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What effect might having too many or too few chromosomes have on an organism?

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The term aneuploidy refers to the incorrect number of chromosomes for that particular species--too many or too few. Usually, a chromosomal aberration has far-reaching effects on the organism. Depending on which chromosome is involved, it usually causes a spontaneous abortion to occur during pregnancy, or the child may have a birth defect of some type, if it survives to full term. Aneuploidy can occur during abnormal cell division that produces the gametes. The gamete may have too few or too many chromosomes. An example is trisomy 21. This produces the condition known as Down Syndrome and usually is due to the female's egg having an extra chromosome 21. There is a link between the age of the mother and Down syndrome. This condition affects the health and lifespan of the child and is a cause of mental retardation. In this example, the individual has 2n+1 chromosomes--in other words, 46(the diploid or 2n amount)plus one additional chromosome, producing a person with 47 chromosomes.

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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.