Cause of birth of Mentally retarded child...3

Cause of birth of Mentally retarded child...3

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4.  What Causes Down Syndrome?
According to the National Down Syndrome Society (NDSS), about 1 in 700 babies in the United States is born with Down syndrome. It’s the most common genetic disorder in the United States.
A quick explanation of basic genetics can help you understand how it happens. In all cases of reproduction, both parents pass their genes on to their children. These genes are carried in chromosomes. When the baby’s cells develop, each cell is supposed to receive 23 pairs of chromosomes (46 chromosomes total). Half of the chromosomes are from the mother and half are from the father.
In children with Down syndrome, one of the chromosomes doesn’t separate properly. The baby ends up with three copies, or an extra partial copy, of chromosome 21, instead of two. This extra chromosome causes problems as the brain and physical features develop.
5.   Types of Down Syndrome
Trisomy 21::Trisomy 21 means there is an extra copy of chromosome 21 in every cell. This is the most common form of Down syndrome
Mosaicism::Mosaicism occurs when a child is born with an extra chromosome in some but not all of their cells. People with mosaic Down syndrome tend to have fewer symptoms than those with trisomy 21.
Translocation::In this type of Down syndrome, children have only an extra part of chromosome 21. There are 46 total chromosomes. However, one of them has an extra piece of chromosome 21 attached.


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