Genetics problem help?

A woman's genotype is XRXr (XX is female and Rr is the phenotype, creating incomplete dominance), man's genotype is XrY (sex-linked usually means only X chromosome carries the gene). When the two have a child, it will be a boy if it has a Y chromosome and either XR or Xr from the mother. So, genotype XRY can happen 50% of the time, which will result in severe rolling.

Let N = nystagmus
Let S = normal

Note that in incomplete dominance, no allele is completely dominant over the other.

XX = female
XY = male

Because this is an example of incomplete dominance, a woman who exhibits slight nystagmus has a phenotype that is intermediate between two other phenotypes.  Therefore, a woman with slight nystagmus must be a heterozygote, and therefore carries the nystagmus allele (N) and the normal allele (S).  In this case, the male is hemizygous for the normal allele (S).

XN XS = slightly nystagmus female
XS Y = normal male

Sorry, I can't use subscripts for the alleles.

Therefore, your cross is between parents with the above genotypes (XN XS x XS Y).

Based on this crossing, you can conclude that:
50% of the female offspring will be heterozygous and slightly nystagmus (XN XS)
50% of the female offspring will be homozygous and normal (XS XS)
50% of the male offspring will be hemizygous for the normal allele (XS Y)
50% of the male offspring will be hemizygous for the nystagmus allele (XN Y)

Therefore, there is a 50% (1/2) chance of having a nystagmus male (severe rolling)