Chromosome disorders are condition where there is either a structural abnormality, an extra abnormal portion or an abnormal number of chromosomes compared to normal.
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 missing portion of chromosome 5. Patients have learning, developmental and speech and language difficulties and a characteristic “cat like cry” as infants.
Duplication disorders occur where 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 a duplication of the short arm of chromosome 17. Patients suffer with sensory and motor neuropathy and have characteristic pes cavus (high arching foot).
Translocation disorders occur where a portion of one chromosome is directly swapped with a portion of another chromosome. The swap can be balanced (reciprocal translocations), where a portion of one chromosome is swapped with a portion of another. Alternatively they can be unbalanced (nonreciprocal translocations), where a portion of one chromosome leaves the first chromosome and attaches to the other without any exchange taking place.
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 acute myeloid leukaemia, which is a reciprocal translocation between chromosome 9 and chromosome 22.
Robertsonian translocations occur in acrocentric chromosomes. These chromosomes are 13, 14, 15, 21 and 22. They have a longer long arm, which contains most of the genetic material, and a very short short arm with very little genetic information on it. When a person has a Robertsonian translocation, they loose the short arm completely, and the two long arms connect to each other at the centromere, essentially loosing a chromosome when they get rid of the two short arms. In this scenario the person is usually phenotypically normal, but has 45 chromosomes when counted and has a risk of problems in their offspring.
Trisomy is where the person has an extra chromosome. They have a total of 47 chromosomes. They have three copies of a particular chromosome. There are three conditions worth knowing about for your exams:
Patau syndrome: This 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. Look out for rocker bottom feet in exams.
Edwards syndrome: This 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’s syndrome: This is trisomy 21. This is the most common trisomy condition. See the full section on Down’s.
Mosaicism is an interesting scenario where the chromosomal abnormality actually happens after conception. The abnormality occurs in a portion of cells in the body and not in others. The person therefore has different genetic material in different cells in their body. Each case is unique and the effects are unpredictable.
Last updated January 2020