Chromosomal Disorders
குரோமோசோம் கோளாறுகள்
Do not confuse chromosomal disorders with Mendelian disorders. Mendelian disorders are gene-level mutations; chromosomal disorders are structural or numerical errors in the genome itself. One extra chromosome changes the entire phenotype. Know the difference, or lose the mark.
The idea
Chromosomal disorders arise from the failure of segregation of chromatids during cell division, resulting in aneuploidy or polyploidy. Aneuploidy involves the gain or loss of specific chromosomes, such as Trisomy (2n+1) or Monosomy (2n-1). Polyploidy occurs when cytokinesis fails after telophase, increasing the whole set of chromosomes. These errors are permanent and systemic.
Why it matters
They result from errors in cell division (nondisjunction) during meiosis, leading to an incorrect number of chromosomes in the zygote.
NEET lens
NEET focuses heavily on the karyotype of these syndromes. Memorize the exact chromosome count for Down's, Klinefelter's, and Turner's. Expect matching-type questions or 'select the incorrect statement' based on symptoms. Do not ignore the distinction between aneuploidy and polyploidy; it is a favorite trap for examiners.
Clinical significance
Down's syndrome results from Trisomy 21, causing physical and mental retardation. Klinefelter's syndrome (47, XXY) leads to sterile males with feminine traits, while Turner's syndrome (45, X0) results in sterile females with rudimentary ovaries. These conditions demonstrate how precise chromosome counts are essential for human development.
Introduction
Chromosomal disorders arise from changes in chromosome number (aneuploidy — e.g., trisomy, monosomy) or structure (deletions, duplications, inversions, translocations), resulting in characteristic phenotypic syndromes often involving intellectual disability, physical abnormalities, and infertility.
Core concept
Aneuploidy (wrong chromosome number): Caused by nondisjunction during meiosis. Trisomy (+1 chromosome): Trisomy 21 (Down syndrome): 47,XX/XY+21. Features: intellectual disability, characteristic facial features, short stature, simian crease, heart defects, Alzheimer's risk. Trisomy 18 (Edwards syndrome): 47,+18. Severe, usually fatal in infancy. Trisomy 13 (Patau syndrome): 47,+13. Severe. Sex chromosome aneuploidies: Klinefelter syndrome: 47,XXY. Males with extra X. Tall, hypogonadism, infertility, gynaecomastia, low testosterone, reduced intelligence. Turner syndrome: 45,X (monosomy X). Female missing one X. Short stature, webbed neck, primary amenorrhoea, streak ovaries, infertility, normal intelligence usually. Structural abnormalities: Deletions, duplications, inversions, translocations → gene dosage effects. Cri-du-chat: 5p deletion. Philadelphia chromosome: t(9;22) → BCR-ABL → CML (leukemia).
Go deeper
Trisomy 21 mechanism: maternal nondisjunction at meiosis I (most common, ~80%). Both homologous chromosome 21s go to same oocyte → n+1 egg → fertilized by normal sperm → trisomy. Frequency increases with maternal age (egg quality, cohesin degradation over decades of arrest). Robertsonian translocation: translocation of chromosome 21 to chromosome 14 → carrier parent has 45 chromosomes (functional trisomy 21 possible in offspring). Familial Down syndrome from Robertsonian translocation — NOT maternal age related.
From Class 10
Sometimes errors in cell division lead to offspring with the wrong number of chromosomes. This causes chromosomal disorders like Down syndrome (extra chromosome 21).