Genetics · MU Sofia

15. Inheritance of Chromosomal Aberrations: Translocations, Inversions, and Mosaicism

Explains how structural chromosome changes and mosaicism are passed on, starting with translocations: reciprocal exchanges with their meiotic quadrivalent and gamete outcomes, and Robertsonian fusions of acrocentric chromosomes with recurrence-risk figures. Next covers paracentric and pericentric inversions and the crossing-over inside the inversion loop, then somatic versus gonadal mosaicism with examples. Closes with clinical relevance, including recurrent miscarriage and counselling of balanced carriers.

Inside: 6 sections · 4 labelled pictures · 14 flashcards · 3 min

Topic 15 of 65 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 6 sections, about 372 words (3 min of reading), with 14 flashcards.

In this note

  1. Inheritance of Chromosomal Aberrations: Translocations, Inversions, and Mosaicism
  2. I. Introduction
  3. II. Inheritance in Translocations
  4. III. Inheritance in Inversions
  5. IV. Inheritance in Mosaicism
  6. V. Clinical Relevance

I. Introduction

  • Chromosomal aberrations can be numerical or structural.
  • Structural aberrations (translocations, inversions) may be balanced (no gain/loss of material, carrier often normal) or unbalanced (extra/missing material → disease/abnormalities).
  • Balanced carriers may be healthy but produce unbalanced gametes → infertility, miscarriage, or affected offspring.
  • Mosaicism introduces variability since not all cells carry the same chromosomal makeup.

II. Inheritance in Translocations

Definition

  • Exchange of material between chromosomes.
  • Occurs due to breaks and abnormal rejoining.

Types

  1. 1. Reciprocal translocations
  • Mutual exchange between two non-homologous chromosomes.
  • Balanced carriers usually healthy.
  • During meiosis → chromosomes form a quadrivalent → segregation may produce:
  • Normal gametes.
  • Balanced gametes (carriers).
  • Unbalanced gametes (partial monosomy/trisomy).

Clinical outcome:

  • Infertility, recurrent pregnancy loss.
  • Offspring with congenital malformations if unbalanced.
  1. 2. Robertsonian translocations
  • Fusion of two acrocentric chromosomes (13,14,15,21,22).
  • Balanced carriers → phenotypically normal but have 45 chromosomes.
  • Gametes may result in trisomy or monosomy.

Clinical example:

  • t(14;21) → Familial Down syndrome.
  • Carrier mother: ~10–15% recurrence risk.
  • Carrier father: ~1–5% recurrence risk.

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