Chromosomal Aberrations and Structure Changes

Introduction

• Structural chromosome rearrangements are changes in the physical structure of chromosomes.
• These changes may result in birth defects, mental retardation, increased risk for infertility.
• Chromosomal aberrations are substantial changes in chromosome structure.
  • They typically affect more than one gene.
  • They are also called chromosomal mutations.

Alterations in Chromosome Structure

• Duplication
  • The total amount of genetic information in the chromosome can change.
  • A chromosomal fragment is attached to a chromosome, adding one or more genes.
• Deletions
  • The total amount of genetic information in the chromosome can change.
  • A portion of a chromosome is deleted.
• Inversion
  • The genetic material may remain the same but rearranged.
  • A segment of a chromosome rotates 180 degrees.
• Translocation
  • The genetic material may remain the same but rearranged.
  • A segment of chromosome integrates into a non-homologous chromosome.

Deletion

• Loss of genetic material
  • A deficiency is a loss of one or more genes.
  • Can be terminal (end of chromosome) or interstitial (middle of chromosome).
• Causes
  • Caused by various agents such as radiation, chemicals, drugs, or viruses.
• Consequences
  • Deletion of the centromere results in an acentric chromosome that is lost, usually with serious or lethal consequences.
• Examples of human disorders caused by large chromosomal deletions:
  • Cri-du-chat (“cry of the cat”) syndrome, resulting from deletion of part of the short arm of chromosome 5.
  • The deletion results in severe mental retardation and physical abnormalities.

Duplication

• Gain of genetic material
  • A duplication involves attachment of a chromosomal fragment resulting in addition of one or more genes to a chromosome.
• Origin
  • Unequal crossing over during meiosis:
    • Homologous chromosomes are paired in a perfect manner so that identical loci lie exactly opposite each other.
    • If paired chromosomes are misaligned, equal exchange products are not formed resulting in duplications.
• Consequences
  • Do not produce any drastic consequences as like deletion in terms of phenotype and survival.

Inversion

• Reversal of chromosome segment
  • A chromosome segment excises and reintegrates oriented 180° from the original orientation.
  • Pericentric inversion: includes the centromere.
  • Paracentric inversion: does not include the centromere.
• Consequences
  • Linked genes are often inverted together.
  • Meiotic consequences depend on the inversion occurring in a homozygote or heterozygote.
    • Homozygote will have normal meiosis.
    • Heterozygote may produce unequal crossover products with serious consequences.

Translocation

• Transfer of chromosome segment
  • Integration of chromosome segment into a non-homologous chromosome.
  • Simple Translocation: A small terminal segment of a chromosome attaches to the end of a homologous chromosome or to a non-homologous chromosome.
  • Reciprocal Translocation: The breakage takes place in two non-homologous chromosomes and is followed by the reunion of broken segments to different chromosomes.

History

• 1923: C.B. Bridges discovered duplications, deletions, and translocations in chromosomes.
• 1926: A.H. Sturtevant discovered inversions in chromosomes.

Explore the fascinating world of structural chromosome rearrangements in this quiz. Discover the implications of chromosomal aberrations such as duplications, deletions, inversions, and translocations, and their effects on genetic information. Test your knowledge on how these changes can impact health and fertility.