LECTURE CONNECTIONS 9 | Chromosome Variation © 2009 W. H. Freeman and Company.
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Transcript of LECTURE CONNECTIONS 9 | Chromosome Variation © 2009 W. H. Freeman and Company.
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LECTURE CONNECTIONS
9 | Chromosome Variation
© 2009 W. H. Freeman and Company
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Chapter 9 Outline
• 9.1 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids
• 9.2 Chromosome Rearrangements Alter Chromosome Structure
• 9.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
• 9.4 Polyploidy Is the Presence of More than Two Sets of Chromosomes
• 9.5 Chromosome Variation Plays an Important Role in Evolution
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9.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
• Causes of Aneuploidy
• Deletion of centromere during mitosis and meiosis
• Robertsonian translocation
• Nondisjunction during meiosis and mitosis
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Types of Aneuploidy
• Nullisomy: loss of both members of a homologous pair of chromosomes. 2n − 2
• Monosomy: loss of a single chromosome. 2n − 1
• Trisomy: gain of a single chromosome. 2n + 1
• Tetrasomy: gain of two homologous chromosomes. 2n + 2
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Concept Check 3
A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in
A) a trisomic member of this species?
B) a monosomic member of this species?
C) a nullisomic member of this species?
D) a tetrasomic member of this species?
A) 2n + 1 = 36 + 1 = 37B) 2n – 1 = 36 – 1 = 35C) 2n – 2 = 36 – 2 = 34D) 2n + 2 + 36 + 2 = 38
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Effects of Aneuploidy in Humans
• High percentage of all human embryos that are conceived posses chromosomes abnormalities.
• Aneuploidy in human sex chromosomes is better tolerated that aneuploidy of autosomal chromosomes.
• Sex- chromosome aneuploids:
- Turner syndrome XO
- Klinefelter syndrome XXY
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Effects of Aneuploidy in Humans
• Autosomal aneuploids:
• Trisomy 21 – Down syndrome
• Primary Down syndrome, 75% random nondisjunction in egg formation in meiosis I
• Familial Down syndrome, Robertsonian translocation between chromosomes 14 and 21
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Concept Check 4
Briefly explain why in humans and mammals, sex-chromosome aneuploids are more common than autosomal aneuploids.
No mechanism of dosage compensation for autosomal chromatids. Autosomes carry more genes. Most autosomal aneuploids are spontaneously aborted.
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Effects of Aneuploidy in Humans
Autosomal aneuploids:
• Aneuploidy and maternal age
• Possible interpretation of this connection
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9.4 Polyploidy Is the Presence of More than Two Sets of Chromosomes
• Autopolyploidy:
From single species
• Allopolyploidy:
From two species
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Allopolyploidy – between 2 species that are sufficiently related that hybridization can occur.
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The Significance of Polyploidy
Hexaploid genome of wheat contains chromosomes from three different wild species.
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9.5 Chromosome Variation Plays an Important Role in Evolution
• New and extra copies of genes give rise to new functions.
• New and extra sets of genes may give rise to new species.