الفهرس 
Down syndromeOnly 14 pages are availabe for public view
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Abstract
Down syndrome is most common autosomal abnormality. Which occurs when there are three copies of chromosome 21. It occurs in all ethnic groups, socioeconomic levels, and geographical regions. There are three types of DS free trisomy 21, translocation and mosaicism. It is approximately 1 in 800 births in the general population, but figures in Egypt vary between 1 in 555 to 1in 770. In 88% of cases, the extra copy is maternally derived, through an error in cell division called non-disjunction. There are many risk factors for trisomy 21 as advanced maternal age, maternal recombination, abnormal folate and methyl metabolism, parental mosaicism, mutations in nuclear-encoded genes, mitochondrial mutations and consanguinity.