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Case study 3: Crossing Over
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Case study 3: Crossing Over



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  • 1. Crossing Over Suma Gondi and Kathryn Addabbo
  • 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):
    • Female genitalia: Yes
    • Breasts and pubic hair: Yes
    • Regular menstrual cycle: Never
    • From this information, you conclude that Santhi is:
    • A: Male B: Female
    • The answer is B, Santhi is female.
  • 3. CQ2: A karyotype was performed on an athlete. Here are the results:
    • From these results, the athlete is:
    • A: Male B: Female
    • The answer is A, because Santhi possesses both an X and Y chromosome.
  • 4. CQ3: So if Santhi is a normal female, her karyotype would be:
    • A: XX
    • B: XY
    • C: YY
    • D: XXY
    • The answer would be A, females possess two X
    • chromosomes.
  • 5. CQ4: Assume that this is one of Santhi’s chromosomes. This chromosome is composed of two chromatids joined by a centromere.
    • A: These chromatids make up a diploid chromosome.
    • B: The cell that contains these sister chromatids must be diploid.
    • C: The sister chromatids were formed by replicating a single chromatid.
    • D: The sister chromatids were joined by fertilization, bringing together a maternal and paternal chromatid.
    • The answer is C, because the
    • chromatids are identical.
  • 6. CQ5: What is true after Meiosis I?
    • A: Four cells have been produced.
    • B: The cells are haploid.
    • C: The DNA will be replicated once more.
    • D: The cells are ready to perform as gametes.
    • E: Each chromosome consists of a single strand of DNA.
    • The answer is B.
  • 7. CQ6: During Meiosis II:
    • A: Homologous chromosomes separate.
    • B: The DNA is replicated.
    • C: Gametes fuse.
    • D: Sister chromatids separate.
    • E: All of the above.
    • The answer is D. Sister chromatids are pulled apart.
  • 8. CQ7: How many possible combinations of maternal chromosomes are possible in a human ovum due to independent assortment during meiosis?
    • A: 23 combinations.
    • B: 46 combinations.
    • C: 23 2 = 529 combinations.
    • D: 2 23 = ~ 8 million combinations.
    • The answer is D. There are many different combinations.
  • 9. CQ8: Could Santhi have an XX karyotype and be male?
    • A: No, an XX individual is always female.
    • B: Yes, this is common.
    • C: Yes, if a male-determining control gene is carried on one of her X chromosomes.
    • The answer is C.
  • 10. CQ9: During crossover, all of the following happen EXCEPT:
    • A: The homologous chromosomes line up in pairs.
    • B: The homologous chromosomes swap pieces with their adjacent partners.
    • C: Crossing over decreases the genetic variability in the gametes.
    • D: Each chromatid is unique after it has crossed over.
    • The answer is C.
  • 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?
    • A: She has female genitalia, allow her to keep her medal.
    • B: She is genetically male, take her medal away.
    • C: Perform additional tests.
    • Our personal choice is A.
  • 12. CQ11: What do you think about requiring gender testing for female athletes in international competition?
    • A: It should be banned because gender determination is so complex.
    • B: It is necessary to ensure an even playing field.
    • C: It is necessary, but needs to include a large number of genetic tests to ensure fairness.
    • D: It should be required for all athletes, both male and female.
    • 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.