7. Chromosomal mutations: numerical and structural chromosomal aberrations. Marker chromosomes.
Covers chromosomal mutations, which are large-scale changes in chromosome number or structure, and the two main types. Numerical aberrations are described as aneuploidy, with monosomy and trisomy examples, and polyploidy. Structural aberrations are taken in turn: deletions, duplications, inversions, translocations including Robertsonian ones, isochromosomes and ring chromosomes. A separate section explains marker chromosomes and their harmless or pathogenic effects, and the note closes with clinical significance, including Philadelphia chromosome and MYC translocation examples in cancer.
Topic 7 of 65 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 6 sections, about 474 words (3 min of reading), with 14 flashcards.
In this note
Chromosomal Mutations: Numerical and Structural Aberrations, Marker Chromosomes
I. Introduction
II. Numerical Chromosomal Aberrations
III. Structural Chromosomal Aberrations
IV. Marker Chromosomes
V. Functional and Clinical Significance
I. Introduction
Chromosomal mutations (also called 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).
Structural aberrations (rearrangements within or between chromosomes).
II. Numerical Chromosomal Aberrations
1. Aneuploidy
Definition: gain or loss of individual chromosomes (≠ exact multiple of haploid set).
Mechanism: nondisjunction during meiosis I or II → gametes with abnormal chromosome number.
Types:
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, major cause of early miscarriage.
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