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 a woman who is wild type have a son with Jacob syndrome (XYY) 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, color blindness and hemophilia are both X-linked recessive traits, meaning that males (who have one X and one Y chromosome) will express the trait if their single X carries the recessive allele, while females (with two X chromosomes) must have both Xs carrying the recessive allele to express the trait.

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, resulting in conditions such as Jacob syndrome (XYY) when an extra Y chromosome is present, which can affect male characteristics and fertility.

Meiotic Division

Meiotic division consists of two rounds of cell division (meiosis I and meiosis II) that produce gametes with half the number of chromosomes. In meiosis I, homologous chromosomes are separated, while in meiosis II, sister chromatids are separated. Understanding where nondisjunction occurs—whether in meiosis I or II—is crucial for determining the genetic outcomes in offspring, such as the presence of Jacob syndrome and hemophilia in this scenario.