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 who is color blind and has hemophilia and a woman who is wild type have a daughter with triple X syndrome (XXX) who has hemophilia and normal color vision.

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 both 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 the question, nondisjunction could explain the presence of an extra X chromosome in the daughter with triple X syndrome.

Meiotic Division

Meiotic division consists of two rounds of cell division (meiosis I and meiosis II) that result in four genetically diverse gametes. In meiosis I, homologous chromosomes are separated, while in meiosis II, sister chromatids are separated. Understanding which meiotic division nondisjunction occurs in is crucial for determining the genetic makeup of the offspring, as it affects the distribution of chromosomes in the resulting gametes.