Nondisjunction
A failure of chromosomes to separate in meiosis. A gamete gets an extra chromosome or is missing one. After fertilisation the baby has the wrong number.
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Analogy
Dealing one hand 24 cards and another 22.
In practice
Down syndrome is usually caused by an extra copy of chromosome 21 (trisomy 21).
Practice
Flashcards
- What causes trisomy 21?Nondisjunction, giving an extra chromosome 21.
Questions people ask
Does the risk of Down syndrome increase with maternal age?
Yes. Egg cells begin meiosis before birth and pause for decades, and the chance of chromosomes failing to separate rises with age. The risk is about 1 in 1,500 for a mother aged 20 and around 1 in 100 at age 40, though most babies are born healthy.
Are all chromosome number changes fatal?
No, but most are. Most embryos with the wrong number of chromosomes do not survive and are lost early in pregnancy. A few, such as trisomy 21, survive. Extra or missing sex chromosomes, like XXY or XO, are often less severe than errors in other chromosomes.