4.2 & 10.1 meiosis


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4.2 & 10.1 meiosis

  1. 1. Genetics 4.2 & 10.1 Meiosis
  2. 2. Meiosis <ul><li>Diploid – two sets of each chromosome present </li></ul><ul><ul><li>In humans, where can you find diploid cells? </li></ul></ul><ul><li>Haploid – one set of each chromosome present </li></ul><ul><ul><li>In humans, where can you find haploid cells? </li></ul></ul><ul><li>Meiosis is a reduction division. This means that a diploid nucleus is divided into 4 haploid nuclei. </li></ul><ul><li>These become the nuclei for gametes (egg and sperm cells). </li></ul>
  3. 3. Homologous chromosome <ul><li>Homologous chromosomes are the chromosomes that are the same size and show the same banding pattern. They contain the same set of genes but their alleles are not identical . </li></ul>
  4. 4. Meiosis <ul><li>Summary of Meiosis: </li></ul><ul><ul><li>Meiosis occurs after interphase </li></ul></ul><ul><ul><li>(replication has occurred). </li></ul></ul><ul><ul><li>There are 2 divisions of the nucleus (each with prophase, metaphase, anaphase, telophase, cytokinesis). </li></ul></ul><ul><ul><li>The homologous chromosomes are paired in prophase 1. </li></ul></ul><ul><ul><li>The result of the process is four haploid cells. </li></ul></ul>
  5. 6. Crossing Over <ul><li>Crossing over (i.e. recombination or synapsis) allows for genetic variability </li></ul><ul><li>Only happens in meiosis (prophase I) </li></ul><ul><li>Forms a chiasmata between non-sister chromatids on homologous chromosomes. </li></ul><ul><ul><li>Homologous chromosomes pair loosely along their lengths </li></ul></ul><ul><ul><li>Homologues align with each other gene by gene. </li></ul></ul><ul><ul><li>Homologous portions of 2 non-sister chromatids trade places (2-3 per chromosome pair for humans) </li></ul></ul><ul><ul><li>Locations where these genetic exchanges have occurred are visible as chiasmata. </li></ul></ul>
  6. 7. Crossing over
  7. 9. Genetic Variability <ul><li>Gametes have genetic variability through </li></ul><ul><ul><li>Crossing over during prophase I </li></ul></ul><ul><ul><li>Independent Assortment- Random orientation during metaphase I </li></ul></ul><ul><li>More genetic variability comes with random fertilization of gametes. </li></ul><ul><li>Egg x Sperm </li></ul><ul><li>(2 23 combinations of chromosomes) x (2 23 combinations of chromosomes ) </li></ul><ul><li>2 23 x 2 23 diploid combinations </li></ul><ul><li>1 in 70 trillion chance for the same genes- not even considering crossing over. </li></ul>
  8. 10. Non-Disjunction <ul><li>Non-disjunction occurs when the centromeres do not uncouple which leads to one nucleus with a missing chromosome and one nucleus with an extra chromosome. </li></ul><ul><li>This is what causes genetic disorders like Down Syndrome (trisomy 21) </li></ul>
  9. 11. Karyotyping <ul><li>A karyotype is an image of a set of chromosomes that is taken during metaphase. The chromosomes are lined up in homologous pairs based on size and structure. </li></ul>
  10. 12. Karyotyping <ul><li>Karyotyping is performed using cells located by chorionic villus sampling or amniocentesis. This allows pre-natal (before birth) diagnosis of chromosome abnormalities. </li></ul>
  11. 13. Genetic Screening Using Karyotyping <ul><li>Karyotyping can be used to screen people for genetic diseases. </li></ul><ul><li>This allows for: </li></ul><ul><ul><li>The probability of a couple having a child with genetic disease can be determined beforehand. </li></ul></ul><ul><ul><li>Fewer children to be born with genetic diseases because fetuses can be aborted or people carrying the diseases can avoid reproduction. </li></ul></ul><ul><li>Therefore there can be; </li></ul><ul><ul><li>Less cost for long-term health care </li></ul></ul><ul><ul><li>Less frequency of harmful alleles in the gene pool </li></ul></ul>
  12. 14. Genetic Screening Using Karyotyping <ul><ul><li>Early detection of a genetic disease in a fetus also allows… </li></ul></ul><ul><ul><li>Treatment to begin early for the genetic disease </li></ul></ul><ul><ul><li>Parents of affected children can be prepared psychologically for the arrival of their baby. </li></ul></ul>
  13. 15. Questions about Karyotyping <ul><li>Risks of side effects from karyotyping (e.g. abortion of the fetus). </li></ul><ul><li>Who should make the decisions about karyotyping and abortion? </li></ul><ul><li>Should national governments interfere with personal freedom to abort fetuses that have genetic disease? </li></ul><ul><li>Should abortion based on gender be allowed? How could it be stopped? </li></ul>
  14. 16. Analyze this karyotype: <ul><li>What is the gender? Has non-disjunction occurred? </li></ul>
  15. 17. Analyze this karyotype: