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Chromosome diseases

Definition

Genetic diseases are caused by DNA alteration and they are classified in chromosome diseases if there is an abnormality in the number or chromosome’s structure (Down syndrome, Turner syndrome, etc.), monogenic diseases, if there is an alteration in a single gene (cystic fibrosis, fragile X syndrome) or multifactorial polygenic diseases, due to the interaction of more modified genes with the environmental factors (cardiovascular diseases, diabetes,etc.).

What is a chromosome?

Long DNA strands, inside cell nucleus, twist in oneself such as a spiral staircase,
until to compact in a structure, called chromosome (fig.1).

Fig.1 – Struttura di un cromosoma

Fig.1 –Chromosome structure

Fig.2 – Cromosomi al microscopio


Fig.2 – Metaphasic chromosomes


In these strands, there are written the "words" (genetic code) encoding for cell function.
In all of of the human cells, there are 46 chromosomes, and through the microscope they appear in a casual order (fig.2). It’s possible, using an appropriate computer program, put them in order by form and dimension: the orderly set is named karyotype.
Chromosomes are split in 23 copies, 22 are make up by two identical chromosome, called
autosomes (numbered from 1 to 22), one from paternal origin and the other from maternal
origin; the 23th copy it's instead formed by sexual chromosomes, different in the
male (XY) (fig.3) and equal in the female (XX) (Fig.4).

Fig.3 - Cariotipo maschile (46,XY): 22 coppie di autosomi e una coppia di cromosomi sessuali, XY.


Fig.3 - Male karyotype (46,XY): 22 couples of autosomes and one couple of sexual chromosomes, XY.

Fig.4 - Cariotipo femminile (46,XX): 22 coppie di autosomi e una coppia di cromosomi sessuali, XX.

 

Fig.4 - Female karyotype (46,XX): 22 couples of autosomes and one couple of sexual chromosomes, XX.

Which is the aim of a karyotype?

Each chromosome, undergone to opportune process of the cytogenetics, a genetic topic that study the chromosomes, assumes a particular aspects which is given from the light and dark zone sequence (banding). The banding pattern is equal between two chromosomes of a couple, but different among chromosomes of different couples (Fig. 3-4). Banding is essential to recognize the chromosomes. The karyotype analysis (or chromosomal map) is helpful to identify individual chromosome aberrations; sometime this exam have a limit so some aberrations (little deletions, genetic mosaic in a low percentage, cryptic chromosome traslocations) can't be identified. Our laboratory performs analysis of prenatal and postnatal cytogenetics.

Molecular karyotype

Thanks to the collaborations with the other centers it’s possible carry out a “molecular Karyotype” through the Array-CGH technique (Array Comparative Genomic Hybridization) in the prenatal and postnatal diagnosis, to allow a detailed chromosomal exam and underline any anomalies not detectable with the standard karyotype.

How are classified chromosomal diseases?

In the population there are people (1% live births) afflicted by pathologies associated by chromosomal alterations. The chromosomal anomalies are classified in:
numerical
structural