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Non Disjunction in the 1st Meiotic Division
A trisomy describes a cell (or individual) that contains an entire extra chromosome (three) rather than the normal pair of (two) chromosomes. Humans with a trisomy have a total of 47 chromosomes rather than the normal 46 chromosomes per cell. Most trisomies are lethal mutations and result in miscarriage (spontaneous abortion). Trisomy 21 (Down’s syndrome) individuals can survive into adulthood. Down’s syndrome has an incidence rate that varies with maternal age (about 1 per 1000 at 30 years old, about 1 per 100 at 40 years old, and up to 1 in 35 at 45 years old). About 95% of trisomy 21 occurs due to nondisjunction at the time of meiosis within the egg. In this illustration, trisomy 21 (Down’s syndrome) occurs (in the fertilized egg) due to nondisjunction of chromosome 21 within the maturing egg during the 1st meiotic division (which is completed at the time of ovulation). | |||||||||||||
