Autosome

An autosome is any chromosome other than the sex chromosomes. For example, the human genome has 24 chromosomes, including 22 autosomes plus the X- and the Y-chromosomes. This is described as the haploid set. If a cell contains a complete haploid set of chromosomes, it is described as euploid. If the chromosome set is altered by duplication or deletion then the cell is aneuploid. If a cell contains two sets of genomes per nucleus, it is diploid, and if it contains more than two, it is described as polyploid.

Organisms that have a largely vegetative existence, such as some fungi and algae, have mainly haploid cells. In metazoans the gametes, such as the spermatozoa and oocytes in humans, are haploid. Sexual reproduction involves the fusion of two haploid gametes to form a diploid egg in an animal. The vast majority of the cells in an animal that develop from an egg are diploid. Thus there are two active copies of each autosome in most animal cells. In plants, a polyploid state is common where the number of genomic copies in somatic cells may range from 3 to 10 (1). The number of genomic copies can actually be polymorphic within a particular species (2). Thus plants are relatively insensitive to the number of copies of a particular gene in a given cell. This is not the case with animals, where somatic cells are almost always diploid. The only exceptions are those animals with parthenogenetic life styles in which either males or females develop as a default state from an unfertilized egg. In general, animal cells cannot accommodate more than two copies of a particular chromosomal region.

Aneuploidy occurs when parts of chromosomes or whole chromosomes are absent from the genome. In humans the nondisjunction of homologous chromosomes during meiosis or of sister chromatids during mitosis can lead to aneuploidy. When a chromosome is in excess of the normal euploid number, the condition is called hyperploidy. For example, a hyperploid state in humans occurs when there are three copies of a particular chromosome, a condition known as trisomy. All human autosomes except chromosome 1 have been found as trisomies (3). Most trisomic embryos are spontaneously aborted, but live births occur with trisomies of chromosomes 13, 18, and 21 (4). The most common human trisomy to reach term is that of chromosome 21, resulting in Down's syndrome. These children are mentally retarded, predisposed to cancer and have a shortened life span. Presumably the deleterious effects of three copies of chromosome 21 follow from an imbalance in the expression of one or several sets of genes on chromosome 21 relative to the other genes on the other chromosomes in the cell.

 Experiments in Drosophila melanogaster indicate that changes in the dosage of a single gene might be tolerated, but a deletion that includes several genes or a large section of a chromosome is likely to cause sterility (5). Surprisingly in trisomies of D. melanogaster involving whole chromosome arms, many of the duplicated autosomal genes express only diploid levels of gene product. The expression of genes outside the duplicated region is also affected, however, indicating the pleiotropic effects of altering gene copy number. An interesting aspect of sexual reproduction requiring the use of two distinct haploid gametes is that there are two copies of each autosome in a diploid cell, whereas there are be one or two copies of each sex chromosome. Dosage compensation is the phenomenon whereby the transcriptional activity of genes on the sex chromosome is adjusted to similar levels independent of the number of copies, so that it is constant relative to the activity of a gene on an autosome.

Humans, again have two X-chromosomes in one sex (female) and one X-chromosome in the other (male). To achieve dosage compensation, an entire X-chromosome is silenced in female diploid cells. This process is called X-chromosome inactivation and results in the formation of the Barr body. The capacity to monitor the activity of genes on autosomes relative to the sex chromosomes has facilitated the discovery of novel mechanisms of regulating genes at the level of whole chromosomes.

References

1. J. E. Averett (1980) "Polyploidy in plant taxa: summary". In Polyploidy (W. H. Lewis, ed.,( Plenum, New York. 

2. K. Irifune (1990) J. Sci. Hiroshima Univ. 23, 163–181. 

3. T. J. Hassold (1986) Trends Genet. 2, 105–109. 

4. E. B. Hook, B. B. Topol, and P. K. Cross (1989) Am. J. Hum. Genet. 45, 855–861. 

5. D. L. Lindsley et al. (1972) Genetics 71, 157–184.