Chromosome disorders occur where there is either a structural abnormality, missing or additional chromosomal material or an abnormal number of chromosomes compared to normal.
Deletion Disorders
Deletion disorders occur where a portion of a chromosome is missing. These syndromes are very rare, and you are unlikely to come across them, particularly in medical school exams. One example is cri du chat, which is caused by a deletion in part of chromosome 5. Patients have learning, developmental and speech and language difficulties and a characteristic “cat-like cry” as infants.
Duplication Disorders
Duplication disorders occur when a portion of a chromosome is duplicated. The chromosome contains twice the number of copies of that gene. One example is Charcot-Marie-Tooth, which can be caused by duplication of a specific region of chromosome 17. Patients suffer from sensory and motor neuropathy and have a characteristic pes cavus (high-arched foot).
Translocation Disorders
Translocation disorders occur where a portion of one chromosome is directly swapped with a portion of another chromosome. The swap can be balanced (reciprocal translocation), in which a portion of one chromosome is exchanged with a portion of another. Alternatively, they can be unbalanced (nonreciprocal translocations), in which a portion of one chromosome leaves the first chromosome and attaches to the other without an exchange.
Translocation does not usually lead to a specific genetic syndrome, but often predisposes to other conditions, such as cancer and infertility. One example is the “Philadelphia chromosome” translocation in chronic myeloid leukaemia, a reciprocal translocation between chromosomes 9 and 22.
Robertsonian translocations occur in acrocentric chromosomes, which are chromosomes 13, 14, 15, 21 and 22. These chromosomes have a longer long arm, which contains most of the genetic material, and a shorter short arm, which contains very little genetic information. When a person has a Robertsonian translocation, they lose the short arms completely, and the two long arms fuse at the centromere, effectively losing a chromosome. In this scenario, the person is usually phenotypically normal but has 45 chromosomes and is at risk of problems in their offspring.
Trisomy Disorders
Trisomy disorders involve an extra chromosome, resulting in a total of 47 chromosomes. There are three copies of a particular chromosome. The three to remember are Patau syndrome, Edward syndrome and Down syndrome.
Patau syndrome is trisomy 13. The syndrome varies in severity. Patients have dysmorphic features, structural abnormalities affecting almost all areas of their body, and learning disability. They have characteristic “rocker bottom feet”, where the soles of the feet are convex (rounded outwards) in shape.
Edward syndrome is trisomy 18. The syndrome varies in severity and affects almost all areas of the body, resulting in dysmorphic features and learning disability. They also have “rocker bottom feet”.
Down syndrome is trisomy 21. This is the most common trisomy condition.
Mosaicism
Mosaicism is a condition in which a chromosomal abnormality occurs after conception. The abnormality occurs in some cells of the body and not others. The person has different genetic material in various cells of their body. Each case is unique, and the effects are unpredictable.
Last updated November 2025
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