1. Mistakes In Meiosis

2. 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

3. 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

4. Cause of Aneuploidy Non-disjuction
Homologous chromosomes fail to separate in meiosis I
all gametes are incorrect
Sister chromatids fail to separate during meiosis II
Half of the gametes are incorrect

5. Example of Aneuploidy Down Syndrome
3 copies (instead of 2) of chromosome 21
Trisomy 21

6. Risk of Down Syndrome by Maternal Age

7. 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)

8. 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

9. 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 non-homologous chromosomes