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Autosomal Aneuploidies

Autosomal Aneuploidies

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Down's syndrome

Congenital disease characterized by organ malformations (heart, kidneys), moderate to severe mental retardation, thick tongue, small feet and hands, changes in features. It results from an abnormality in the chromosomal constitution: affected individuals feature an extra chromosome - which adds to the pair number 21 - in their cells (for this reason the disease is also called trisomy 21). The term Mongolism is a common synonym: the presence of oblique eyelid slits reminds individuals of the eastern races.

The frequency with which this syndrome manifests itself is one for every 500 live born children and is superior for conceptions in women over 40 years of age.

This syndrome was described in 1866 by the English physician John Langdon Haydon Down (1828 - 1896). THE Down's syndrome or trisomy 21is undoubtedly the most common chromosomal disorder and the most common form of congenital mental disability. It can usually be diagnosed at birth or shortly thereafter by its dysmorphic characteristics, which vary among patients but produce a distinctive phenotype.

The patients have short stature and the skull has brachycephaly, with the occipital flattened. The ear pavilion is small and dysmorphic. The face is flat and rounded, the eyes show eyelid slits and brushfield spots around the iris margin. The mouth is open, often showing the ridged and protruding tongue. The hands are short and broad, often with a single transverse palmar fold ("simian fold").

Trisomy of the 13 - Patau

The trisomy of 13 is clinically severe and lethal in almost all cases surviving to 6 months of age. The extra chromosome comes from non-disjunction of maternal meiosis I and about 20% of cases result from unbalanced translocation. The phenotype includes severe central nervous system malformations such as arrinencephaly. A marked mental retardation is present. There are usually congenital heart defects and urigenital defects. Often there are cleft lip and palate, eye abnormalities, polydactyly, clenched fists and arched plants.

18-Edwards Trisomy

Most patients with chromosome 18 trisomy have regular trisomy without mosaicism, ie karyotype 47 XX or XY, +18. Of the rest, about half are mosaic cases and the other complex cases, such as double aneuploidies, translocations.

Manifestations of trisomy 18 always include mental retardation and growth retardation and sometimes severe malformations of the heart. The skull is excessively elongated in the occipital region. The ear pavilion is dysmorphic, with few grooves. The mouth is small. The neck is short. There is a great intermamilar distance. The external genitals are anomalous. The index finger is larger than the others and flexed over the middle finger. The feet have arched plants. The nails are usually hypoplastic.