Question

The genes encoding the red-and green-color-detecting proteins of the human eye are located next to one another on the \(\mathrm{X}\) 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).

Short answer

Answer: The chromosomal explanation lies in an unequal crossover event during the mother's meiosis, which resulted in the loss of one of her red-detecting genes and the duplication of the green-detecting gene on one of her X chromosomes. This abnormal X chromosome was then passed on to her son during fertilization, resulting in his red color blindness.

Step-by-step solution

Understand the genetic background of color vision

The genes responsible for red and green color detection are located on the X chromosome. This means that color vision is a sex-linked trait. Since males have only one X chromosome, they can be more prone to color blindness if they inherit a defective gene from their mother.

Examine the mother's chromosomal background

The mother has normal vision, which means that both of her X chromosomes have the red and green color-detecting genes. Her genetic background can be represented as: \(X^{R}X^{G}X^{R}X^{G}\), where \(R\) stands for red, and \(G\) stands for green.

Understand the son's genetic background

The son has red color blindness, which means that he has no copies of the red-detecting gene. He has one copy of the green-detecting gene, which he inherited from his mother. His genetic makeup can be represented as: \(X^{G}Y\), where \(Y\) represents his Y chromosome.

Determine the cause of the chromosomal abnormality

During meiosis, the mother's X chromosomes undergo crossing over, which is the exchange of genetic material between non-sister chromatids of homologous chromosomes. In this case, there must have been an unequal crossover event that resulted in the loss of one of the mother's red-detecting genes and the duplication of the green-detecting gene on one of her X chromosomes. This abnormal chromosome was then passed on to the son during fertilization, resulting in his red color blindness.

Visualize the crossover event

The unequal crossing over event in the mother's X chromosomes resulted in the following genetic makeup: Normal Chromosome: \(X^{R}X^{G}\) Abnormal Chromosome: \(X^{G}X^{G}\) When the abnormal chromosome is passed to the son, his genetic makeup becomes: \(X^{G}Y\), which explains his red color blindness.

Key concepts

Sex-Linked Trait

When we talk about sex-linked traits, we refer to genetic characteristics that are associated with genes located on sex chromosomes. In humans, the sex chromosomes are X and Y, with females having two X chromosomes (XX) and males having one X and one Y chromosome (XY). Since color blindness primarily affects the X chromosome, and males have only one X chromosome, they are more likely to express color blindness if the X chromosome they inherit carries the mutation.

This form of inheritance is crucial in understanding why diseases like color blindness occur more frequently in males. Women, having two X chromosomes, would require mutations in both copies of the gene to express the trait, making them less likely to be colorblind, although they can still carry and pass on the trait, which can be seen in the exercise where a woman with normal vision has a son with red color blindness.

X Chromosome Inheritance

Diving deeper into the X chromosome inheritance, it is important to recognize that males inherit their single X chromosome from their mother, while their Y chromosome comes from their father. This is a key aspect in the inheritance of sex-linked traits like color vision, contributing to why the son in the given exercise inherited red color blindness.

A female's genetic makeup for color vision could be symbolized as XX, where each 'X' represents an X chromosome with the normal vision genes, while a male's genotype would be denoted as XGY, indicating he inherited color blindness. The X chromosome's inheritance patterns are the very reason why certain traits can be passed from mother to son, often skipping the daughters.

Unequal Crossover During Meiosis

Meiosis is a specialized type of cell division that occurs to produce gametes, such as sperm and egg cells, with half the number of chromosomes of the parent. An interesting event that can occur during meiosis is unequal crossover. This happens when homologous chromosomes exchange genetic material unevenly. In the context of color blindness,

During the formation of the mother's egg cells, an unequal crossover could have resulted in one chromosome with both green-detecting gene copies (XGXG) and another with neither. Consequently, when the mother's abnormal X chromosome with double green genes (XGXG) combines with a normal Y from the father, you end up with a son who carries XGY — a genetic makeup that explains his red color blindness.