Ask A Scientist Molecular Biology Archive

name         Nisha Z.
age          15
Question:  Is it more harmful for a person to have too few or too many chromosomes?
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Answer: "More harmful" - what does this mean? Most chromosome abnormalities are so harmful 
that the embryo or fetus does not survive long enough to be born. Many many pregnancies 
end in miscarriage, and many of these are caused by chromosome abnormalities - either 
numerical differences (too many or too few) or structural differences (damaged chromosomes 
with missing or extra pieces, or pieces in the wrong order). Some of these miscarriages 
are so early that the woman may not even realize she has been pregnant.

  a.. The perfect number of chromosomes for humans is 46. We have 46 chromosomes in almost 
  all of our cells - that is, 22 pairs of autosomes, numbered 1 through 22, plus the two 
  sex chromosomes, called X and Y. Normally, females contain two X chromosomes in their 
  cells in addition to the 44 autosomes. We describe female chromosomes as 46,XX (46 
  including 2 X' s). The cells of a male contain an X chromosome and a Y chromosome plus 
  44 autosomes; we describe their chromosomes as 46,XY.
Can a person survive with too many or too few chromosomes? Since the chromosomes contain 
genes that are needed for our growth and development, it makes sense that we cannot grow 
and develop successfully if any chromosomes are missing.

Only in the case of the X chromosome is it possible for a person to survive with a missing 
chromosome, even though the X chromosome is a fairly large chromosome. Considering that a 
normal, healthy male can manage with only one X chromosome (and a Y, of course), it is not 
surprising that a woman with only one X chromosome may be able to survive at all.

A person with only one sex chromosome, an X, will be a female with Turner syndrome, 
perhaps the mildest of all the chromosome abnormalities. We describe her chromosomes 
as 45,X (meaning that each cell contains 45 chromosomes, including one X chromosome). 
Even with Turner syndrome, most pregnancies will not continue to term but will end in 
a miscarriage - only some will survive. The symptoms of Turner syndrome are quite 
significant for those who have to live with them.

A person with only one Y chromsome (45,Y) could not survive or be born alive. The X 
chromsome ahs important genes on it that would be completely missing in such a person.

As to why it is harmful to have an extra chromosome, this is harder to understand. If 
chromosomes are good, wouldn't additional chromosomes be even better? The answer is no, 
they would not. The genes on the chromosomes are needed in exactly the correct amount in 
order to work correctly. (As with medicine, you need the right dose!)

Many genes cause the cell to make proteins of various kinds, and these are needed in the 
correct amounts. Some genes function by instructing other genes to turn on and off at the 
right moment, and these instructions need to be precise, accurate and sensitive. Having an 
extra chromosome is like having a business with too many managers or a family with too 
many parents or a car with two drivers or a country with two presidents, perhaps giving 
contradictory instructions.

The smallest of all the chromosomes is chromosome 21. A person who has, not two but three 
copies of chromosome 21 has Down syndrome, a well-known form of mental retardation.

Are people ever found with other extra chromosomes besides chromosome 21? Hardly ever. 
Those babies who are born with extra copies of larger chromosomes may not survive for 
long; they have many different health problems and birth defects.

Once again, the exception is the sex chromosomes. It is possible for males to survive with 
two Y chromosomes (47, XYY instead of 46, XY). It is also possible to survive with an 
extra X chromosome - 47, XXX females (triple-X syndrome) and 47, XXY males (Klinefelter 
syndrome).

By doing research into chromosome differences and the symptoms they cause, we learn about 
how the genes and chromosomes function most of the time. A syndrome is a set of 
recognizable symptoms that tend to occur together. New research into the many genes on 
chromosome 21 is helping scientists to understand which symptoms of Down syndrome are 
caused by which extra genes. This type of research is also helped by studying people who 
having extra or missing pieces of chromosomes, as opposed to a whole chromosome being 
extra or missing. These patients help scientists to narrow down which genes on the 
chromosomes are responsible for which function in our bodies. It is fascinating!

Sarina Kopinsky, MSc, H.Dip.Ed.
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