Mistakes In Meiosis
Sources of Error
• The millions of meiotic divisions are
susceptible to error due to:
– Mistakes in separation of chromosomes during
division
– Incorrect exchange of genetic info during crossing
over
Errors in Chromosome Separation
• Result in abnormal number of chromosomes
in a gamete
– If these gamete cells are involved in fertilization,
the zygote will have an abnormal number of
chromosomes
• This is termed aneuploidy
Cause of Aneuploidy
• Non-disjuction
1) Homologous chromosomes fail to separate in
meiosis I

all gametes are incorrect
2) Sister chromatids fail to separate during meiosis II

Half of the gametes are incorrect
Example of Aneuploidy
• Down Syndrome
– 3 copies (instead of 2) of chromosome 21
• Trisomy 21
Risk of Down Syndrome by Maternal Age
Some Terms
• Monosomy – a zygote with one missing
homologous chromosome
– in humans = 45 total chromosomes
• Trisomy – a zygote that has one extra
homologous chromosome
– In humans = 47 total chromosomes
• Triploidy – non-disjunction of all chromosomes
– In humans 3n = 69, rare, more common in plants
(wheat example)
Abnormal Structure
• Due to errors in crossing over (metaphase I)
Types: deletions, duplications, inversions,
translocations
• A child produced from these chromosomes
will have a wide range of symptoms of
varying severity
Abnormal Structure Cont’d
• Deletions – missing part of a chromosome
– Fragment does not attach during crossing over
• Duplication – extra section of chromosome
– Attachment of chromosome onto a complete
homologue
• Inversions – reversed order of genes on a
chromosome
• Translocations – exchange between nonhomologous chromosomes