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Chromosome NumberChromosome NumberAll cells in the human body (SOMATIC CELLS) have 46 or 23 pairs of chromosomes
Called the DIPLOID or 2n number
GAMETES (eggs & sperm) have only 23 chromosomes
Called the MONOPLOID or 1n number
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NondisjunctionNondisjunctionChromosomes may fail to separate during meiosis
Resulting gametes may have too few or too many chromosomes
Disorders:Down Syndrome – three 21st chromosomes
Turner Syndrome – single X chromosomeKlinefelter’s Syndrome – XXY chromosomes
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52n = 46
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62n = 46copyright cmassengale
72n = 47copyright cmassengale
82n = 47copyright cmassengale
Trisomy 21—Down’s SyndromeThree copies of
chromosome 21Maternal
AgeIncidence at
Birth20 1 in 1500
30 1 in 900
35 1 in 400
40 1 in100
45 1 in 30
102n = 47copyright cmassengale
Klinefelter’s SyndromeMale—XXY 1 in 1000 malesSterileTall, thin, and slightly
lower IQ
12
2n = 45copyright cmassengale
Other DisordersTurner’s Syndrome—XO1 in 500 female births (most
are aborted before birth)Very short, infertile, broad
chest and may have a webbed neck
Edward’s Syndrome—Trisomy 181 in 3000Most babies die within the
first month to the first year