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 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

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 non-homologous chromosomes