Abstract
The phenotypic expression in DS is determined by the type of underlying cytogenetic abnormality. Almost 90-95% cases of DS are due to pure trisomy of the 21st chromosome; 6-7% is the result of mosaicism and in only 3-5% of cases it results from Robertsonian translocation (ROB). About 1/3rd cases of unbalanced Robertsonian translocation causing DS are inherited. We report a 1-year-old-boy with DS secondary to rea(21;21) ROB.
Highlights
Aneuploidy is the most frequently observed chromosomal abnormality in humans with Down syndrome (DS) being the commonest autosomal aneuploidy
The overall prevalence varies from 1 per 800-1,200 live births depending upon the acceptability and availability of prenatal screening, foetal survival and medical termination of Down syndrome pregnancies
Almost 90-95% cases of DS are due to pure trisomy of the 21st chromosome; 6-7% is the result of mosaicism with varying percentage of normal and trisomy cells
Summary
Aneuploidy is the most frequently observed chromosomal abnormality in humans with Down syndrome (DS) being the commonest autosomal aneuploidy. Almost 90-95% cases of DS are due to pure trisomy of the 21st chromosome; 6-7% is the result of mosaicism with varying percentage of normal and trisomy cells. In a small proportion (3-5%) of cases the extra chromosome 21 attaches itself to another acrocentric chromosome; this is known as Robertsonian translocation (ROB)2.
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