Down Syndrome

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Down Syndrome is a condition caused by trisomy 21 due to nondisjunction in meiosis. As a result, instead of normal 46 chromosomes, an affected person will have 47 chromosomes. Nondisjunction of the chromosome number 21 normally occurs in the gamete of the ovum of the mother in her forties. Down syndrome also occurs due to an unusual chromosome structure but at a very small percentage. The similar phenotypic feature of people with Down Syndrome is that they have broad and flat heads and flattened noses. Affected people have a lower intellectual capacity and defective health with a shortened life expectancy which is less than 50 years[1].

However, it is expected that the life expectancy for individuals with Down syndrome will drastically increase in years to come as there is a new treatment being developed to create effectively a chromosome 21 Barr body. This would be completed by using a zinc finger nuclease to insert an inducible Xist transgene into embryos with trisomy 21. This gene could then be induced to inactivate the extra chromosome and make up for the extra dosage of these genes, thus hopefully eliminating the effects of the condition.


  1. Hartl, D.L., Jones, E.W. (2009) Genetics: Analysis of Genes and Genomes, United States of America: Jones and Bartlet
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