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Chapter 5: Problem 24

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 on each of her two X chromosomes has a redcolor-blind son who was shown to have one copy of the greendetecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).

Short Answer

Expert verified
Answer: Unequal crossing over during meiosis in the woman's egg cells could explain a normal-visioned woman having a red-color-blind son. This event can lead to the deletion of the red-detecting gene on one X chromosome, which is inherited by the son, causing his red-color-blindness.

Step by step solution

01

Understand the genetic information provided

The genes encoding the red and green color-detecting proteins are located next to each other on the X chromosome and have 76 percent homology in their amino acid sequences. This means that they are similar genes and might have evolved from a common ancestral pigment gene. The woman has both genes on each of her two X chromosomes and has normal vision.
02

Analyze the son's genetic makeup

The woman's son has one copy of the green-detecting gene and no copies of the red-detecting gene, making him red-color-blind. Since males inherit one X chromosome from their mother and one Y chromosome from their father, it means that the son inherited an X chromosome from his mother that carries one copy of the green-detecting gene and no copies of the red-detecting gene.
03

Investigate the possible chromosomal event during meiosis

A possible explanation for these observations can be a chromosomal event like unequal crossing over during meiosis in the woman's egg cells. Unequal crossing over occurs when homologous chromosomes exchange genetic material in an unequal manner, resulting in the duplication of a region on one chromosome and deletion of that same region on the other chromosome.
04

Explain the cause of red-color-blindness in the son

In our case, it is likely that unequal crossing over happened between the red and green genes on the X chromosomes in the woman. This event led to the deletion of the red-detecting gene on one X chromosome, which was inherited by the son, and a possible duplication of the green-detecting gene on the other X chromosome, which was not inherited by the son. As a result, the son lacks the red-detecting gene, causing his red-color-blindness.

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Most popular questions from this chapter

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Review the Chapter Concepts list on p. \(83 .\) These all center on sex determination or the expression of genes encoded on sex chromosomes. Write a short essay that discusses sex chromosomes as they contrast with autosomes.

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