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.
A man who is color blind and a woman who is wild type have a son with Jacob syndrome (XYY) who has hemophilia.

Verified step by step guidance

1. Understand the genetic basis of the traits: Both color blindness and hemophilia are X-linked recessive traits, meaning the genes responsible for these conditions are located on the X chromosome. Males (XY) inherit their X chromosome from their mother and their Y chromosome from their father, while females (XX) inherit one X chromosome from each parent.
2. Analyze the parents' genotypes: The man is color blind, so his X chromosome must carry the allele for color blindness (X^c). The woman is wild type, meaning her X chromosomes do not carry the alleles for color blindness or hemophilia (X⁺).
3. Determine the son's genotype: The son has Jacob syndrome (XYY), meaning he has two Y chromosomes and one X chromosome. He also has hemophilia, so his X chromosome must carry the allele for hemophilia (X^h).
4. Identify the source of nondisjunction: Nondisjunction is the failure of chromosomes to separate properly during meiosis. Since the son has two Y chromosomes, nondisjunction must have occurred in the father during spermatogenesis. This resulted in a sperm cell with both Y chromosomes.
5. Determine the stage of nondisjunction: If nondisjunction occurred during the first meiotic division, the two Y chromosomes would fail to separate, leading to a sperm cell with two Y chromosomes. If nondisjunction occurred during the second meiotic division, a single Y chromosome would fail to separate, but this would not result in a sperm cell with two Y chromosomes. Therefore, nondisjunction must have occurred during the first meiotic division in the father.

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) are more likely to express these traits if they inherit the affected X chromosome. Females, having two X chromosomes, can be carriers without expressing the traits if they have one normal X chromosome.

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, identifying where nondisjunction occurs is crucial for understanding the genetic makeup of the offspring, particularly in relation to the Jacob syndrome (XYY) condition.

Meiotic Division

Meiotic division consists of two rounds of cell division (meiosis I and meiosis II) that result in four genetically diverse gametes. The first meiotic division separates homologous chromosomes, while the second separates sister chromatids. The timing of nondisjunction—whether it occurs in the first or second meiotic division—affects the resulting gametes and can lead to conditions like Jacob syndrome, which involves an extra Y chromosome.