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 man and a woman who each has the wild-type phenotype have a son with Klinefelter syndrome (XXY) who has hemophilia.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In this case, traits such as color blindness and hemophilia are X-linked recessive, 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. In meiosis, nondisjunction can occur in either the first or second meiotic division, resulting in aneuploidy, such as Klinefelter syndrome (XXY). Identifying when nondisjunction occurs is crucial for understanding the genetic outcomes in offspring.

Klinefelter Syndrome

Klinefelter syndrome is a genetic condition in males characterized by the presence of an extra X chromosome, resulting in a 47,XXY karyotype. This condition can lead to various physical and developmental features, including infertility and certain health issues. In the context of the question, the presence of Klinefelter syndrome in the son indicates that nondisjunction likely occurred in one of the parents during gamete formation.