For the following crosses, determine as accurately as possible the genotypes of each parent, the parent in whom nondisjunction occurs, and whether nondisjunction takes place in the first or second meiotic division. Both color blindness and hemophilia, a blood-clotting disorder, are X-linked recessive traits. In each case, assume the parents have normal karyotypes (see Table 10.2).

A color-blind man and a woman who is wild type have a daughter with Turner syndrome (XO) who has normal color vision and blood clotting.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In this case, color blindness and hemophilia are X-linked recessive traits, meaning that males (XY) are more likely to express these traits since they have only one X chromosome. Females (XX) can be carriers if they have one affected X chromosome but typically do not express the trait unless both X chromosomes are affected.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to gametes with an abnormal number of chromosomes. This can occur during either the first or second meiotic division. In the context of Turner syndrome (XO), nondisjunction likely occurred during the formation of the egg or sperm, resulting in a missing X chromosome in the daughter.

Turner Syndrome

Turner syndrome is a chromosomal disorder characterized by the presence of a single X chromosome (XO) in females, leading to various developmental and physical features. Individuals with Turner syndrome often have normal color vision and blood clotting if they inherit a normal X chromosome from one parent. Understanding this condition is crucial for analyzing the genetic implications of the cross described in the question.