Summary
Failure of chromosomes to separate in meiosis, producing gametes with extra or missing chromosomes.
In depth
Homologues fail to separate in meiosis I or sister chromatids in meiosis II, giving gametes with n+1 or n-1 chromosomes. Fertilisation then produces trisomy or monosomy.
Most autosomal aneuploidies are lethal in the embryo, except for chromosomes 13, 18 and 21.
Maternal age is the strongest known risk factor for trisomies because eggs remain arrested in meiosis I for decades, and cohesion between chromosomes weakens with time: the risk of Down syndrome is about 1 in 1500 at age 20 and 1 in 100 at age 40. In males, errors are more often paternal for sex chromosome conditions.
Nondisjunction in mitosis causes mosaicism, with some cells normal and others abnormal, which may produce milder disease. Polyploidy, whole extra sets of chromosomes, is another type and is common in plants such as wheat and bananas.
Remember: trisomy has three copies of a chromosome (2n+1), monosomy has one (2n-1); only a few autosomal trisomies, such as 13, 18 and 21, allow survival to birth.
Frequently asked questions
What is Nondisjunction?
Failure of chromosomes to separate in meiosis, producing gametes with extra or missing chromosomes.
Which area of biology covers Nondisjunction, and how advanced is it?
Nondisjunction is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include X-Chromosome Inactivation, Law of Segregation, Turner and Klinefelter Syndromes.
Can you give an example of Nondisjunction?
Risk of Down syndrome increases with maternal age, from about 1 in 1500 at age 20 to 1 in 100 at 40.
What should I learn before Nondisjunction?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Nondisjunction. Intermediate topics assume the Beginner vocabulary.
What topics are related to Nondisjunction?
The closest topics to Nondisjunction are X-Chromosome Inactivation, Law of Segregation, Turner and Klinefelter Syndromes, Mitochondrial Inheritance. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Nondisjunction?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Nondisjunction. To practise the whole of Genetics, use the Genetics quiz.
How is Nondisjunction different from X-Chromosome Inactivation?
Nondisjunction: Failure of chromosomes to separate in meiosis, producing gametes with extra or missing chromosomes. X-Chromosome Inactivation: One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage. Read both side by side and note what each one explains.
How should I study Nondisjunction step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with X-Chromosome Inactivation and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.