It is impossible to identify the types of Down Syndrome someone has unless you study their chromosome karyotype. This is because the physical features and behaviours are similar to all types
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Understanding Down Syndrome Karyotype:
In general, everybody has 23 pair of chromosomes in each cell of our body. Down Syndrome is caused when chromosome 21 has an extra copy, making it a pair of three rather than two.
There Are Three Types of Down Syndrome:
1. Trisomy 21:
This is the most common type of Down Syndrome as 95% of people with Down syndrome have Trisomy 21.
With this type of Down syndrome, every cell in the body has three copies of chromosome 21 instead of two, i.e., each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.
How to diagnose Trisomy 21:
There are two types of Down Syndrome tests – Screening tests and Diagnostic tests.
A screening test is done during pregnancy that helps the doctor identify the possible chances of your baby having Down Syndrome.
Diagnostic tests are confirmatory tests to detect whether or not your baby has Down syndrome by studying its chromosome karyotype.
2. Translocation Down Syndrome:
Translocation Down Syndrome happens when a segment or a full extra copy of chromosome 21 gets attached to another chromosome pair instead of being on its own.
This usually gets attached to chromosome 14 or 15, but there are cases where it would be attached to some other pair as well. This type occurs in a small percentage of people with Down Syndrome, roughly 3 to 4%.
Translocation Down syndrome is further classified into two types,
In this case, the parent will carry a rearrangement of genetic material. But this rearrangement would not result in any gain or loss of any genetic material in the parent and is called Balanced Translocation.
The parent will have only 45 chromosomes in each cell of the body but will be normal and healthy.
However, if this unbalanced genetic material gets inherited by the baby from a parent, an abnormal translocation of chromosome 21 is formed, leading to Down Syndrome in the baby.
In de nova type, parents have the normal karyotype (chromosome arrangement), but the abnormal chromosome karyotype occurs spontaneously during maternal meiosis (occurs when the chromosome pairs fail to separate).
How to diagnose Translocation Down Syndrome:
The doctor studies your fetal chromosomes to identify the presence of an extra copy of chromosome 21.
This also allows your doctor to identify which chromosomal pair has the extra copy and whether it is present in all the cells.
If the extra copy of 21 is attached to any other pair of chromosomes instead of its own, it is diagnosed as a translocated syndrome.
Whenever a baby is diagnosed with translocation syndrome, the parent’s chromosomes are studied to identify if its inherited or not.
3. Mosaic Down Syndrome
Mosaic Down syndrome is the rarest type (only 1%) of Down syndrome.
Babies with this type of syndrome have a mixture of cells.
This means that some cells have the extra copy of chromosome 21, making it 47 in total, and others have the normal 46 chromosomes.
How is Mosaic Down syndrome caused?
This is caused when the extra copy of chromosome 21 is passed from the parent shortly after conception. How early this occurs determines the number of cells to be affected.
American Journal of Medical Genetics says a baby with this type of syndrome shows fewer symptoms when compared with babies having trisomy 21 and translocation.
That’s because only some of the cells are affected. However, there is ongoing comparative research on this.
How to diagnose Mosaic Down Syndrome:
If only some cells carry an extra copy of the 21st chromosome, the doctor may recommend other confirmatory tests involving skin or brain cells.
Mosaic down syndrome is described as percentage.
For this, your doctor would analyze chromosomes from 20 cells. For instance, out of this, if 5 cells have 46 chromosomes and the others have 47, the baby is said to have a 75% level of mosaicism.
Being Unique and Happy:
There is no complete treatment for any type of Down syndrome. But there are many organizations available to support them and their families to raise awareness. They all come together to take forward the task and vision that makes these individuals lead a comfortable and successful life just like us.