do Females Have 46 Autosomes?
Ans: A normal cell in a human being consists of 23 pairs of chromosomes in total 46 chromosomes. Of these 22 pairs, the autosomes do not play any role in the determination of the sex of a person. The sex is determined by the remaining one pair of chromosomes that are known as the sex chromosomes and is different in both the males and females. Hence, a female has 22 pairs or 44 autosomal chromosomes. Because these autosomal chromosomes are for the normal bodily and cellular functions and are so equally important for both the males and females and are present in both of them irrespective of the sex. Only in some cases of disorders, there can be a difference in the number of autosomal chromosomes from the typically normal human being.