Biology · MU Pleven

21.Structural and numerical chromosomal mutations. Human pathology examples.

Covers structural and numerical chromosome mutations, starting with the two main categories and how aberrations are inherited through gametes, mosaicism or de novo arising. Deletions, duplications, inversions and translocations are explained alongside the three DSB repair pathways, Robertsonian translocations, and aneuploidy, anaphase lag and polyploidy. It ends with karyotype evolution and speciation, including a comparison of human and great ape karyotypes and a list of chromosomes with shared banding patterns. Human pathology examples such as Down and Turner syndromes are named.

Inside: 7 sections · 16 flashcards · 4 min

Topic 21 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 503 words (4 min of reading), with 16 flashcards.

In this note

  1. Structural and numerical chromosomal mutations. Human pathology examples.
  2. CHROMOSOMAL MUTATIONS
  3. Types of Chromosomal Mutations
  4. Inheritance of Chromosomal Aberrations
  5. Structural Chromosome Aberrations Explained
  6. Numerical Chromosome Aberrations
  7. Karyotype Evolution & Speciation

CHROMOSOMAL MUTATIONS

  • Chromosomal mutations (chromosome aberrations) are changes in the number or structure of chromosomes.
  • They often affect large DNA segments and can be seen during metaphase in cell division.

Types of Chromosomal Mutations

Chromosomal changes fall into two major categories:

A. Numerical Disorders

  • Changes in chromosome number.
  • Examples: Down syndrome, Turner syndrome.

B. Structural Aberrations

  • Changes in chromosome structure.
  • Include deletions, duplications, inversions, translocations.

Inheritance of Chromosomal Aberrations

  • If a mutation occurs in gametes (egg or sperm) → inherited → present in every cell of the child.
  • If it occurs after fertilization → mosaicism (only some cells affected).
  • Can be inherited from parents or arise de novo.
  • Parental karyotyping is often performed to check inheritance.

Structural Chromosome Aberrations Explained

A. Deletions & Duplications — Causes

  • Often due to unequal crossing over during meiosis, especially in regions with repetitive DNA.
  • Also caused by faulty repair of double-strand DNA breaks (DSBs).

Mechanisms of DSB Repair

  1. 1. Non-homologous end joining (NHEJ)
  2. a. No sequence similarity required.
  3. b. Error-prone: may trim/add nucleotides.
  4. 2. Microhomology-mediated end joining (MMEJ)
  5. a. Uses short matching sequences to align ends.
  6. 3. Homology-directed repair (HDR)
  7. a. Occurs in S/G2 phase.
  8. b. More accurate.

B. Inversions

  • A chromosome segment reverses orientation.
  • Types:
  • Paracentric: does not include centromere.
  • Pericentric: includes centromere.
  • Forms loops during pairing in meiosis.
  • Heterozygous inversions → misalignment → reduced fertility.
  • Homozygous inversions → usually normal fertility.
  • Important in reproductive isolation → contributes to speciation.

C. Translocations

  • Exchange of chromosome segments between non-homologous chromosomes.
  • Can disrupt proper pairing & segregation.
  • Heterozygotes → fertility issues.
  • Homozygotes → generally normal fertility; can contribute to isolation.

Robertsonian Translocations

  • Fusion of two acrocentric chromosomes → reduces chromosome count.
  • Associated with fertility issues & some genetic conditions (e.g., Down syndrome).
  • Human chromosome 2 formed by such a fusion.

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