Medical Genetics · MU Plovdiv

4. Main disorders of the karyotype. Mechanisms of occurrence of chromosomal aberrations, designation.

Inside: 8 sections · 12 labelled pictures · 28 flashcards · 8 min

Topic 4 of 33 in the MU Plovdiv syllabus for Medical Genetics (Medicine, Year 4). The free AceMed note covers it in 8 sections, about 1,100 words (8 min of reading), with 28 flashcards.

In this note

  1. Introduction
  2. I. Introduction
  3. II. Numerical chromosomal aberrations
  4. III. Structural chromosomal aberrations
  5. IV. Marker chromosomes
  6. V. Inheritance of chromosomal aberrations
  7. VI. Functional and clinical significance
  8. VII. Chromosomal abnormalities in practice

Introduction

This topic covers the main disorders of the karyotype: numerical and structural chromosomal aberrations, marker chromosomes, how translocations, inversions and mosaicism are inherited, and their clinical significance.

I. Introduction

Chromosomal mutations (chromosomal aberrations) are large-scale changes in chromosome number or structure. Unlike single-gene mutations, they affect large regions of the genome, sometimes involving hundreds of genes.

  • They arise due to errors in meiosis, mitosis, or DNA repair.
  • Consequences: miscarriage, congenital malformations, infertility, developmental delay, or cancer.
  • Types: numerical aberrations (changes in whole chromosome number) and structural aberrations (rearrangements within or between chromosomes).
  • Structural aberrations (translocations, inversions) may be balanced (no gain/loss of material, carrier often normal) or unbalanced (extra/missing material, causing disease or abnormalities).
  • Balanced carriers may be healthy but produce unbalanced gametes, leading to infertility, miscarriage, or affected offspring.
  • Mosaicism introduces variability since not all cells carry the same chromosomal makeup.

II. Numerical chromosomal aberrations

1. Aneuploidy

  • Definition: gain or loss of individual chromosomes (not an exact multiple of the haploid set).
  • Mechanism: nondisjunction during meiosis I or II, giving gametes with an abnormal chromosome number.
  • Monosomy (2n–1): loss of one chromosome. Example: Turner syndrome (45,X).
  • Trisomy (2n+1): extra chromosome. Examples:
  • Trisomy 21: Down syndrome (47,XX,+21 or 47,XY,+21).
  • Trisomy 18: Edwards syndrome.
  • Trisomy 13: Patau syndrome.
  • Clinical note: most autosomal monosomies are lethal in embryogenesis.

2. Polyploidy

  • Definition: presence of extra full haploid sets (3n, 4n).
  • Mechanism: failure of meiotic cytokinesis or dispermy (two sperm fertilizing one egg).
  • Examples: triploidy (3n = 69 chromosomes), tetraploidy (4n = 92 chromosomes).
  • Clinical outcome: usually lethal, a major cause of early miscarriage.

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