Chromosomal abnormalities

Radiation, diet or conditions inside the body can damage chromosomes or change their number. A change in a chromosome's structure is a structural aberration; a change in the number of chromosomes is a numerical one.

Numerical changes:

• Aneuploidy changes the count of one chromosome: monosomy (2n - 1, one copy missing) or trisomy (2n + 1, three copies instead of two).
• Polyploidy multiplies whole sets: in humans 69 chromosomes (23 × 3, triploid) or 92 (23 × 4, tetraploid). It is rare and serious in humans, but common in crops: bread wheat is hexaploid.

Structural changes, which you can remember as lose it, repeat it, flip it or move it:

• Deletion: a segment is lost, as on chromosome 13 in retinoblastoma (an eye cancer). If both ends are lost, the broken ends can join into a ring chromosome.
• Duplication: a segment is repeated, as on chromosome 17 in Charcot-Marie-Tooth disease (a nerve disorder).
• Inversion: a segment is cut out and put back turned through 180°. The chromosome keeps its length, but the gene order is reversed (ABCD becomes DCBA).
• Translocation: a segment moves to a different chromosome. A two-way swap is reciprocal, as between chromosomes 8 and 14 in Burkitt's lymphoma (a cancer).

Two words to tell apart: a syndrome is a set of signs and symptoms that occur together; a disease is a broader abnormal condition of the body.