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Case study 3: Crossing Over


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Case study 3: Crossing Over

  1. 1. Crossing Over Suma Gondi and Kathryn Addabbo
  2. 2. CQ1: Imagine you are a member of the committee assigned to determine whether Santhi is female. Here are possible results of the initial tests (we don’t know the real results): <ul><li>Female genitalia: Yes </li></ul><ul><li>Breasts and pubic hair: Yes </li></ul><ul><li>Regular menstrual cycle: Never </li></ul><ul><li>From this information, you conclude that Santhi is: </li></ul><ul><li>A: Male B: Female </li></ul><ul><li>The answer is B, Santhi is female. </li></ul>
  3. 3. CQ2: A karyotype was performed on an athlete. Here are the results: <ul><li>From these results, the athlete is: </li></ul><ul><li>A: Male B: Female </li></ul><ul><li>The answer is A, because Santhi possesses both an X and Y chromosome. </li></ul>
  4. 4. CQ3: So if Santhi is a normal female, her karyotype would be: <ul><li>A: XX </li></ul><ul><li>B: XY </li></ul><ul><li>C: YY </li></ul><ul><li>D: XXY </li></ul><ul><li>The answer would be A, females possess two X </li></ul><ul><li>chromosomes. </li></ul>
  5. 5. CQ4: Assume that this is one of Santhi’s chromosomes. This chromosome is composed of two chromatids joined by a centromere. <ul><li>A: These chromatids make up a diploid chromosome. </li></ul><ul><li>B: The cell that contains these sister chromatids must be diploid. </li></ul><ul><li>C: The sister chromatids were formed by replicating a single chromatid. </li></ul><ul><li>D: The sister chromatids were joined by fertilization, bringing together a maternal and paternal chromatid. </li></ul><ul><li>The answer is C, because the </li></ul><ul><li>chromatids are identical. </li></ul>
  6. 6. CQ5: What is true after Meiosis I? <ul><li>A: Four cells have been produced. </li></ul><ul><li>B: The cells are haploid. </li></ul><ul><li>C: The DNA will be replicated once more. </li></ul><ul><li>D: The cells are ready to perform as gametes. </li></ul><ul><li>E: Each chromosome consists of a single strand of DNA. </li></ul><ul><li>The answer is B. </li></ul>
  7. 7. CQ6: During Meiosis II: <ul><li>A: Homologous chromosomes separate. </li></ul><ul><li>B: The DNA is replicated. </li></ul><ul><li>C: Gametes fuse. </li></ul><ul><li>D: Sister chromatids separate. </li></ul><ul><li>E: All of the above. </li></ul><ul><li>The answer is D. Sister chromatids are pulled apart. </li></ul>
  8. 8. CQ7: How many possible combinations of maternal chromosomes are possible in a human ovum due to independent assortment during meiosis? <ul><li>A: 23 combinations. </li></ul><ul><li>B: 46 combinations. </li></ul><ul><li>C: 23 2 = 529 combinations. </li></ul><ul><li>D: 2 23 = ~ 8 million combinations. </li></ul><ul><li>The answer is D. There are many different combinations. </li></ul>
  9. 9. CQ8: Could Santhi have an XX karyotype and be male? <ul><li>A: No, an XX individual is always female. </li></ul><ul><li>B: Yes, this is common. </li></ul><ul><li>C: Yes, if a male-determining control gene is carried on one of her X chromosomes. </li></ul><ul><li>The answer is C. </li></ul>
  10. 10. CQ9: During crossover, all of the following happen EXCEPT: <ul><li>A: The homologous chromosomes line up in pairs. </li></ul><ul><li>B: The homologous chromosomes swap pieces with their adjacent partners. </li></ul><ul><li>C: Crossing over decreases the genetic variability in the gametes. </li></ul><ul><li>D: Each chromatid is unique after it has crossed over. </li></ul><ul><li>The answer is C. </li></ul>
  11. 11. CQ10: If you were a member of the Asian Games medal committee and Santhi’s karyotype revealed that she is XY and SRY+ , what would you do? <ul><li>A: She has female genitalia, allow her to keep her medal. </li></ul><ul><li>B: She is genetically male, take her medal away. </li></ul><ul><li>C: Perform additional tests. </li></ul><ul><li>Our personal choice is A. </li></ul>
  12. 12. CQ11: What do you think about requiring gender testing for female athletes in international competition? <ul><li>A: It should be banned because gender determination is so complex. </li></ul><ul><li>B: It is necessary to ensure an even playing field. </li></ul><ul><li>C: It is necessary, but needs to include a large number of genetic tests to ensure fairness. </li></ul><ul><li>D: It should be required for all athletes, both male and female. </li></ul><ul><li>Our personal answer is both A and C. Gender determination is very complex, as the possibilities are endless. Yet if testing continues, it should be so fairly. </li></ul>