Haemophilia is a sex-linked recessive trait. A son born to phenotypically normal parents has Klinefelter syndrome and hemophilia. In which meiotic division of which parent is non-disjunction of the X chromosome most likely to have occurred? Explain with diagrams and state your assumptions.
To determine the most likely scenario for non-disjunction of the X chromosome leading to Klinefelter syndrome and hemophilia in the son, we need to understand the normal meiotic divisions involved in the formation of sperm and eggs.
Assumptions:
1. The mother is phenotypically normal, which means she does not have hemophilia or Klinefelter syndrome.
2. The father is also phenotypically normal, which means he does not have hemophilia or Klinefelter syndrome.
3. Klinefelter syndrome is caused by an extra X chromosome (XXY genotype), resulting from non-disjunction during meiosis.
4. Hemophilia is a sex-linked recessive trait, meaning it is carried on the X chromosome.
Now, let's break down the possible scenarios:
1. Non-disjunction in the mother's meiosis I:
In this case, during the mother's meiosis I division, the X chromosomes fail to separate correctly. As a result, one daughter cell could receive both X chromosomes, while the other daughter cell would have no X chromosome. If the egg with no X chromosome is fertilized by a normal sperm (with an X chromosome), the resulting zygote would be missing an X chromosome and would not develop into a viable embryo.
Diagram:
Mother's Meiosis I: XX (mother's chromosome pair) ⟶ X0 (no X chromosome) + XX (normal X)
Father's Meiosis: XY (father's chromosome pair) ⟶ X (normal X) + Y
Resulting Zygote: X0 (no X chromosome) + X (normal X) + Y
This scenario is not viable.
2. Non-disjunction in the mother's meiosis II:
In this case, during the mother's meiosis II division, the X chromosomes fail to separate correctly. This would result in two eggs with an extra X chromosome. If one of these eggs is fertilized by a normal sperm, the resulting zygote would have Klinefelter syndrome (XXY).
Diagram:
Mother's Meiosis I: XX (mother's chromosome pair) ⟶ X + X
Mother's Meiosis II: X (extra X) + X ⟶ XX (normal X) + X (extra X)
Father's Meiosis: XY (father's chromosome pair) ⟶ X (normal X) + Y
Resulting Zygote: XX (normal X) + X (extra X) + X (normal X) + Y
This scenario leads to Klinefelter syndrome (XXY) and is feasible.
3. Non-disjunction in the father's meiosis I or II:
Since hemophilia is a sex-linked recessive trait, it must be inherited from the mother, who is phenotypically normal. Therefore, non-disjunction in the father's meiosis leading to hemophilia is not possible.
Based on the assumptions and the possible scenarios, it is most likely that non-disjunction of the X chromosome occurred during the mother's meiosis II division, resulting in the son having both Klinefelter syndrome (XXY) and hemophilia (inherited from the mother), as shown in scenario 2.