Chromosomes and their aberrations

The essentials

Chromosomes are thread-like structures in the nucleus made of DNA wound around proteins. A normal human body cell has 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). Gametes carry 23 chromosomes. The arrangement of a person's chromosomes by size and shape is called a karyotype.

Key points

• Structure: each replicated chromosome has two sister chromatids joined at the centromere; ends are called telomeres. By centromere position, chromosomes are metacentric (middle), submetacentric, acrocentric (near one end) and telocentric (at the end; not found in normal humans).
• Numerical aberrations arise mostly from non-disjunction (failure of chromosomes to separate in meiosis). Aneuploidy means gain or loss of single chromosomes; polyploidy means extra whole sets.
• Down syndrome: trisomy 21, 47 chromosomes; risk rises with maternal age.
• Edwards syndrome: trisomy 18. Patau syndrome: trisomy 13.
• Turner syndrome: 45,X (XO) female; short stature, underdeveloped ovaries.
• Klinefelter syndrome: 47,XXY male; tall, small testes, often infertile.
• Structural aberrations: deletion (loss of a segment, e.g. cri-du-chat, deletion on chromosome 5), duplication, inversion (segment reversed) and translocation (segment moved to a non-homologous chromosome).

Exam tips

• Memorise the chromosome count and formula for each syndrome.
• Monosomy means 2n−1 (45); trisomy means 2n+1 (47).

Common mistakes

• Thinking Turner syndrome is in males; it occurs in females with one X.
• Confusing a gene mutation (change in DNA sequence) with a chromosomal aberration (change in number or structure).