Chromosomes

• Karyotype = an individual's complete set of chromosomes
• Deletion - a section of chromosome is removed

  e.g. Cri-du-chat in humans (tip of chromosome 5 is lost)

  notch wing in drosophila
• Deletion in tsetse fly control - sterile males released into the wild and females cannot produce viable offspring to reduce fly numbers and eliminate the disease
• Duplication - a section of chromosome is doubled up

  e.g. Huntington's disease in humans (duplication of CAG on chromosome 4)

  bar eye in drosophila (eye reduced to a slit)
• Pseudogene = a DNA sequence that resembles a gene but has been mutated into an inactive form over the course of evolution

  e.g. the odorant receptor gene family
• Inversion - a section of chromosome has been rotated and replaced in position

  e.g. antennapedia in drosophila (legs grow where antenna is meant to be)

  white eye variegation in drosophila (white eyes)
• Inversion gives a dicentric (two centromeres) and acrocentric (no centromeres) fragment which breaks and get lost at cell division, not present in gametes so inversions are crossover suppressors
• Translocation - two non-homologous chromosomes exchange parts

  e.g. Burkitt's lymphoma in humans (8-14 translocation), chronic myeloid leukaemia (9-22 translocation)
• Aneuploidy = individuals with chromosome number different from normal (excess or deficiency)
• Nondisjunction = an error in cell division in which homologous chromosomes fail to separate and migrate to opposite poles, responsible for trisomy and monosomy
• Trisomy = having three copies of one chromosome instead of the usual pair
• Monosomy = having one copy of a particular chromosome instead of the usual pair
• Trisomy 21 (down's syndrome), caused by an extra copy of chromosome 21 and also associated with 14-21 translocation
• Klinefelter's syndrome - males have an extra X chromosome so are XXY and have a number of female characteristics
• Turner's syndrome - females lack an X chromosome so are XO
• Trisomy 18 (Edward's syndrome) causing clenched fingers, and organ abnormalities
• Haplodiploidy - sex is determined by the number of chromosomes, females are diploid and develop from fertilised eggs and males are haploid and develop from unfertilised eggs

  e.g. ants, bees and wasps
• Polyploidisation - changes in number of chromosome sets, odd numbers of chromosomes associated with infertility as they cannot pair during meiosis
• Autopolyploidy - duplication of one diploid set of chromosomes
• Allopolyploidy - having two or more complete sets of chromosomes derived from different species
• Allopolyploidy examples:

  • triploid - banana, ginger
  • tetraploid - maize, cabbage, tobacco
  • hexaploid - wheat, chrysanthemum
  • octaploid - strawberry, sugar cane
• Somatic aneuploidy - drosophila gynandromorphs which are half female, half male due to non-disjunction