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An extra chromosome is due to nondisjunction. This is when, during meiosis, homologous chromosomes fail to segregate. The result is one gamete has a missing a chromosome (called a monosmy) while the other has an extra (trisomy).
The question posed is pretty broad because humans have 23 pairs of chromosomes, so it really depends which pair has the extra chromosome. However, if we look at some more common trisomies we can start with Trisomy 21 also known as Down's syndrome. Typical signs/symptoms of this disorder include delayed physical development, impaired cognitive/intellectual ability, and recognizable physical characteristics of the face.
Klinefelter's syndrome is when a male has an extra X sex chromosome (genotype XXY). These individuals will have poor muscle tone and in general weak secondary male sex characteristics due to low testosterone. They may also experience infertility.
There are other trisomy disorders such as trisomy 18 or Edward's Syndrome in which most individuals do not survive complications from the disease. So once again, it truly depends on which trisomy is being examined when evaluating how it effects an individual.
Having more or less than 46 chromosomes is a genetic disorder and can often be fatal. It occurs when a chromosome fails to separate completely during meiosis. Having an extra chromosome is called a trisomy with the numbered pair of chromosome afterwards. "Tri-" means three. For example, Down Syndrome, which has three chromosomes in the normally 21st pair is called Trisomy 21. Trisomy 13 and Trisomy 18 are more detrimental to an infant's health and usually results in death.
Another type of trisomy is when a person has an additional chromosome in their sex chromosomes. Triple X syndrome is where a female has three X chromosomes. XYY is when a male has two Y chromosomes and Klinefelter's is when a male has two X chromosomes (XXY).
Since every pair of chromosomes hold different genetic content, it really depends on which pair the trisomy is occurring at.
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