Structural Abberations

• 4 types of abberations:

  • duplications
  • deletions
  • inversions
  • translocations
• deletions: loss of chromosomal material
• Most deletions are internal
• There are DNA repair mechanisms that recognize broken ends of chromosomes (sticky ends) and attempt to make repairs
• Missing genes generally behave as recessieve "null" alleles meaning no mRNA or protein can be made from the missing genes
• Heterozygotes (one normal chromosome and one deleted chromosome) are generally okay as long as the deletion is not too large
• If a gene needed for gamete function is deleted, the individual may have lowered fertility
• Homozygosity for a large deletion is often lethal
• Cri du chat: a condition where a deletion in the short arm of chromosome 5 leads to extreme mental retardation, microcephaly, and a mewing like cry
• Philadelphia chromosome: deletion on chromosome 22 that leads to a high rate of leukemia
• Deletion of genes involved in regulation of cell division leads to an increased risk of cancer in heterozygotes
• duplication: gain of genetic material
• Since duplications provide extra copies of genes, the consequences are usually not so severe
• gene families: copies of genes with very similar sequences that may be differentially expressed
• inversions: an internal segment of chromosome is inverted
• If a chromosome is broken in two places, the sticky ends can heal such that the internal segment is "flipped over" from its normal arrangment
• paracentric inversion: if both breaks occur in the same chromosome arm so the centromere is not included
• Problems arise during meiosis only in individuals heterozygous for the translocation
• Crossovers within the loop of a pericentric inversion heterozygote still cause duplications and deletions, but do not cause a bridge
• translocations: aberrations involving 2 different chromosomes
• If the ends of two different broken chromosomes heal by attaching to the wrong partner, a translocation has occurred
• Inversions and translocations seem to be involved in the evolution of new species
• If two populations differ by several rearrangements, hybrids between the two may be sterile
• Robertsonian Translocation: two short one-armed chromosomes fuse at the centromere to form one bi-armed chromosome