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Chromosome Number Abnormalities

A genetically normal human has a maternal copy and a paternal copy of 23 different chromosomes. With 23 pairs of each chromosome, a genetically normal human has 46 chromosomes total. There are exceptions -- for example, sperm cells are haploid and contain only one copy of each chromosome (23 chromosomes total). The number of chromosomes within a haploid cell is referred to as n. Thus, a cell with 2 copies of each chromosome (a diploid cell) has 2n chromosomes; a cell with 3 copies of each chromosome (a triploid cell) has 3n chromosomes; and a cell with 4 copies of each chromosome (a tetraploid cell) has 4n chromosomes.

Not including reproductive cells, if an individual has any other than 2n chromosomes then they are heteroploid. If a heteroploid has an exact multiple of n chromosomes (such as 69 or 92, in humans) then that individual is euploid. If a heteroploid does not have an exact multiple of n chromosomes (such as 45, 47 or any value ≠ 46) then that individual is aneuploid. Although rare, triploidy and tetraploidy are forms of euploidy. Paternal chromosome triploids (individuals with an extra copy of every paternal chromosome) have abnormal placentas and are known as partial hydatiform moles; however, maternal chromosome triploids are spontaneously aborted early during gestation.

Aneuploidy is more common than euploidy. Most aneuploidy is trisomy (three copies of an individual chromosome, instead of a pair) or monosomy (one copy of an individual chromosome). The most common trisomy is trisomy 21, which occurs in 95% of Down Syndrome patients. Chromosomes 21, 13 and 18 contain the fewest genes of any other chromosome -- trisomy of any chromosome other than these three deadly. Monosomy of any chromosome except Chromosome X (a condition called Turner Syndrome) is lethal. Aneuploidy is usually caused by meiotic nondisjunction, meaning that (usually during meiosis I) chromosomes fail to disjoin properly and daughter cells either have too many or too few chromosomes.

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