Two brothers have X‑linked red–green colorblind vision, and their parents have normal color vision. The first brother's karyotype is 47,XXY (Klinefelter syndrome), and the second brother's karyotype is 46,XY. In which parent and in what cell division phase did the first brother's chromosomal nondisjunction occur? Assume no recombination.

The answer is mother in meiosis II

Explanation:

nondisjunction can be defined as the failure of the segregation of the child chromosomes in meiosis I and meiosis II resulting in gametogenes. This results in abnormal gametes with some chromosomal imbalance being formed and subsequent fertilization of these gametes results in a generation of abnormal individuals.

According to exercise, red-green color blindness follows an X-linked recessive pattern and the phenotype is only expressed. the father cannot bring the possibility of color blindness to his children, and the nondisjunction, defined earlier, cannot occur in either meiosis I or meiosis II. As a conclusion we can say that it may have a place in meiosis I or meiosis II corresponding to the mother and although the mother's condition may be normal, the first child would be compromised with color blindness, therefore, the mother is the carrier.