Genetics Ch. 8

• Metacentric

  Chromosome with the centromere in the middle
• Submetacentric

  Chromosome with the centromere slightly off-center
• Acrocentric


  Chromosome with the centromere very close to one end
• Telocentric

  Chromosome with the centromere at the very end
• Karyotyping

  1. Chromosomes prepared from actively dividing cells

  2. Halted in metaphase

  3. Chromosomes arranged according to size
• G bands

  Giemsa stain
• Q bands


  Quinacrine mustard
• C bands

  Centromeric heterochromatin
• R bands

  Rich in cytosine-guanine base pairs
• Aneuploids


  Alters the number of chromosomes
• Polyploids


  One or more complete sets of chromosomes are added
• Chromosome rearrangements

  Alter the structure of chromosomes
• Types of chromosome mutations

  • Chromosome rearrangements
  • Aneuploidy
  • Polyploidy
• Four types of chromosomal rearrangements

  • Duplication
  • Deletion
  • Inversion
  • Translocation
• Unbalanced gene dosage results from chromosome duplications
• Deletions


  Loss of a chromosomal segment
• Pseudodominance

  The dominant wild-type allele in a heterozygous individual is absent due to a deletion on one chromosome
• Haploinsufficiency

  Imbalances in gene product
• Paracentric inversion


  Inversion that does not include the centromere
• Pericentric inversion


  Inversion that includes the centromere
• Individuals homozygous for inversions have no problems during meiosis
• Individuals heterozygous for inversions demonstrate reduced recombination and form abnormal gametes
• Nonreciprocal translocation


  One chromosome breaks and attaches to another
• Reciprocal translocation

  Two chromosomes exchange segments
• Robertsonian translocation

  Two acrocentric chromosomes fuse at the centromere to form one large metacentric chromosome
• Fragile sites are chromosomal regions susceptible to breakage
• Aneuploidy

  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
• Nullisomy

  Loss of both members of a homologous pair of chromosomes
• Monosomy

  Loss of a single chromosome
• Trisomy


  Gain of a single chromosome
• Tetrasomy


  Gain of two homologous chromosomes
• Aneuploidy

  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
• Types of Aneuploidy

  • Nullisomy: loss of both members of a homologous pair of chromosomes
  • Monosomy: loss of a single chromosome
  • Trisomy: gain of a single chromosome
  • Tetrasomy: gain of two homologous chromosomes
• Effects of aneuploidy in humans

  • Sex-chromosome aneuploids: Turner syndrome (XO), Klinefelter syndrome (XXY)
  • Autosomal aneuploids: Trisomy 21 (Down syndrome), Trisomy 18 (Edward syndrome), Trisomy 13 (Patau syndrome), Trisomy 8
• Maternal age

  Increased risk of aneuploidy
• Uniparental disomy


  Both chromosomes inherited from the same parent
• Mosaicism

  Nondisjunction in mitotic division
• Autopolyploidy

  Polyploidy in which extra chromosome sets are derived from the same species