About Chromosomes & Trisomy

Chromosome disorders are caused by an alteration in the number or genetic structure of chromosomes.  A trisomy ('three bodies') is a chromosomal disorder characterized by an additional chromosome, so the child has 47 instead of the usual 46.  And when people speak of a "Full" Trisomy this is usually what they mean.  Down's Syndrome (Trisomy 21), Edward's Syndrome (Trisomy 18) and Patau's Syndrome (Trisomy 13) are the most common forms of trisomy.  

By definition, a Rare TRISOMY disorder, can occur in any Chromosome other then the sex determining one (23rd), and may have one of the following descriptions:  Duplication, Partial Trisomy, or a "Full" Trisomy of any of the Autosomes (Chromosomes #'s 1-22), other than a full 21st (Down Syndrome), which is not considered to be rare.  In addition to the extra Genetic material designation, they may also contain such terms as Deletion (lost material), De Novo (new, not inherited from a parent), Inversion (flipped upside down), Mosaic (not in every cell of the body), Ring (ends have stuck together forming a circle), or a Balanced, Unbalanced, or Robertsonian Translocation.  

Now a chromosome is made up of two parts.  A smaller upper part which scientists named the "P" for petite leg, and the larger bottom part, or "Q" leg, because scientist aren't known for their creativity, and "Q" was just the next letter of the alphabet.  (Personally however, I am creative, and like to think of it as "Q" for queen size.)  These legs are then divided up into areas called bands, which contain all of the assorted genes found on a particular chromosome.  So a partial trisomy will usually include one of these two letters, indicating which leg of the chromosome has been duplicated, and there will also likely  be some numbers included as well, which tells how many and which bands of that chromosome's leg has been added, or subtracted.

A further note on Translocations:

When an individual has all of the necessary genetic material, but part of one chromosome has switched places with part of another one, this is a Balanced Translocation.  While this can be spontaneous (de novo) it is far more likely to be inherited from a parent.  And this then becomes the route of inherited chromosomal abnormalities.    Because the individual has the right amount of material, even if it's not where it normally would be, the individual usually doesn't show any symptoms.  And unless they've been tested due to a family history of chromosomal abnormalities, are probably totally unaware that they even have this.  That is until perhaps they start having children of their own, and end up passing only part of their Balanced Translocation on to a baby (an Unbalanced Translocation).  This child  then doesn't have the right amount of genetic material, and which ever genes they have extra or missing will determine just what health and developmental problems the child will encounter in life.  Now obviously this can vary a great deal from case to case.

So why did I list a Balanced Translocation as being a possible additional Trisomy term?  Because it's only had to happen once.  And there is at least one documented case, where a child who inherited a parent's Balanced Translocation, went on to win the chromosome lottery of a whole extra chromosome of his own as well. And that gave him a total of three #13 chromosomes, even though one of them is in 2 pieces.   Now a Robertsonian Translocation can produce a similar result to that, but in a different way.  To start with, they can only effect certain chromosomes (#'s 13, 14, 15, 21 & 22).  These particular chromosomes contain very short "P" arms.  And sometimes a whole one of these, will attach itself to a whole other one of these, forming what then looks like one very long chromosome.  So the child who inherits a Robertsonian Translocation from a parent, will in fact also end up with a "Full" Trisomy condition instead of a Partial one.