# Feedback

As always, we will have to read the problem carefully and know that it is a disease controlled by a recessive allele linked to sex. That is, linked to the X chromosome or the Y chromosome. As it is also transmitted by daughters, even if they do not suffer from it, we will have to think that it is a recessive allele linked to the X chromosome.

As the father has normal vision and the mother also, we will assume that she is a carrier. They can have children with the following genotypes.

Parents: X^{N}Y x X^{N}X^{n}

Gametes X ^{N} Y X^{N} X ^{n}

Children (possible genotypes) X^{N}Y X^{n}Y X^{N}X^{N} X^{N}X^{n}

Let's go back to the problem:

**The first male child** has nystagmus, but he has a normal daughter. Therefore, it seems clear that this son is XnY and has a daughter with a woman who contributed X^{N}, we assume normal (X^{N}X^{N}), although she could also be a carrier (X^{N}X^{n}).

X^{n}Y x X^{N}X^{N}

Gametes X^{n} YX^{N}

Normal daughter X^{N}X^{n} She is a carrier of the disease, although her vision is normal.

**The second son** is normal and has a normal daughter and son. This child has to be X^{N}Y, and his wife had to contribute another X^{N} chromosome, we assume normal (X^{N}X^{N}), although she could also be a carrier (X^{N}X^{n}).

X^{N}Y x X^{N}X^{N}

Gametes X^{N} Y X ^{N}

Normal son and daughter X^{N}Y X^{N}X^{N}

**The first daughter** is normal and she has 8 children all normal. As she has so many children and all with normal vision, we must think that she is X^{N}X^{N}, and her partner, X^{N}Y.

X^{N}X^{N} x X^{N}Y

Gametes X^{N} X^{N} Y

Normal sons and daughters X^{N}Y X^{N}X^{N}

**The second daughter** has two normal daughters and two sons, one normal and one affected. Since she does not say anything, we assume that her partner has normal vision, X^{N}Y, and she, although she also has normal vision, is a carrier of the disease (X^{N}X^{n}).

X^{N}X^{n} x X^{N}Y

Gametes X^{N} N^{n} X^{N} Y

Child genotype X^{N}Y N^{n}Y X^{N}X^{N} X^{N}X^{n}

Normal son Sick son Normal daughters (50% carriers)