Question

A child is born with hemophilia (inherited as an X-linked recessive) and Klinefelter's syndrome to parents who both have normal blood clotting. In which parent (mother or father) and during which round of meiosis (I or II) did the nondisjunction event occur?

* note that Klinefelter syndrome is 47 chromosomes, so 2n+1 (47,XXY)

Answer 1

For X chromosome

Females are homozygous - means they have two X chromosomes and

Males are hemizygous - means they have only one X chromosome.

As the females are homozygous, if they have recesseive allele on one X chromsome, the expression of recessieve allele depends on the allele of the other X chromsome, if that X chormosome have dominant allele, normal phenotype will be expressed. If both X chromsomes have recessive alleles then female organism will be effected. Incase of males, as males have only one X chromosome irrespective of the status dominant or recessieve allele, it will be expressed.

The non disjunction occurs in Mother.

As hemophilia is an X linked recesseive, if it is inherited from father, the father should express this phenotype, as the father expressed normal phenotype which means the mother is heterozygous for this gene which means she is a carrier.

The nondisjuction occurs in Meiosis II.

If the non disjuction occurs at meiosis I, the gametes has both alleles (normal and recessive), due to this child should be a normal phenotype, as the child expressed hemophilia which means the disjuction occurs at meiosis II.

Find the below image for explanation...