Turner syndrome can result from sex chromosome non-disjunction

in the germ line of either parent. If an individual with Turner

syndrome is red-green color-blind and both parents have normal

vision, in which parent did the non-disjunction most likely

occur?

The father

Explanation:

Turner syndrome is a monosony condition resulting from the fusion of normal gamete with a gamete lacking a chromosome. The resulting zygote lacks in one chromosome. In human, Turner syndrome is deleterious in males and only occur in female.

Color blindness is a X-linked recessive trait. A male with XY chromosome will need just a copy of the allele while a normal XX female will need 2 copies (one from each parent). A X-0 female (Turner syndrome) only need a copy of the recessive allele.

Since both parents have normal vision, it means that the father was not affected for the disease and the affected X chromosome must have been from the carrier mother. It thus means that the missing X chromosome is the fault of the father.

Therefore non-disjunction of the XY chromosome must have happened in the father leading to inability to contribute a X chromosome to the daughter.