Why monosomy X (Turner syndrome) is survivable
The full 45,X pattern (45 chromosomes with a single X) seen in Turner syndrome is survivable because one X is already largely switched off in normal female cells.
Remember
A cell needs two working copies of an autosome (a chromosome other than X or Y) but only one working X. Monosomy, where a cell has only one copy of a chromosome instead of the usual two, is usually lethal for an autosome (the pregnancy is usually lost early) but survivable for the X.
Monosomy means a cell has only one copy of a chromosome instead of the usual two. Having just one copy of an autosome is usually lethal. What matters is gene dosage, the amount of each gene's product that a cell makes. Losing one of a pair of autosomes halves the output of hundreds of genes at once, so the pregnancy is usually lost early.
The X chromosome is a special case. In normal female cells, one of the two X chromosomes is largely switched off in a process called X-inactivation, so a female cell already works with a single active X. Males (XY) have one X and survive. A cell therefore needs only one working X, and a person with 45,X can survive.
Survival still comes at a cost. The condition is called Turner syndrome. Females with it are typically shorter than average and typically have streak ovaries, which are underdeveloped ovaries that appear as thin streaks of tissue. About 15% of the genes on the X escape inactivation, meaning they are active on both X chromosomes, so with one X a person gets only half the usual dose of those genes. Loss of one copy of one of them, the SHOX gene, is thought to cause the short stature.
Key facts
- Monosomy
- a cell has only one copy of a chromosome instead of the usual two.
- Autosomal monosomy
- usually lethal, because the genes on the missing chromosome are left at half dosage.
- X-inactivation
- in normal female cells one X is largely switched off, so one active X is enough.
- Chromosome count
- written 45,X, which is 45 chromosomes in total with one X, as in Turner syndrome.
- Cost of survival
- people with Turner syndrome are typically shorter than average and have underdeveloped ovaries, so survival is possible but not trouble-free.
- SHOX
- a gene on the X that escapes inactivation, where loss of one copy is thought to cause the short stature.
Exam traps
- A single X does not double its gene output to compensate for a missing X. Normal female cells function with one active X because of X-inactivation, and male cells have one X to begin with.
- A Barr body, which is the inactivated X chromosome in a normal female cell, does not explain survival with a single X. Forming one from the only X would silence the one active X and worsen gene dosage.
- A small autosome such as chromosome 21 is not made survivable by its small size. Complete monosomy of any autosome is usually lethal, because half the gene dosage is lost.
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