Can you please explain why the answers A) and C) are correct? I dont understand
ID: 262519 • Letter: C
Question
Can you please explain why the answers A) and C) are correct? I dont understand why nondisjunction only occured with the mom and how im supposed to distinguish between nondisjunction in meiosis 1 and 2. Please help. Thank you!
Consider the pedigree below from a human family. The affected male has Klinefelter syndrome, a genetic condition that results when a child is born with XXY chromosomes. Phenotypes shown in the pedigree (A, B, or AB) are determined by the co?dominant alleles XA and XB.
The mom has an X^A, X^A (unaffected) and the father has an X^BY (unaffected). Their child has X^A X^A Y and is affected with Klinefelter syndrome.
Based on the pedigree, nondisjunction must have occurred:
A. In meiosis 1 of the mother
B. In meiosis 1 of the father
C. In meiosis 2 of the mother
D. In meiosis 2 of the father
Explanation / Answer
Since the condition of Klinefelter syndrome is XXY making total of 47 chromosomes. It is only possible when there is nondisjunction in egg (ovum) of female during meiosis. It is not possible in father (male) because sperm containing Y chromosome can create YY when undergone nondisjunction, and during fertilization it will result in XYY condition which is almost normal.
It may be either meiosis I or II nondisjunction because both can generate gametes with XX condition. In case of meiosis I nondisjunction, all the four gametes are with altered chromosome numbers while meiosis II nondisjunction create only 2 gametes with altered chromosome numbers and 2 gametes with normal chromosome number.