The genes encoding the red- and green-color-detecting proteins of the human eye are located next to one another on the X chromosome and probably evolved from a common ancestral pigment gene. The two proteins demonstrate 76 percent homology in their amino acid sequences. A normal-visioned woman (with both genes present on each of her two X chromosomes) has a red-color-blind son who was shown to have one copy of the green-detecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).

Key Concepts

X-Linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. Since males have one X and one Y chromosome, they are more likely to express recessive traits linked to the X chromosome, such as color blindness. In this case, the son inherits his X chromosome from his mother, which carries the green-detecting gene but not the red-detecting gene, leading to red-color blindness.

Meiosis and Genetic Recombination

Meiosis is the process of cell division that produces gametes (sperm and eggs) with half the number of chromosomes. During meiosis, homologous chromosomes can exchange genetic material through a process called recombination. This can result in gametes with different combinations of alleles, which explains how the son could inherit the green-detecting gene from his mother while lacking the red-detecting gene.

Gene Homology and Evolution

Gene homology refers to the similarity in sequence between genes due to shared ancestry. The 76 percent homology between the red- and green-color-detecting proteins suggests that they evolved from a common ancestral gene. Understanding this evolutionary relationship helps explain why these genes are located close together on the X chromosome and how mutations in one can affect the function of the other.