Each cellin human body has 46 chromosomes, 44 of which are always in pairs – 22 pairs of similar form and length. In female, the chromosomes in chromosome pair 23(the sex chromosome) are also the same – the so called Х-chromosomes. In male, this pair consists of dissimilarchromosomes – one Х and oneY chromosome. The mature sex chromosomes containonly onechromosomeofeach pairor a total of 23 chromosomes in the course of their maturity. The egg cell has always only onesex chromosome – the Х-chromosome. The mature sperm cell containsone Х or one Y sex chromosome. When the sex cells pair up during reproduction, again a full set of 46 chromosomes is formed.

What is trisomy?

Trisomy is an abnormality in the number of human chromosomes, where during reproduction 3 instead of 2 chromosomes appear in one chromosome pair. In humans, the carriers of chromosomal numerical disorders, the trisomic cell contains 47 chromosomes.

The most frequent trisomy types

Patau’s Syndrome (Trisomy 13)

Patau’s syndrome is a genetic abnormality, caused by the presence of an extra chromosome in the 13th chromosome pair. This disorder is associated with problematic child development, as the most common manifestations are serious mental retardation and physical deformations in different parts of the body. The characteristic anatomical abnormalities of the babies affected by the Patau’s Syndrome often facilitate the diagnosis, e.g. too small circumference of the head (microcephaly), small eyes (microphthalmia), and the common retinal dysplasia. About 60% of the babies have the so called cleft lip, as well as other anatomic defects.

Edward’s syndrome (Trisomy 18)

This genetic disorder occurs during conception when the foetus receives one additional chromosome in the chromosome pair 18. It is a severe condition associated with mental retardation, dysfunction and abnormalities of many internal organs. Not more than 10% of the infants born with this condition survive beyond the first 6 months of life and more than half of the affected by the Edward’s syndrome do not survive beyond the first week of life. Edward’s syndrome occurs in approximately 1 in 6000 live births, while the median lifespan is 5–15 days. Major causes of death include apnoea and heart abnormalities.

Down’s Syndrome (Trosomy 21)

Down’s syndrome is named after Dr. Langdon Down, who first described the condition as a disorder in 1866. Although his observations were correct, he has not found the cause for this disorder. It was not until 1959, however, that scientists prove that a genetic disorder in chromosome pair 21 is the most common cause for the syndrome. On average, Down’s syndrome occurs about once in every 700 births and is the major cause for the mental retardation of the affected children. Some of the manifestations of this disorder include learning difficulties, retarded development, specific facial features and muscle weakness in early childhood. Many children affected by Down’s syndrome suffer from congenital heart defects, leukaemia, early manifestations of Alzheimer disease, gastro-intestinal problems, immunological, respiratory, eye, and orthopaedic disorders. Due to recent advances in medical care and social inclusion, life expectancy has increased dramatically for individuals with Down’s syndrome. However, 7% of the newborn babies do not survive the first 6 months of life, 25% of them never reach full age, while 36% die before reaching the age of 5 years. There is a close relation between the high mortality rates among children affected by Down’s syndrome andthe common congenital heart disease. Many would die, or be severely disabled if the condition wasn’t treated. Therefore, it is considered that the life expectancy and the quality of life of these individuals could significantly be improved if timely and corrective cardiovascular surgery is undertaken in early childhood. People with Down’s syndrome can live up to the age of 55 years and longer, if adequate care is taken for their health. Due to the genetic nature of this disorder, the most common risk factor for Down’s syndrome is the age of the parents, and predominantly the maternal age, which directly affects the quality of the genetic material.

The risk varies depending on the age

20 1:1527
25 1:1352
30 1:895
32 1:659
34 1:446
36 1:280
38 1:167
40 1:97
42 1:55

The extra chromosome in chromosome pair 21 is responsible for the characteristic abnormalities associated with Down’s syndrome. About 2% to 4% of the affected children inherit additional genes from chromosome 21, but not in every cell of the body. This is known as mosaic Down’s syndrome. They often don’t have all the typical physical characteristics and may not be as severely intellectually impaired as people with full Trisomy 21.