Correct option is B
Klinefelter syndrome occurs when a male has two X chromosomes (XXY), one more than the typical XY configuration. For Son 1 to have an XXY genotype, there must have been a nondisjunction event during the formation of the gametes, specifically during female meiosis II.
Explanation:
Nondisjunction in female meiosis II occurs when the sister chromatids of an X chromosome do not separate properly during meiosis II. This can lead to an egg with two X chromosomes instead of one.
When this egg (with two X chromosomes) is fertilized by a normal Y-carrying sperm from the father, the resulting offspring will have an XXY chromosome composition, which is characteristic of Klinefelter syndrome.
Meiosis I in females involves the separation of homologous chromosomes (X and X), and nondisjunction here could also result in abnormalities, but it would not lead to the XXY configuration, as we would expect a chromosome set with either XX or XY, but not XXY.
Thus, nondisjunction in female meiosis II is the correct event that caused Klinefelter syndrome in Son 1.
Son with Klinefelter syndrome and colour Blindness: Both the parents have normal vision but both the sons are colorblind. Since colorblindness is X linked recessive disorder, the sons have obtained the allele for colorblindness from mother. This makes the mother carrier for colorblindness. The genotype of the mother is X^cX. The young man with Klinefelter syndrome is colorblind which means that he is homozygous recessive for the allele of colorblindness. His genotype is X^cX^cY. Since this man has obtained two copies of the allele of colorblindness, the mother must have experienced nondisjunction at meiosis II during gamete formation.
Anaphase-II of meiosis II separates and segregates the sister chromatids (now called daughter chromosomes) to opposite poles. Failure of segregation of two copies of X^c chromosome of mother to opposite poles and their distribution to the same egg cell resulted in the formation of an egg with two copies of X^c chromosome. Fertilization of this egg with a sperm carrying "Y" chromosome as sex chromosome resulted in a zygote with X^cX^cY that developed into the man with Klinefelter syndrome and colorblindness.
