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Inheritance patterns and prenatal diagnosis
Inheritance patterns
In 22q11 Deletion syndromes, each chromosome has a long (q) and short (p) arm. In this syndrome a tiny part is missing (deleted) from the long arm (q) of one of the two chromosomes 22's at position 11 on that chromosome.
In many affected children the deletion has started in the particular egg or sperm which went to form them, just by chance when the chromosomes were being copied to pass on, but in a small number (ten per cent) one parent has it and has passed it on. The deletion is shown by the FISH (fluorescence in-situ hybridisation) test, when instead of both chromosome 22's 'lighting up' with a special fluorescent DNA tag only one does. A new genetic technique MLPA (multiplex ligand-dependent probe amplification) can now identify the small proportion (about 5%) of individuals who could not previously be detected by the FISH test, and is likely to become the test of choice in the future. If a parent carries the 22q11 deletion, the inheritance mode is autosomal dominant. When neither parent shows the deletion there is still a one to two per cent risk of another affected child as it may only be carried in the germinal cells of the parents ovaries or testes.
22q11 deletion occurs in around 1 in 4,000 of the population. The very variable ways it affects individuals can result in a lengthy time to obtain a diagnosis.
Prenatal diagnosis
Amniocentesis or Chorionic villus sampling will detect the chromosome 22 deletion. Ultrasound scans can detect most heart defects.
What are the symptoms?
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