Question

x^{b = color blind}

x⁺ = normal vision

In which parent did nondisjuntion occur? Was it in Meiosis 1 or Meiosis 2 ?

yx^{b   X} x^b x⁺  -----> x⁺  

please draw a diagram of how you found the nondisjuntion and any methods you used to figure out in which parent the nondisjuntion occured.

Answer 1

Y Xb X Xb X+   X+ O(Null / NO chromosome )

- The daughter is color-blind, got the X+ chromosome from her mother. Thus her mother contributed a normal gamete in that it had one X chromosome. She is X+ O which means that she did not get any sex chromosomes from her father. Thus, during gamete production in her father, Nondisjunction occurred in such away that he produced a gamete with no sex chromosomes.

- Nondisjunction:-   It can occur during either meiosis I or II, with differing results.

If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.

If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome. if a gamete with no copies of the chromosomes combines with a normal gamete during fertilization, the result is monosomy.

- Th above cross clearly shows in which parent disjunction occurred.