Autosomes
Autosomes are all chromosomes other than the sex chromosomes. In humans, all chromosomes, except the X and Y chromosomes are autosomes.
Normal human cells carry 44 autosomal chromosomes in 22 corresponding (homologous) pairs. In humans, the homologous autosomal pair is comprised of one autosome derived from mother, and the corresponding chromosome contributed by the father. Homologous autosomal chromosomes contain similar, but not identical genes. Although normal homologous autosomes contain the same genes (e.g., the gene for a particular protein) each may carry a different form of the gene (allele).
In addition to autosomes, normal human cells carry a pair of sex chromosomes. Females carry a pair of X chromosomes and males carry an X and Y chromosome).
During typical karyotype analysis, the autosomal pairs are ordered and numbered, from 1 to 22, in descending order of length (i.e., the longest homologous pair is labeled pair 1 and the shortest are labeled pair 22). Scientists refer to autosomal chromosomes by their placement in this grouping. According, in normal human cells there are two "chromosome 1" chromosomes, two "chromosome 2" chromosomes, etc. Autosomes are further divided into groups based upon other gross morphological characteristics such as the position of the centromere, and the position and size of bands that result from staining.
In diploid species, the autosomal set of chromosomes consists of all non-sex chromosomes. During meiosis in diploid organisms, including humans, the homologous autosomal chromosomes align and allow a crossing over of genetic material that causes a reassortment of maternal and paternal genes (genes from the mother and father). This reassortment of genes in autosomal chromosomes leads to specific forms of inheritance that can be predicted by Mendellian genetics. There are several different types of autosomal inheritance, including autosomal dominant inheritance and autosomal recessive inheritance.
The number of autosomes depends upon the species. Drosophila melanogaster cells contain three autosomes (a fourth pair of sex chromosomes), some species of grasshoppers carry 11 pairs of Autosomes, and humans 22 pairs. One theory of sex determination termed geneic theory assets that the ratio of the number of X chromosomes to the number of autosomes that determine sex in some species (e.g., Drosophila melanogaster).
Autosomally inherited diseases are inherited through the non-sex chromosomes. Correspondingly, sex chromosomes transmit sex-linked diseases. Approximately half of genetic diseases and abnormalities involve autosomes. Autosomal trisomy, for example, occurs whenever a gamete participating in fertilization carries an extra autosomal chromosome. Although most autosomal abnormalities result in a lack of complete embryological development (fetal death), some trisomes may result in genetic defects. Perhaps the best known example of live birth, autosomal trisomy is Down syndrome, where each cell carries three copies of chromosome 21.
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