31 for 21: Day 14


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Liz asked me to explain the translocation stuff again.

A ‘typical’ individual has 46 chromosomes, or 23 pairs of chromosomes, 23 from the mom and 23 from the dad. 2 of each chromosome from chromosome 1 – 23. Down syndrome occurs when there are three of the 21st chromosome, hence the name Trisomy 21. There are 4 types of Down syndrome, all of which are a type of T21 (since all DS have 3 21 chromosomes in one way or another):

1) Non-disjunctional DS: this is the most common type of DS, ca 95% of the time when there is a Trisomy 21. Here there are simply 3 21 chromosomes instead of the ‘typical’ 2 that are present in every cell of the body. Total of 47 chromosomes. Think of it like three spaghettis laying next to each other.

2) Mosaic DS: this occurs in 1-2% of the DS diagnoses. In this case there are three 21 chromosomes, but not in all the cells of the body. So this is like three spaghettis laying next to each other, but not in every single cell in the body i.e less than 100% of the cells. Total of 47 chromosomes.

3) Partial DS: this type of DS is very uncommon. Basically a little part of a 3d 21st chromosome has broken and is attached to each cell of the body. Think of it as 2 spaghettis laying next to each other with a little spaghetti next to them. So more than 2, but not 3 complete 21 chromosomes. Still 47 chromosomes.

4) Translocation DS: this is occurs in 2-4% of the time when a person has DS. This is also the kind that COULD be inherited. Translocation Trisomy 21  occurs when two 21 chromosomes have grown together, and a third 21st chromosome is attached. When the 23 chromosomes  from the mother meets the 23 chromosomes from the father, one of the 21 chromosomes have grown together with another chromosome, causing a so-called Robertsonian Translocation. So when the 23 meets the other 23, there is 3 chromosome 21, one extra 21 chromosome has simply grown attached to another chromosome, and a third is added. In this case of DS, the person with DS actually has 46 chromosomes in total instead of the DS typical 47. Most often the 3 21st chromosome is attached to chromosome 14, but it could also be attached to chromosome 13,15,21 and 22 as well. The most uncommon of these Robertsonian translocation is for the extra 21st chromosome to be attached to a 21st chromosome (21;21). This is what Vince has.
Think of this type as two spaghettis laying attached to each other on one side and then one additional spaghetti next to this one.

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Vince karyotype, look under 21 and there you see how the 21st chromosomes have two 21st chromosomes grown together, and one more om the side of it. Hence, total of 46 chromosomes, but still DS/Trisomy 21.

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Boring person’s karyotype (aka mine), only 46 spaghettis, nothing fancy.

About the inheritance:
The Translocation 21; 21 is the only case of DS, that when a parent is a carrier of a grown together chromosome, you can only have babies w DS. Each pregnancy will lead to a baby with DS. This means that one of the parents is a so-called balanced carrier, in this case, the person who is the carrier actually only has 45 chromosome. Because the two 21st chromosomes are already attached, and when you add a set of each chromosome from the other parent, this will ALWAYS lead to three chromosomes 21, aka Trisomy 21/Down syndrome.

2 Responses

  1. […] 1. The 2010 version of Translocation Trisomy 21;21 […]

  2. […] The 2010 version of Translocation Trisomy 21;21 (incl. karyotype of […]

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