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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