Red-green color blindness is an X-linked recessive disorder. Evan has a 47,XXY karyotype and is color blind. His 46,XY brother, Samuel, is also color blind. Both of their parents have normal color vision. Where did the nondisjunction occur that caused Evan to be color blind and have Klinefelter Syndrome?
The Correct Answer and Explanation is :
The nondisjunction that caused Evan to have Klinefelter Syndrome (47,XXY) and be color blind likely occurred in Evan’s father during meiosis I.
Explanation:
- Key Facts About Color Blindness:
- Red-green color blindness is an X-linked recessive disorder.
- Males are typically affected if they inherit an X chromosome with the defective gene because they have only one X chromosome (46,XY).
- Females are typically carriers (46,XX) unless they inherit two defective X chromosomes.
- Evan’s Karyotype and Condition:
- Evan has a 47,XXY karyotype, indicating Klinefelter Syndrome caused by nondisjunction during gamete formation.
- He is color blind, meaning he inherited an X chromosome with the defective gene.
- Samuel’s Karyotype and Condition:
- Samuel is 46,XY and color blind, meaning he inherited an X chromosome with the defective gene and a normal Y chromosome.
- Parents’ Phenotype:
- Both parents have normal color vision, so Evan and Samuel’s defective X chromosome must have been inherited from their mother, who is a carrier.
- Nondisjunction Analysis:
- For Evan to have Klinefelter Syndrome (47,XXY), he must have inherited two sex chromosomes from one parent and one from the other.
- His defective X chromosome came from his mother, so the additional sex chromosome must have come from his father.
- The father contributed both an X and a Y chromosome due to nondisjunction during meiosis I, where homologous chromosomes (X and Y) failed to separate.
- Why Meiosis I?
- If nondisjunction occurred in meiosis II, the father would contribute either two X chromosomes or two Y chromosomes, not both.
Thus, Evan’s condition results from paternal nondisjunction in meiosis I.