Concept of Autosome
The term Autosome (or somatic chromosome) refers to any of the chromosomes that make up the genetic heritage of a living being, with the exception of sex chromosomes (also called heterosomes or gonosomes) (Fig.1).
Each species has a characteristic number of chromosomes. In the case of humans, the normal karyotype (i.e. the set of chromosomes arranged according to different parameters) consists of 23 pairs of chromosomes: 22 pairs of autosomes, identical copies of each other in all individuals, and a pair of heterosomes different depending on the sex of the individual (XX in females and XY in males), each pair consisting of a chromosome of maternal origin and another of paternal origin.
Figure 1. Normal male karyotype
Classification of chromosomes
The classification of chromosomes is done according to two main characteristics:
The chromosome size (large, medium and small) and the centromere index:
Metacentric chromosomes with a centromere positioned centrally and arms of equal size;
Submetacentric chromosomes with a centromere slightly far from the centre and arms slightly different in size;
Acrocentric chromosomes with a centromere far from the centre and asymmetrical arms (one long and one short);
Telocentric chromosomes with a centromere very close to the telomeres (this type of chromosomes does not exist in humans).
Aside from the size and the centromere index, banding techniques are also used, allowing the identification of each pair of chromosomes.
Human chromosomes are organized in decreasing order of size and classified into 7 groups:
Group A: chromosomes 1, 3 (big metacentrics) and 2 (big submetacentric)
Group B: chromosomes 4 and 5 (big submetacentrics)
Group C: chromosomes 6 to 12 and X chromosome (medium meta and submetacentric)
Group D: chromosomes 13, 14 and 15 (big acrocentrics)
Group E: chromosomes 16, 17 and 18 (small meta and submetacentrics)
Group F: chromosomes 19 and 20 (very small metacentric)
Group G: chromosomes 21, 22 and Y chromosome (small acrocentrics)
Chromosomal abnormalities
Whether autosomes or sex chromosomes may show abnormalities.
Chromosomal abnormalities are numerical anomalies (aneuploidies), when there is a change in the number of copies of chromosomes that results from a non-disjunction of homologous chromosomes either in mitosis or meiosis divisions, or structural anomalies, when there is an internal rearrangement of genetic material on the chromosome.
The more frequent viable autosomal aneuploidies are the trisomies (2n +1) of chromosomes 13, 18 and 21 (Patau, Edwards and Down syndromes, respectively).
Structural anomalies can affect one or more chromosomes simultaneously.
The types of anomalies that occur in a single chromosome are:
Pericentric inversion: When a break occurs on either side of the centromere and inversion of the fragment;
Paracentric inversion: Two breaks occur in the same chromosome arm and inversion of the fragment;
Deletion: Loss of a chromosome fragment whether of a terminal portion or of an intermediate portion;
Duplication: Duplication of a chromosome fragment which can be direct if the fragment is in the same sense or reversed, otherwise;
Ring chromosome: As the name implies, it’s the formation of a circular chromosome after a break in the telomeric region followed by fusion of the two arms;
Structural abnormalities that occur in more than one chromosome are:
Reciprocal translocation: An exchange of fragments occurs between two non-homologous chromosomes (chromosomes from different pairs);
Robertsonian translocation: This is a type of translocation which happens exclusively between two acrocentric chromosomes (13, 14 15, 21, 22 and Y chromosome), consists in the loss of the short arms and fusion of the chromosomes;
Insertion: There is an insertion of a chromosome fragment inside of another chromosome.
References:
- Azevedo, C. (1999). Biologia Celular e Molecular. 3rd ed. Lisboa: Lidel. p125-127.
- Read, A. & Donnai, D. (2008). Génétique médicale: de la biologie à la pratique. Paris: De Boeck Supérieur. p27-56.
