Chromosomal disorders
How errors in the number or structure of chromosomes arise, mostly when eggs and sperm form, why an extra or missing chromosome disturbs so many organs, and the main syndromes, from trisomy 21 to Turner and Klinefelter syndromes.
By the end, you can
- Explain how nondisjunction in meiosis produces trisomy and monosomy, and why maternal age matters.
- Describe deletions, duplications and translocations, and why balanced carriers are usually healthy.
- Relate the gene-dosage effect of trisomy 21 to its heart, gut, brain and blood features.
- Explain why sex-chromosome aneuploidies are milder, using Turner and Klinefelter syndromes.
- Recognise the classic features of Down, Edwards, Turner and Klinefelter syndromes and what karyotype, FISH, microarray and cell-free DNA screening show, and explain how supportive and hormone treatment act on the consequences.
What it is
Chromosomal disorders result from a change in the number (aneuploidy) or structure of chromosomes large enough to alter the dose of many genes at once. Most arise as new events when eggs or sperm form; many affected pregnancies end early. Those compatible with birth include trisomies 21, 18 and 13, sex-chromosome aneuploidies and many small deletions and duplications.
3 types to tell apart
- Numerical (aneuploidy)
- A whole chromosome extra (trisomy) or missing (monosomy), usually from nondisjunction in meiosis.
- Structural
- A segment lost (deletion), doubled (duplication), moved (translocation) or reversed (inversion). Balanced rearrangements lose nothing and the carrier is usually healthy.
- Mosaicism
- An error after fertilisation gives two cell lines with different chromosomes; features are often milder.
Normal function
Forty-six chromosomes, halved and restored
A body cell has 46 chromosomes in 23 pairs: 22 pairs of autosomes and the sex chromosomes, XX or XY. Photographed in pairs during cell division, they form the karyotype, written as 46,XX or 46,XY.
Eggs and sperm are made by meiosis, two divisions that halve the number. In meiosis I the two chromosomes of each pair separate; in meiosis II the two sister chromatids of each chromosome separate. The egg casts off its unused sets as . Egg development pauses in meiosis I from before the mother’s own birth until ovulation, which may be decades later.
At fertilisation the egg and the sperm each bring 23 chromosomes, seen as the two of the , restoring 46.
Underlying change · Step 1 of 4
Nondisjunction: an extra or missing chromosome
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Underlying change · Step 2 of 4
Broken and rejoined chromosomes
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 4
Trisomy 21: too much of many genes
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 4 of 4
Other trisomies and the sex chromosomes
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .