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).

Verified step by step guidance

1. Understand that the genes for red and green color detection are located on the X chromosome and are closely linked, meaning they are likely to be inherited together unless a recombination event occurs.
2. Recognize that the son is red-color-blind, which indicates he lacks the red-detecting gene on his X chromosome. Since males have only one X chromosome, this gene must be missing from the X chromosome he inherited from his mother.
3. Consider the process of meiosis in the mother, where homologous chromosomes (the two X chromosomes) can undergo recombination. This is where segments of DNA are exchanged between homologous chromosomes.
4. Propose that during meiosis in the mother, a recombination event occurred between the red and green pigment genes on her X chromosomes. This recombination could have resulted in one X chromosome with two copies of the green-detecting gene and no red-detecting gene.
5. Conclude that the son inherited the X chromosome from his mother that had undergone this recombination event, resulting in him having one copy of the green-detecting gene and no red-detecting gene, leading to his red-color-blindness.

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.