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Incomplete (unbalanced) chromosome sets: monosomy , trisomy , etc.

Complete chromosome sets: 1N , 2N, 3N, etc. Incomplete (unbalanced) chromosome sets: monosomy , trisomy , etc. Changes in chromosome number. Euploidy vs. aneuploidy. Autosomal trisomies and monosomies. Monosomy (2N - 1 ) … invariably fatal in humans.

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Incomplete (unbalanced) chromosome sets: monosomy , trisomy , etc.

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  1. Complete chromosome sets: 1N, 2N, 3N, etc. Incomplete (unbalanced) chromosome sets: monosomy, trisomy, etc. Changes in chromosome number Euploidy vs. aneuploidy

  2. Autosomal trisomies and monosomies Monosomy(2N - 1) … invariably fatal in humans Trisomy (2N + 1) … only three kinds tolerated in humans: # base pairs # genes # base pairs # genes Most common at birth—trisomy 21 (Down syndrome) 1 in 750 live births; 85% survive >1 year Less common Trisomy 18 (Edward syndrome, 1 in 10,000); <5% survive >1 year Trisomy 13 (Patau syndrome, 1 in 20,000); <5% survive >1year Why are trisomy 21, 18 & 13 tolerated better than other trisomies?

  3. sex chromosomal aneuploidy Sex chromosome aneuploidies are better tolerated than autosomalaneuploidies # base pairs # genes # base pairs # genes • XXY -- Klinefelter’s Syndrome • X0 -- Turner’s Syndrome • XXX -- Normal females • XYY -- Taller males, some mental retardation • Y0 -- Inviable Only one X-chromosome is active in males, females, and in individuals with an X-chromosomal aneuploidy XX XXY XXX XY Barr bodies

  4. X-chromosome inactivation • One X chromosome is packaged away. • Visible as a spot in female cells (Barr body). • The inactive X is randomly selected at about 32 cell stage of embryonic development. • In about half the cells, the maternal X is active; in the other half the paternal X is active. In some diseases involving genes on the X-chromosome this can yield a mosaic pattern of disease expression

  5. AnhidroticEctodermalDysplasia (caused by a mutation on the X chromosome)

  6. What’s unusual about this pedigree? XDXd XDY filled = colorblind X-linked recessive XDXd Xd=colorblind XD=color vision XdY -Identical twins with non-identical phenotypes -XDXd heterozygote is showing recessive phenotype X-inactivation phenotype… all-or-none (or in-between)?

  7. Meiosis II nondisjunction Meiosis I nondisjunction All 4 products defective 2 normal 2 defective How does aneuploidy arise? Major cause of aneuploidy—meiosis nondisjunction

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