Genetic factors

Genetic disorders can be caused by defects in single genes or through chromosomal aberrations.
Single-gene defects arise through mutations. The defective genes become active when they are inherited in an autosomal dominant manner. In autosomal recessive inheritance, they are expressed only if they are present on both homologous chromosomes. Recessive X-chromosomal disorders affect only males.
Chromosomal aberrations are very diverse and common. They can be divided into the following forms: supernumerary or missing chromosomes (aneuploidy); breakage or loss of a piece of a chromosome (deletion); transfer of broken fragments between two chromosomes (translocation); abnormal splitting of the centromere during mitosis, which leads to a loss of one arm and to duplication of the other arm of the chromosome (formation of an isochromosome); chromosomal mosaicism.
Aneuploidy (missing or supernumerary chromosomes) is the clinically most important chromosomal aberration. It occurs when homologous pairs of chromosomes fail to separate at the anaphase of maternal meiosis (non-disjunction). The frequency of this condition increases as the mother gets older.
In monosomy, a particular chromosome is singular. All the others are diploid (example: Turner Syndrome). In trisomy, a particular chromosome is tripled while all the others are diploid (example: Klinefelter syndrome). Trisomies are the most frequent chromosomal anomalies and are the most common cause of prenatal death. Infants with trisomy of chromosome 13, 18 and 21 can survive after birth.