When Meiosis goes wrong

Important terms Deletion Translocation Duplication Inversion Non-disjunction Monosomy Trisomy Prenatal Karyotype

Learning goals To understand the ways that meiosis can go wrong To be familiar with some of the more common genetic disorders that occur due to chromosomal malfunction

Review Meiosis is used to make gametes When it works (and it usually does) each gamete ends up with a haploid number of cells (23 for humans What do you notice about the 4 gametes?

Meiosis errors But sometimes it doesn’t go according to plan Mistakes happen There are two main types of errors Errors caused by changes in chromosome structure Errors caused by changes in chromosome number

Errors in chromosome structure Usually occurs during the process of crossing over Sometimes when parts of chromosomes detach and switch places, small pieces get removed or inverted (put in upside down)or switched to a completely different chromosome. Sometimes only one chromosome loses a piece and another chromosome gets one

Errors in Structure When any of these things happen there is a chance that a baby that forms from one of the gametes will be deformed

Errors in Structure Structural disorders can lead to many abnormalities and increase the likelihood of some diseases, including cancer Table 4.1 on page 177 in your textbook describes some of these READ IT!!!! Cri du chat Syndrome is a deletion disorder

Errors in number Occurs when homologous chromosomes or sister chromatids don’t separate properly in Anaphase (I or II) This is called nondisjunction

Errors in chromosome number When non-disjunction occurs, one gamete will lose a chromosome and one will get an extra one Both cases can cause defects As you can see from the karyotype chart, the later chromosomes are shorter Since they’re shorter, they’re more likely to get moved to the wrong place

Non-disjunction disorders Trisomy 21 – Down Syndrome- Trisomy 18 -Edward Syndrome- Cause – 3 chromosome #21 Chances – 1 in 800 births (more common with older mothers) Signs- flattened face, intellectual disabilities, short stature Cause – 3 chromosome #18 Chances – 1 in 18000 Signs- facial abnormalities, intellectual and physical disabilities

Non-disjunction of sex chromosomes Klinefelter Syndrome XXY Turner Syndrome XO 1 in 1000 males Unable to produce sperm Female features (e.g. breast development) 1 in 5000 females Short stature, webbed neck, sexual immaturity