Chromosomal disorders: Down's, Turner's and Klinefelter's syndromes
Chromosomal disorders come from a missing, extra or wrongly arranged chromosome. Aneuploidy causes Down's (trisomy 21), Klinefelter's (XXY) and Turner's (XO) syndromes.
How chromosomal disorders arise
A chromosomal disorder is caused by the absence, excess or abnormal arrangement of one or more chromosomes. Sometimes the chromatids or homologous chromosomes fail to separate during cell division (non-disjunction). Then one cell gets an extra chromosome and another gets one too few. A person with three copies of one chromosome has trisomy (2n + 1); one with only one copy has monosomy (2n − 1).
Aneuploidy vs polyploidy
Three syndromes compared
| Feature | Down's syndrome | Klinefelter's syndrome | Turner's syndrome |
|---|---|---|---|
| Cause | Extra copy of chromosome 21 (trisomy 21) | Extra X in a male | One X missing in a female |
| Chromosomes | 47 (45 autosomes + XX or XY) | 47 (44 autosomes + XXY) | 45 (44 autosomes + XO) |
| Sex | Male or female | Male | Female |
| Main features | Short, small round head, furrowed tongue, partly open mouth; broad palm with a characteristic palm crease; delayed physical, psychomotor and mental development | Mostly masculine, but some feminine development such as breast development (gynaecomastia); sterile | Short stature; sterile because the ovaries are rudimentary; other secondary sexual characters lacking |
How an extra chromosome 21 arises
- 1During meiosis in a parent, the two copies of chromosome 21 fail to separate.
- 2One gamete gets two copies of chromosome 21 (24 chromosomes); another gets none (22).
- 3The 24-chromosome gamete fuses with a normal gamete (23 chromosomes).
- 4★The zygote has 47 chromosomes, with three copies of chromosome 21: trisomy 21.
- 5In the same way, an XX egg + Y sperm gives XXY (Klinefelter's), and an egg with no X + X sperm gives XO (Turner's).
★ key step
Counting chromosomes in each syndrome
How many chromosomes, and how many autosomes, are present in a body cell of a person with (a) Down's syndrome, (b) Klinefelter's syndrome, (c) Turner's syndrome?
- A normal cell has 46 = 44 autosomes + 2 sex chromosomes.
- (a) Down's: one extra autosome (21), so 47 in all, with 45 autosomes + XX or XY.
- (b) Klinefelter's: one extra X, so 47 in all, with 44 autosomes + XXY.
- (c) Turner's: one X missing, so 45 in all, with 44 autosomes + X (written XO).
(a) 47, with 45 autosomes; (b) 47, with 44 autosomes; (c) 45, with 44 autosomes.
Doctors may offer screening and amniocentesis in pregnancy to check the chromosomes of the foetus. Children with Down's syndrome do best with early therapy, good nutrition and inclusive schooling.
2 marks: “Distinguish between aneuploidy and polyploidy.” Aneuploidy: gain or loss of one or a few chromosomes, from failure of chromatids to separate; e.g. Down's syndrome. Polyploidy: increase in whole sets of chromosomes, from failure of cytokinesis after telophase; common in plants.
3 marks: “Give the chromosomal cause and two features each of Down's, Klinefelter's and Turner's syndromes.” Down's: trisomy 21; short, furrowed tongue, palm crease, delayed development. Klinefelter's: XXY male; gynaecomastia, sterile. Turner's: XO female; rudimentary ovaries, sterile, no secondary sexual characters.
Down's: trisomy 21. Klinefelter: eXtra X in a male (XXY). Turner: Takes away an X (XO).